The R216 is a portable
"Intercept Receiver" used in the 1950s for example
for eavesdropping on VHF radio signals emanating from East Germany.
Several types of receiver were used for this general purpose
including various Eddystone models but, comparing the two makes,
the R216 (made by Ekco in the early 1950s) I think is superior
to the Eddystone types (and which anyway do not offer the portability
of the R216)and looks more professionally made... no doubt to
an unrestricted "cost plus" budget. Frequency coverage
is pretty wide and the receiver can demodulate AM, MCW and FM
or CW with selectivity at 30 or 120KHz. (The R216 looks similar
to the R210, a short-wave receiver).
During 1965 the R216 receiver was fitted in a new Canberra variant,
the B Mk6 which was effectively a sort of GCHQ outstation (ref
RAF Strike Command 1968-2007: Aircraft, Men and Action).
As you can see in the
above picture the set uses a 4-section turret tuner, seen below
from a couple of views.
The use of a turret tuner allows
the coils for the different wavebands to offered up to the RF
valves without the need for extraneous wiring (like that used
in the R208).
Above is a view showing the
chassis of the IF/AF unit (note V12 is missing ... perhaps used
to replace another in a more important role). V12 is used to
provide an IF output (at 4.86MHz) for
connection to alternative demodulation equipment.
Below right is the RF chassis
which includes the front-end plus the crystal calibrator (top)
and left the IF/AF chassis. The curved aluminium panel marked
"Serial 593" encloses the rear of the dial. Should
the serial number originally have matched that of the receiver
which is 597? Perhaps an inadvertent switch was made when mods
1 & 2 were being incorporated (see mod record panel below)?
The valve line up is slightly odd
as it uses a mixture of both battery and mains valves possibly
because VHF battery valves were relatively uncommon, although
one might query the use of the EF91 as crystal calibrator and
1st IF amplifier (the simple explanation may have been to just
keep the valve heater types common to each of the two chassis,
but then... why use the EB91.. was there no suitable 1.4 volt
equivalent?). Note that the valve listing doesn't include an
RF mixer. For some odd reason the designer chose MR1, referred
to as a "Westinghouse silicon diode rated at 19 volts and
80uA" which carries the code "KMSA" which I'm
assuming was the manufacturer's part number:-
1st RF Amplifier
2nd RF Amplifier
1st IF Amplifier
2nd IF Amplifier
3rd IF Amplifier
4th IF Amplifier
IF Output buffer
AM AF Amplifier
Detector & Noise Limiter
Microwave silicon diode
Above and below... RF and IF/AF
circuits of the R216. Click to see a larger image complete with
Of course you'll have noticed
the R216 uses an external power supply: Two versions were used,
AC and DC connecting via the multi-way "Plessey" connector
PLE on the front panel (and SKE on the PSU). Below is an enlarged
view of the plug with its connections.
Note that the external DC power
supply includes a second battery switch made by a link in the
receiver across pins M and L which is seen once the lead from
the DC power supply is plugged into the receiver. M is fed by
the fuse following the power supply main on/off switch connected
to battery positive terminal and L is the input to the battery
power supply main circuit. Maybe a method of saving battery life
when the DC power supply is left switched on when a receiver
isn't plugged in?
You'll note that the wiring
takes account of sharing currents to minimise cable losses and
the resultant voltage drops.
400mA DC 900mA AC
Ground return for V6, V7, V8, V9, V10, V11, V12, V13, V14, V15,
Filament supply for V7, V8, V9, V10, V11, V12, V14, V16
900mA AC max
Heater for V3 + V5 + dial lamp return
HT supply for valves V1, V2, V3, V5, V6
HT supply for valves V7, V8, V9, V10, V11, V12, V14, V16
Grid bias supply for valves V6, V7, V8, V14, V16
Heater for V1, V2, V6, V13, V15
Ground return for V1, V2, V3, V5
300mA AC max
Dial lamp supply voltage (return via Pin C)
Not shown in circuit diagram, not connected
DC power supply battery positive linked to Pin M
DC power supply battery feed linked to Pin L
This is a close up of the R216
power connector. You'll note that there are two polarising techniques
used. The plastic insert has a couple of protruding pips and
the outer aluminium shell has five slots cut in the inner section.
Within the assembly there are also six slots allowing the inner
plastic part to be located correctly. The five slots give six
These connectors were made originally
by Plessey and their code number for the "Free Socket"
mating connector to this is 508-1-40009-320. The corresponding
NATO code is 5821-99-638-1648.
Weald Electronics now make these connectors
under their LMF/LMG range. The code for the mating part for the
R216 plug is LMF/1/40009/320 clearly lined up with the original
Plessey code to avoid confusion.
The relationship between those five
shell slots and orientations is given by the last digit in the
final 3-digit part of the code number. So you can have 320/321/322/323/324/325.
The picture opposite has the polarisation pip between Pin A and
Pin B symmetrical between the two pairs of shell slots. This
is given the 320 orientation number. As the pip rotates through
60/120/180/240/300 degrees the number increases by one 321/322/323/324/325.
For those unfamiliar with battery valves
I'll explain the need for a grid bias supply. Most valves, battery
or those with indirectly heated cathodes ("mains" valves)
draw anode current when an HT supply is connected. The value
of this current is determined by the valve characteristics which
call for a voltage between control grid and cathode. In the case
of "mains" valves the addition of a cathode resistor
will result in a negative control grid voltage (bias). The higher
the resistor, the greater the bias and the less the anode current.
This method of biasing is usually termed automatic bias and roughly
maintains a constant average anode current. With battery valves
the use of a cathode resistor isn't practical so each valve may
need a separate grid bias voltage to set its anode current. The
designer will work out the optimum value for bias voltages. One
could just ground the bias lines but this would result in increased
anode currents, and although may not affect operation much (it
might produce distortion), it will reduce the life of the HT
battery. Another point to mention is the decoupling capacitor
used in auto-bias. One can tailor the effect of auto-bias on
the frequency of the signal being amplified by altering the cathode
resistor decoupling capacitor, making it anything from nothing
to several tens of microfarads. Although the anode current of
the valve is roughly governed by the cathode resistor (say 600
ohms) working at DC, a 25uF capacitor will have an impedance
of about 6 ohms at 1KHz thus shunting the 600 ohm cathode resistor
and much lessening the instantaneous biasing of the control grid
and producing a 1KHz current at the valve anode in excess of
the DC value.
A bias voltage can not only modify the
valve anode current, but with some specially designed valves
called variable mu valves, increasing the bias will reduce the
gain of the valve. This technique is very useful in receivers
which might have to deal with signals ranging from 1 microvolt
to hundreds of millivolts. To handle this range of signals an
AGC or AVC circuit is usually employed. The amplitude of the
received signal is converted to a negative control voltage and
applied as feedback to the amplifying circuits. Their gain is
reduced, the amplitude drops and the amount of feedback is reduced.
Very rapidly the receiver will establish a more or less constant
output no matter what the received signal strength. This protects
an operators ears but there should ideally be an S-Meter as all
signals will sound much the same. It depends on exactly what
the receiver was designed for as to whether an S-Meter was included
or not. For some reason the R216 does not use AGC, instead it
has an RF gain control. This sets the overall bias level for
those valves arranged to be controllable.
In the R216 you'll see CV138, EF91 valves
which are variable mu types, as are the CV785, DF91. Type CV1758,
DF92 is not variable mu and if you examine the circuit diagram
you'll see that these are operated under zero-bias condition
(control grids DC-wise tied to chassis).
The first step in getting the
receiver working is to build a power supply. As I already have
a general purpose power unit (see below) for running an R1155
separately from the T1154, I decided to modify this so it would
be suitable for the R216 whilst still enabling it to be used
with the R1155 (this means isolating the HT negative from ground
because the R1155 uses this to develop an extensive network of
bias voltages.. perhaps via a front panel switch which can be
used to ground or open HT negative from common ground).
The R216 requires several voltages
which I'll summarise...
1.4 volts DC
@ 400mA. This supply is extremely
critical as it's used to feed the filaments of the battery valves.
I chose to use a linear voltage regulator type LMS1585ACT-1.5
whose output is a nominal 1.5 volts (1.47 to 1.53 volts) at up
to 5 Amps and which requires an input voltage of 2.9 volts to
14.5 volts. The device requires only input and output smoothing
capacitors plus a "minimum drain" resistor of about
150 ohms across the output. The circuit is fitted to the left
of the right hand mains transformer above and is driven from
the 6.3 volt winding of the transformer on the left and comprises
a full wave bridge rectifier, the LMS1585ACT-1.5/NOPB, two 2200uF
smoothing capacitors and a small 47uF output capacitor (needed
for stability). A 150 ohm load resistor is fitted across the
choc block terminals carrying the output voltage. Because of
the low voltage it's important to minimise the output wiring
length to avoid voltage loss when current is drawn. Using four
3.3 ohm resistors wired in parallel this supply drew 1.8 Amps
with the output voltage maintaining over 1.47 volts.
100 volts DC @
26mA. This is the HT voltage
for the battery valves and is supplied from the basic HT line
of 300 volts. In the centre of the baseboard you'll see a tagstrip
carrying two plastic power transistors type BUT11. The left transistor
is a series pass regulator with its base connected to a pair
of 51 volt zener diodes connected in series and fed with a 22Kohm
feed resistor from the 300 volt HT line. The emitter of the BUT11
is maintained at 100 volts. I tested this supply using a 1.1Kohm
load resistor drawing 88mA with the output dropping from 100
to 99.8 volts.
250 volts DC @
43mA. Using the same technique
as the 100 volt stabiliser, the base of the second BUT11 is fed
with a chain of five 51 volt zener diodes and a 8.2Kohm resistor
to provide an output of 250 volts at its emitter.
6.3 volts AC @
3A. This is supplied directly
by the right hand transformer, above.
-19 volts DC @
15uA. This voltage needs to
be perfectly hum-free. I fitted a third mains transformer to
the baseboard for the bias supply (and also the dial lamp supply).
The total power drain for this circuit is pretty low so the transformer
needs a rating of no more than say 10VA. To ensure compatibility
with the dial lamp AC supply (as in the official R216 power supply)
the DC output will be provided by a half-wave rectifier followed
by a twin pi-filter using two 22 Kohm resistors and three 2200uF
19 volts AC @ 300mA
max. In the official R216 power
supply this voltage is supplied by the same winding as the bias
supply and I used a small standard transformer with twin windings
to match the requirement.
Metering of the various
voltages is carried out via the rotary switch in the centre of
the front panel connected to a voltmeter and to monitor HT current
I used a milliammeter. This indicates the drain of the main 300
volt HT supply, but because the HT supply also provides feeds
to the 100V and 250V stabilisers I arranged the wiring to take
the feed for the zener diodes from the input of the milliammeter
rather than the output. This ensures that only the HT consumption
of external equipment will be monitored. The latest part of construction
is the addition of stabilized voltages minus 19 volts, 100 volts
and 250 volts (left to right above). I used an 18 volt zener
for the 19 volt supply (a PNP series pass transistor) and 51
volt zener diodes using two for the 100 volt supply and five
for the 250 volt supply (both with NPN series pass transistors).
A permanent R1155 supply cable
is fitted with a Jones plug and the supply to the R216 is fitted
with a 12-way Plessey plug.
The right hand transformer (top
picture) carries twin 6.3 volt 3 Amp output windings tapped at
4 volts and 5 volts. The twin outputs will be wired so that they
can supply either 6.3 volts 6 Amp or 12.6 volts 3 Amps. The main
HT rail is wired for the R1155 to supply outputs from before
and after the HT choke. Here's a circuit diagram (liable to change).
Note the need to have
dual switches for the voltmeter because of the need to monitor
one negative supply as well as the four positive supplies. The
meter has a 0 to 3 scale which is read as 300V for the HT supplies,
-30V for the grid bias voltage and 3V for the filament voltage.
The movement required 300Kohm for each HT range, 30Kohm for the
grid bias and 3Kohm for the filament voltage.
Because the R1155 has an isolated
HT requirement (+250V including -30 volts bias) as well as needing
a little extra voltage (not necessary for the R216) I used a
2-pole switch and a relay, the relay is activated by the switch
when R1155 operation is required and disconnects the 6.3V heater
winding from HT negative (that connection is required for the
R216) with the second pole of the switch adding a small reservoir
capacitor to the swinging choke circuit used for the R216. I
used the swinging choke circuit to keep the HT voltage down as
the transformer secondary winding was a little high for the task,
probably capable of a maximum, off load, of around 370 volts
with a typical reservoir capacitor. The position of the switch
in the circuit is important as there is a risk of damaging the
HT regulator circuits if their HT return is removed, so I placed
the switch in the 6.3 volt grounding connection (Black) as shown
above. The R1155 heater circuit wiring connects one side of the
6.3 volt supply to the R1155 chassis which is some tens of volts
higher than HT negative so disconnecting the PSU heater ground
connection in this power unit eliminates the risk of the R1155
bias circuits shorting out (note: when a T1154 is used in conjunction
with the R1155 the heater supply has to be DC rather than AC
because that same circuit operates the T1154 send-receive relay
which has a 6 volt DC coil). Unless I add this 2-pole switch
the cabling carrying power to the R216 might need to be disconnected
from the PSU terminals before plugging in an R1155 because R216
Plug Pin A carries HT ground and a grounded heater connection
as well as the filament return and the dial lamp supply. The
relay (which may not have been strictly necessary but gives some
flexibility for later modifications) is driven from the 30 volt
supply used for the -19V R216 grid bias. I added a 220 ohm resistor
to reduce the coil voltage to exactly 24 volts.
The power supply is built on a piece
of chipboard and the front metal panel is isolated, so circuit
ground hasn't any real meaning, other than the interconnection
circuitry between the various output supply voltages. If the
PSU used a metal chassis and this was connected to HT negative
it would have a potential difference of some tens of volts compared
with the R1155. A significant advantage of using wood is that
parts can be moved around easily as design changes are made.
Some more work is required to fit the R1155/R216 switch and the
addition of the voltmeter wiring for monitoring the bias supply.
Once everything is working I can tidy up the rats nest of wiring.
Here's a picture of the R216 plug
wired up. I found a drum of cable with 4-cores plus screen, so
two lengths of cable will do the job as the R216 needs 10 connections
with two being duplicates of earth return for HT and LT. I used
the screens of the two cables connected together giving me enough
wires for carrying the various power connections. The ends of
the cable were soldered to tails secured in the choc bloc.
Once completed I decided to test the
Note that the plugs and sockets use
a fine thread not the very coarse type used in later equipments.
After checking over as much as
I could I plugged in the receiver to the new power supply. There
was a slight crackling sound and smoke rose from somewhere inside
one of the two chassis. Clearly something was amiss but I couldn't
work out what component was burning so I disconnected the various
tails and measured the current for each. The 1.4 volt supply
was drawing 360mA and as one valve is missing (it's only the
IF output buffer so not critical) this current is correct. Next
I connected back the 6.3 volt supply and this stayed at the same
level so I judged it to be OK. Next the bias supply which measured
53uA and the voltage remained constant. The 19 volt AC supply
drew 0.9A which is too high but it didn't change when the illumination
control was turned. I suspect the phase is wrong. Looking at
the proper power supply circuit, the 19 volt supply is derived
from the same winding as the 6.3 volt supply, but the dial lamps
are supplied by the voltage between the 19 volt and 6.3 volt
windings (ie. 12.7 volts). I should therefore modify my PSU which
presumably has the opposite phases for the two voltages resulting
in something odd happening. As there's a spare 12 volt winding
on the bias transformer, I'll use this.
Next, I checked the HT currents. 100
volts drew 12mA and 250 volts drew 9mA which might be OK as these
measurement were taken with the valves cold. On the other hand,
only the regulator should draw HT current, and that from the
250 volt rail so what explains the 100 volt drain? I can see
there's two resistors across the 100 volt rail at V11. These
add up to 55K so should draw 1.8mA leaving 10.2mA unaccounted
for. The RF chassis doesn't use the 100 volt rail so the drain
must be in the IF chassis.
As all the measurements, bar that small
100 volt drain, appeared to be fine I plugged in the receiver
once more... no smoke and the combined HT current measured around
half scale on the milliammeter. As the total HT current is 69mA
I reckoned this wasn't too critical. Plugging in a pair of headphones
gave me a loud hiss on the FM settings, dropping to about half
the volume on the narrower bandwidth setting. AM and CW gave
me a loud hum which dropped off a lot when I reduced the AF gain.
Connecting a length of wire to the aerial socket did crackle
slightly but absolutely no signals were present even from the
crystal calibrator. Apparently the IF module is working but not
the front end.... Maybe the problem is associated with the smoke?
A short-circuit condenser may have open-circuited a resistor?
That would explain why the smoke didn't reappear a second time.
What would smell hot... well a drain through a 10K resistor from
250 volts would be 25mA and dissipate 6 watts and if the resistor
turned into a charred mass of say 50K this would dissipate around
a watt or so and get pretty hot. The 5mA HT current wouldn't
really make much difference in the sets consumption.
A second test... this time I fitted
a spare 1T4 into the unoccupied DF92 V12 socket in case I decided
to monitor the IF output. Turning on the power supply gave me
a quarter scale current reading which slowly rose to half scale
(something like 35mA). I guess this is because the battery valves
have a very short warm-up time and the mains valves need up to
20 seconds or so. After a couple of minutes there was a hot smell
but I couldn't easily figure out from where it was coming. I
also turned on my spectrum analyser to see if any RF signals
were present but drew a blank. This implies the RF chassis has
a problem, perhaps a short-circuit condenser decoupling the HT
line? If this had burned out it's associated resistor (the smoke
when first turning on the receiver) it would all make sense.
R15 and R21 are prime candidates especially the latter. A simple
check is of course to simply remove the can from V4 and see if
the voltage stabilizer is lit (but it's in an awkward position..
and I noticed it lighting up when power is applied to the receiver).
Another test could be to remove all
the mains valves and see what current if any is drawn from the
250 volt supply. Unless I've missed something, only the QS150/15
would draw any HT current... I unplugged the interconnecting
cable from the RF chassis and disconnected the filament supply.
I measured 9mA from the 250 volt supply which disappeared when
V6 was unplugged which seems fine, but I noticed that the HT
rail wasn't too high, being only 230 volts dropping to 205 volts
with the RF chassis plugged in, when the HT draw is 33mA (which
equates to an additional 24mA from the RF chassis). Clearly the
power supply isn't producing enough voltage so I'll have to add
extra reservoir capacity, or remove the 250 volt stabilizer because
it's not necessary. I'll do this once I've fixed the fault.
I had noticed something slightly odd
when I carried out the initial checks. Rotating the RF gain control
dramatically increased the HT current. This is a combination
of the 100volt and 250 volt rails. Some of the change is current
drawn from the 100 volt rail because two IF valves are controlled
by the pot although only V6 uses the 250 volt supply. Of course
I did hear a lot of audio noise and this would drive the audio
stages to draw more current.
I then did another test with the filament
supply disconnected. This would of course prevent the battery
valves from drawing current. In fact I measured the 100 volt
draw and it measured 1.8mA (this is the current drawn by the
resistors at V11 as calculated above) and remained at this no
matter what setting was the RF gain control which is correct.
With the RF gain control set at full the 250 volt draw was 33mA,
but if I set the RF gain at zero the 250 volt HT current dropped
to 10mA. V6 drawing 9mA at full gain will drop to maybe 2mA at
minimum gain. But that leaves 8mA from the RF valves rising to
24mA at max gain. This is because the RF valves are fed from
a bias supply which is combination of the RF gain pot setting
plus a potential divider formed by R43, R44 and R46 so their
HT current will change with the setting of the RF gain control.
All this seems about right.
I suppose I can unplug the RF unit and
test the IF chassis by injecting varieties of 4.86MHz signals.
To make things easier I should first find a suitable 2-pin headphone
plug and a suitable RF plug for the IF output. This I did, fitting
two old fashioned wander plugs to a standard quarter inch jack
socket and plugging in a loudspeaker. I turned on my signal generator
and tried listening for signals. I did notice that using FM I
could hear a bleep when the generator was tuned to 4.86MHz, the
IF. Cranking up the FM deviation did however produce a good response
using an input of 50mV to the aerial plug. Switching the receiver
to AM I could hear the FM signal so both AM and FM are working
at least at the IF of 4.86MHz. Nothing could be heard when using
the various wavebands with the generator tuned to corresponding
frequencies such as 28MHz or !00MHz so the local oscillator or
possibly the mixer is not working. The RF gain smoothly changed
the audio volume so my guess is the RF amplifier may be working.
The stabilizer is lit but there's still that burnt smell coming
that seems to be coming from the RF chassis.
Swapping V2 and V3, type EF91
should prove the oscillator valve, unless both are too tired...
, but no that didn't work and it was a real problem replacing
the oscillator valve. It's held in place by a metal clamp and
difficult to see the holes in the socket for lining up the valve
pins. Of course the mixer might be u/s as it's a CV291 a special,
extremely sensitive device (see a sheet
carrying data on this). Also here's some more data including
a picture. I have a few spare ones in the workshop if mine has
been damaged. I prodded the signal generator on the thin end
of the diode and the IF amplifier responded but nothing heard
when prodding the fat end. Duff mixer diode or then again that
burning smell might be relevant?
Frequency of operation
Min back to forward resistance
Min forward resistance
Max amount by which performance is
below "standard best"
Max noise temperature
Nominal IF impedance
I measured some resistors and voltages. Two 15K each measured
18K so not important. V2 anode went from 158 to 79 volts, drawing
7.6mA through 18K to 216 volts (15K reading high) and V1 anode
was 116 volts fed from 230 volts and 18K (15K reading high) drawing
6.3mA at max gain. V3 (triode-connected) anode was 88 volts fed
from 141 volts so is drawing 5.8mA through 9.2K (the 8.2K was
high). All these measurements seem to me to be OK. I wondered
about the three coax cables which mix the calibrator output to
the aerial but placing a signal on the output cable proved they
were probably blameless as there was no improvement. I probed
the oscillator with an oscilloscope probe but no RF could be
Just in case the oscillator
stops working when prodded I'll tune my general coverage Japanese
receiver to listen for the R216 oscillator in case there's something
odd about it. Absolutely nothing heard. Next I fitted a loop
to the end of a lead plugged into my signal generator which I
tuned to 4.86MHz with 200KHz FM and heard a really loud signal.
I then tuned the generator to 105.16MHz then to 95.44MHz connected
an aerial and tried to tune in Classic FM on 100.3MHz but failed.
Switching on the R216 calibrator to 5MHz I listened on my Japanese
receiver to 95MHz and found that I could hear the 19th harmonic
of the crystal tuning nicely when the R216 passed through 95MHz.
This proves the RF amplifier is at least partially operating
so I'm left with the oscillator and the crystal mixer. Could
the latter be faulty and as a consequence killing the oscillator?
Looking at the circuit diagram I can't see that this is likely,
so could there be a failed oscillator component? Something did
smoke when I first turned on the set so was this one of the critical
parts? For example there's a path to ground from C36 connected
to V3 anode.
I found the reason for the local
oscillator not oscillating. I'd noticed the lamp for a particular
scale didn't always come on so I wondered if the turret switch
was bad at the oscillator contacts. There are three contacts
for the oscillator K,L and M and by using a strong light I could
see the last contact M, wasn't touching the turret coil contact
because the spring strip was straighter than its neighbours.
By carefully levering the metal strip I was able to make it look
the same as the others, but testing proved the oscillator still
wasn't working. The three contacts are: a cathode connection
at K where you see 1.2Kohms to ground. That was OK. Next a small
condenser connected to the anode, L, and thirdly a ground connection
M. It's clear then that the centre of the three contacts should
be grounded when turret contacts are good. In fact this wasn't
happening and wiggling the centre strip showed it was fractionally
clear of the turret contact but grounded when it was bent slightly.
I then applied some pressure to the centre strip (with the turret
disengaged) until it looked level with the other two strips.
Whilst doing this I noticed a barely perceptible movement at
the material where the three strips were soldered. As the oscillator
was now working I didn't follow this up until I turned the chassis
through 90 degrees so I could read the dial. The oscillator stopped
and I found that the RF chassis had a tiny amount of play between
one side and the other. By pressing the front towards the back
I noticed the two screws A and B were nowhere near tight (in
fact you can see about 1mm gap). Someone had perhaps been investigating
the oscillator problem and slackened the screws, failed to notice
the contact problem, then just left them. I applied pressure
to line up the two sections of the chassis, tightened the two
screws and now all is well.
Now that the local oscillator is
running I can tackle alignment. Tuning the dial to 100.1 and
with wide FM selected I could hear Classic FM which is supposed
to be 100.3MHz. The sound is badly distorted and using narrow
FM, AM or CW the overall gain is too low to hear the signal.
As I still haven't discovered the source of the smoke when I
first turned on the receiver, no doubt that will account for
at least one problem.
One reason for lack of
audio output and distortion was very poor IF adjustment. The
IF is 4.86MHz and the coils are predominantly anode loads with
condenser coupling to the following stage.
The tuning of each coil is via
a dust core at the top.
Very carefully I managed to
roughly tune each IF stage to the correct frequency and receiver
sensitivity increased dramatically, however a couple of cores
were damaged because of the force needed to adjust them.
On the left are two examples of IF
transformers and above is the BFO coil. Each core has been secured
by masses of wax dripped into the cans so I decided to pull off
each can (just two 8BA screws in the top) and remove the wax.
Each core has a slot at either
end so I removed each one and turned it over. To remove the cores
I first removed each aluminium can (2 screws and just pull the
can vertically upwards) scraped away as much wax as possible,
then applied heat from a soldering iron carefully pressed onto
the top of the core until slight smoke rose from the heated wax,
then unscrewed the core using finger pressure. I then removed
most of the white locking compound from the core. The threads
are the same as those square dexion nuts and I checked each core
freely accepted a nut and each coil former freely accepted a
dexion bolt before re-inserting the core with the better slot
uppermost. It's a long job as there are ten coils needing attention,
however all went well and all cores are now adjustable.
The next stage is to align the
IF strip to 4.86MHz and centre the BFO. I don't have any tuning
instructions so I'll assume that IF alignment is carried out
in the narrower bandwidth setting in AM mode (TR1,
L31, L32, L33, L34, TR2, TR3)
then switching to FM and continuing with the limiter stage
L37 and the Foster-Seeley FM discriminator tuning coil
Below are the results of IF
alignment with a signal injected at V8 Pin 6 with V8 removed
with bandwidth set to 30KHz then 120KHz. The vertical scale is
set to 7dB steps and the horizontal scale 50KHz steps.
Just after I'd checked the IF
response there was a sudden increase in HT current and some smoke
from the HT regulator in my PSU. I traced the latter to cable
insulation touching a hot component, but the surge in HT current
I suspect was a recurrence of an earlier fault I've yet to isolate.
Reducing the HT voltage resulted in the excess current not being
drawn although there seemed to be instability within the front
end of the receiver, perhaps a leaky condenser?
Most of the decoupling condensers
are enclosed in metal tubes soldered to the chassis with their
earth connection soldered at the exposed end so it can easily
be unsoldered. The plan to identify the culprit is to supply
an HT voltage and check for something running warm.
You can see some examples on the left.
There's a power supply connector between the two chassis which
can be unplugged to make the job easier.
Front-end alignment looks tricky.
I know it needs sorting out because dial accuracy was a few hundred
KHz out in the FM broadcast band and signal strength for Classic
FM improving greatly when the main tuning condenser trimmers
were adjusted. Each waveband is adjusted within the turret and
the various trimmers look awkward to access.
A little more investigation
continued. I modified the HT voltage regulators in the home brew
PSU to reduce zener current because once I'd added an extra reservoir
condenser the HT had risen and the zeners were running quite
hot. Connecting the receiver showed it was consuming 60mA once
the mains valves had warmed up. Overall gain was now resulting
in a hiss from the loudspeaker, but with some roughness and hum.
I turned to the FM broadcast band and I was able to hear the
crystal calibrator set to 5MHz at 100MHz or so on the dial, but
Classic FM was weak. After a little while I was aware of smoke
and this was coming from a tiny 10 ohm wirewound resistor which
was running white hot. I'd noticed the lamp for this band wasn't
coming on, although the others were OK when their bands were
selected. Presumably the lamp circuit is short-circuit.
The R216 low voltage wiring is
The dial lamps are switched by contacts
linked to the turret and are arranged to let the operator know
to which band he's listening. The lamps LP1-5 are festoon types
rated at 12 volts 2W (about 167mA) and their brightness can be
varied with RV3. The various resistors are designed to limit
the lamp voltage, perhaps to improve their longevity?
The lamp supply is connected between
the 6.3 volt (Pin C) and the dial lamp supply of 18 volts (Pin
J). Pins A and H are ground connections.
There are a several wires in the power
cable feeding the LT circuits so that resistive losses are kept
You'll note the presence of RF chokes
in some of the filament and heater wiring, used in conjunction
with decoupling condensers, to reduce interaction between stages
and RF leakage.
There seems to be three
faults remaining (plus the odd one or two I haven't yet discovered)...
firstly the short-circuit dial lamp, secondly the low front-end
gain. I'd noticed that putting a long wire on the aerial pin
didn't produce a crackle and thirdly there's still that sudden
surge in HT current I haven't tracked down. One possibility is
a decoupling condenser has failed in the RF stages so that RF
gain is reduced? For example those decoupling R2 and R10 resulting
in a low screen grid voltage. Each time I get set to tackle one
fault, another seems to pop up. Again I tested the set, but this
time I removed the valves from the IF amplifier chassis and monitored
the HT. The HT registered a stable level at less than 10mA but
adding valves suddenly caused the current to shoot up and the
voltage to drop. Next, I disconnected the 100 volt and 250 volt
supplies and fed these from my two Solartron HT PSUs. With the
voltages set at exactly 100 and 250 volts both currents remained
stable at 25mA for the former and 35mA for the latter. I was
able to check the FM broadcast band without any problems arising...
so the fault must lie in the home brew PSU. Time to do some calculations
In fact, when I carried out
load testing on my power supply I discovered the problem was
breakdown of the series pass transistors. These were plastic
versions of the BUT11 (the BUT11AF) designed for pulsed operation
in switch mode power supplies. I substituted two BUT11A mounted
on a pair of heatsinks. During testing I changed the zener diode
resistors as I'd calculated their values without allowing enough
base current for the transistors. Further testing showed the
100 volt regulator transistor was running fairly warm so I added
a 3.3Kohm ballast resistor in its collector feed. This moves
the dissipation from the transistor to the resistor (for example...
running 25mA at 100 volts from an HT rail of 300 volts results
in wastage of 200 volts which equates to 5 watts of heat and
10 watts at 50mA. The ballast resistor absorbs 82 volts at 25mA
and 164 volts at 50mA soaking up about 2 watts and 4 watts respectively).
A genuine R216 power supply,
complete with a proper cable, is due to arrive here in a couple
of days (see below).
When checking the receiver further
it seems that most if not on all the other wavebands the local
oscillator is not working. Maybe it's misalignment of the turret
tuner with respect to the springs located on the RF amplifier
chassis? If the turret and the chassis are not precisely lined
up then the studs will not be in contact with the springs at
one end or the other. I think this is a design weakness because
if things can go wrong they usually will go wrong. Well, after
scrutinising the mechanical assembly I decided the turret could
be removed by slackening and/or removing screws in the end plate.
This I did and all was revealed... for starters I'm not the first
to detach the turret because I spotted black pen lines where
something needed to be marked before removal. It was clear that
the springs which mate with the turret have caused problems before.
I also noted that several 6BA screws were missing and something
that may explain the smoke and the overheating resistor. See
This green resistor is fitted
at the rear of the turret and is provided for dropping the
HT feed to the RF amplifier valves. The smaller 470K resistor
is associated with V4, the voltage regulator valve.
This small tag panel was broken
away from its mountings and this could quite easily short to
I think I can replace it.
The repaired tagstrip. I had difficulty
getting solder to adhere to the tags because the thing was ancient
and oxidised but it serves its purpose and will stop any short
circuits that I attribute to the damaged one. I can't figure
out how the original tagstrip had been damaged. It's possible
the turret adjusting lever, which is visible above, had been
wedged behind it.
The lever can be moved to accurately
position the rotational position of the turret so the coil locating
pips are precisely lined up.
Below is a view showing the pips that
engage with the turret visible once the turret has been detached.
The set of three on the left are the contacts for the oscillator
coil. One or more of these fails to make contact on all but one
Here you can see that the three springs
have lost their set. They're supposed to be bulging slightly
outwards so that pressure is maintained on the turret pips. To
fix the problem I think I'll need to unsolder the right hand
ends, bend the spring slightly then resolder the springs. I suspect
these have been worked on previously because the end spring is
a different colour and the pip looks very slightly different
to the others?
The task was a lttle easier than
I imagined. Before unsoldering anything I placed a dense plastic
pad under each spring to give it less flexibility but then I
discovered that there's enough play in the hole marked 628 to
lift the spring to the same level as others. I did this to the
three oscillator spprings plus one other so that they all now
lie in the same plane.
What about the faulty dial lamp?
I fitted a new 12 volt lamp in place of the duff one (open filament)
and not seen a recurrence of the overheating of R73. Maybe there's
something other than a short-circuit to explain this (but see
the broken tag strip above). It does remind me of bias shorts
in the R1155... Could the burning resistor perhaps be due to
a bad grounding connection in the home brew PSU?
Whilst the turret is out I checked the
various resistors which would normally be inaccessible. All read
high in value so it will be prudent to replace them.
Without a detailed description
of removing and refitting the turret I had to work this out by
trial and error. Removing the endplate securing screws allowed
the turret to just pop out. That part was easy. Refitting involved
struggling with spring tension so I unscrewed the nut on the
spring end. This allowed the turret locating lever to drop away.
The turret could then be placed in postion. I turned the wavechange
knob half way between Range 4 and 5, then fitted the turret so
that it lined up midway between Ranges 4 and 5. I then turned
the knob to check the setting lined up when the coil pips were
located correctly. Then I screwed back the endplate ensuring
the front and rear bearings were in place. The turret then turned
relatively freely. Next the locating spring needed to be refitted.
I screwed the nut back so it was located against the bar (as
above) and checked the lever was located correctly in the slot
at the end of the turret. Next the spring needed to be stretched
and its hooked end positioned over the peg within the metal slot.
To do this I threaded some string through the slot and around
the hook and then tied the ends to form a loop. By gently pulling
the looped end the spring stretched and located on the peg. At
this exact point the string snapped but the hook dropped onto
the peg. The final task is to confirm the correct coilset is
engaged with respect to the knob setting then adjust the lever
at the end of the bar adjacent to the resistor panel (which you
can see by scrolling up to the picture of the damaged tagstrip).
You'll note that the yaxley
switch seen above the spring is correctly selecting the lamp
Also note more missing screws...
this time at the panel covering the filmstrip.
Having got EMER 384 which carries
lots of alignment instructions, I can hopefully now more easily
track down the reason for receiver deafness. After having carried
out more testing, which confirmed the turret is working correctly,
I can use the test data to confirm or otherwise the performance
of the IF amplifier. A test signal of AM at 30% depth should
give me 5mW at the 150 ohm output sockets. What sort of level
should be input to the IF strip? The data doesn't tell me this
in so many words, but it does specify that a 10uV 4.86MHz CW
signal applied to the IF side of the mixer diode should give
me a decent tone using the BFO. If this works out then the deafness
must be attributable to the RF front end (including the mixer
diode), otherwise the problem is in the IF strip. I've already
noticed that all the IF coils tune nicely which is a good sign.
Currently I can hear local FM broadcasts but no other broadcasts
on any waveband. My signal generator can be heard but that needs
to be at best 50uV and sometimes more than 10mW across the different
Part of the receive problem is fixed.
The IF strip is prone to instability when the maximum gain setting
is used and retuning the stages upset the centre frequency. I
found this out when I injected 4.86MHz directly into the IF input
socket on the chassis instead of forcing it through the front
end. I set the spectrum analyser to 4.86MHz and proved the calibration
by adding a signal from my signal generator and found the IF
amplifier was several KHz away from 4.86MHz. It was quite awkward
to shift it back and I soon discovered that I needed to back
off the RF gain control to get the amplifier to centre on 4.86MHz.
Once the shape of the response curve was correct and dead centre
on the correct frequency I gradually turned up the RF gain. Instead
of the shape of the resonse just getting a higher peak as the
gain increased, it flat-topped and got wider and wider as the
control was turned up. Backing off the input signal and increasing
the gain showed that the amplifier began to get unstable. This
seemed to be due to interaction between input and output leads
used for testing. Finally, after checking the amplifier was properly
centred on 4.86MHz I disconnected the monitor lead from the grid
of V11 and re-plugged the lead from the mixer. I was then able
to complete the IF tuning by adjusting V11's input at TR3 and
V6's input transformer TR1.
That flat-topping I'd noticed at V11
grid.... I wonder if it's the so-called "noise limiter"
working its magic?
The FM adjustment method in the EMER
sounded pedantic so I adopted a much easier method. First L37
is tuned for maximum noise then T39 tuned to almost minimum noise,
then L37 retuned slightly for max noise and finally T39 tuned
for minimum noise. This lets me listen to broadcast stations
without distortion if wide bandwidth is selected and careful
Next, I'll need to align the turret
tuner RF circuits. As there are 4 trimmable coils for each waveband
the difference between best and nominal adjustment will be very
significant, but before tackling this I decided to check the
mixer diode. Testing these types of microwave diode is not easy
because a normal test meter would probably destroy it so I looked
in my spare parts box and spotted a couple of Schottky diodes
marked "HP5082-2835". I bought these diodes from a
local Tandy shop that was closing down, probably about 1985 and
checking their datasheet
they look ideal for a new R216 mixer. The new diode was easy
to fit by just soldering across the terminals at the original
diode socket (so if ever a new CV291 becomes available it can
easily be fitted). I turned on the R216 and immediately heard
the familar racket of a shortwave band. That was because of the
last test I'd done... I'd switched to the lowest range with the
BFO on and a long wire connected and heard precisely nothing...
apart from broadcast FM stations on range 4 the receiver had
refused to ackowledge anything on any band (apart from millivolts
from my signal generator). So there's the answer to receiver
deafness. Mixing was presumably taking place at the first IF
amplifier but only if sufficiently strong signals were present...
the duff diode was acting purely as a tiny capacitor and coupling
the RF and local oscillator signals to the grid of the EF91 where
slight non-linearity within the valve produced mixer products.
Essentially the IF amplifier was a valve down and the substitute
mixer only working on strong signals.
I attempted to measure the old diode
now it's clearly u/s and I could see absolutely no trace of anything
except possibly a minute capacitance. It seems the capacitance
of the diode socket and local wiring must have been responsible
for coupling the RF from the front end to the IF amplifier. No
wonder the receiver was deaf... the previous owner probably tested
the mixer diode with a multimeter.
Left, the defunct CV291, early
1950s microwave diode, and on the right a new HP5082-2835 from
my junkbox. This can be used as a mixer diode said to be suitable
for use up to UHF so should perform adequately.
The anode goes to the turret contact
(left) and the diode to the IF input transformer (right).
The R216 power supply designed for
the receiver (shown below) uses a much simpler means of supplying
the correct voltages because the mains transformer has been wound
to supply exactly the right voltages and power.
I've printed two circuit diagrams
here. The first came from the "Illustrated Parts List"
and strangely is wrong. No doubt the technical author misread
a document and no-one bothered to check his work. I've indicated
the correct markings in red... This now lines up with the condenser
numbering ie. C2/C6 are 350V working and C1/C5 are 150V working.
Now the correct circuit
diagram. I wonder how much confusion resulted from the errors?
I opened up the new power
supply but closed it again because of the very strong smell coming
from inside the case. Both the R216 and its PSU are sealed and
it took a couple of weeks to disperse the smell of government
surplus equipment from our conservatory where I first opened
the receiver case. I opened the PSU in the garden and here are
pictures of its interior...
The first view shows the mains
transformer and two HT chokes ZA44291, plus the LT filament choke
ZA44292, on the right. The two HT (250v + 100v) rectifier valves
are type CV493 or 6X4. This has two anodes which are wired together
as a single-phase rectifier in the circuit shown above.
Here's something very odd...
in the view above in the lower left corner you'll see the mains
input socket. If you look closely you can see the connector is
mounted on a metal plate and this is stuck to the front panel
with araldite. I discovered this when fitting a mains cable.
On my lead the 3-pin plug orientation pips were turned one position
away from those on the socket and as the mains lead came with
a mating socket I decided to swap that for the one fitted to
the PSU. Just unsolder three wires, unscrew the securing ring
and fit the new one. I noticed that the straight edge designed
to mate with the flat on the connector was poor and allowed the
connector to turn so I tightened the locking ring a little more
and the result was that the new connector dropped out carrying
the plate with it. Up till then I hadn't noticed the plate was
just glued to the front panel... I weighed up the options and
decided to fit an IEC connector, then decided against this, found
a tube of superglue and used the contents to re-secure the plate.
Whoever fitted the plate took care to touch up the front panel
so that the repair wasn't obvious. It seems the repair was not
done by the Army, but by Andy, G8JAC who in 2007 removed an amateur-fitted
plastic replacement for the original socket.
An alternative solution could
have been to drill shallow holes in the rear of the front panel
and self-tap the aluminium plate in position. I rejected that
idea as it risked penetrating the front so I used superglue.
It's currently setting and if its secure it'll have to do....
There are several electrolytics.
Identifying these led me to discover the errors in the circuit
diagram shown first. The largest is C6, a 32uF x 350V for 250V
smoothing, C2 is an 8uF x 350V for the 250V reservoir, C5 is
16uF 150V for 100V smoothing and C1 16uF 150V for 100V reservoir.
C4, 2uF 150V; C8, 2uF 150V;
and C9, 2uF 150V are for smoothing the bias supply and C3 and
C7 both 1000uF 6V are used for smoothing the filament supply.
The large black rectifier
is used for the filament supply (also in picture below).
The black rectifier is
a Sentercel full-wave selenium device used for the filament supply.
Above you can see a set
of high stability resistors for the metering circuits together
with a tiny mains rectifier MR4 for monitoring the PSU input
On the right you can see a pair
of Yaxley switches incorporating an interlock so that mains voltage
can't be changed when the receiver is turned on.
Also, another view of G8JAC's Araldited
plate, top right. Once the plate had fallen away you could see
there was just a large hole with no metal left for mounting the
socket. The superglue repair worked fine and the mains cable
On the left is the filament
rectifier MR1/MR2 and top centre (end on) the bias supply rectifier
Most people advise reforming
capacitors before running them at full voltage, but I know that
the PSU worked OK in 2007 so I didn't expect a problem. As this
PSU uses valve rectifers it's fine to connect an
external variable voltage HT power supply to the HT line
and very gradually crank up the voltage with an eye on the test
HT current which should be held back to say no more than 10mA
or so (bearing in mind there are bleeder resistors in place).
Once you've reached say 30 to 50% of either 100 or 250 volts
in this example and the feed current has dropped away nicely
the capacitors should be serviceable the full voltages can then
(less) slowly be applied, whilst still montoring the current.
The 100 volt and 250 volt rails have 22K and 620K resistors consuming
about 4.5mA and 0.5mA respectively. If you have a bad capacitor
one of two things will be noticed. First the feed current will
be next to nothing which implies an open circuit or high resistance
capacitor, or the feed current may be erratic and prone to increase
rather than decay.
Large capacitors in old military
equipments can be paper types rather than electrolytics and these
are usually constructed in exactly the same way as those dreaded
wax tubular types. In my experience these large paper types can
either be perfect or leaky and if really bad will run hot, then
very hot then expire.
With 250 volts applied to the
PSU the meter read 7.5 and with 100 volts applied the meter read
about the same. Next, I'll check the low voltages are present
and at the correct pins. Powering the PSU proved it was working.
Off load of course, the voltages were miles away from the levels
specified for the R216. Being used to modern circuitry where
most voltages are regulated to a couple of percent it's a bit
alien to see a 4.13 volt supply destined for 1.4 volt valves,
however when the leads were all connected the voltages settled
down to their correct levels. I noticed the 1.4 volt output was
a little low but gradually asserted itself to 1.3 volts... no
doubt the low voltage electrolytics needed some reforming and
will slowly improve.
I plugged in my VHF aerial and
turned on the set with the proper PSU in place and was pleased
to hear the FM broadcast band full of stations. The highest band
is now receiving a fair number of aircraft transmissions but
I did notice that the local oscillator seems to cut out before
the lower end of the highest band is reached. I'm guessing this
is possibly due to a low emission EF91 because I had a similar
problem wuith a Racal RA14 oscillator. Also noted is an annoying
effect of the audio on AM reducing in amplitude as a signal is
tuned... quite possibly the action of the noise limiter which
is no doubt engaging to prevent too much headphone output.