R1475- Final Alignment & Testing
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I'm now at the stage were I can just plug in
an aerial and a pair of headphones (or loudspeaker) and listen
to QRM, QRN or whatever on short-waves but interested in a sideline
viz. the guard receiver module. This uses a separate 6K8 mixer
valve and has no extra RF amplifier but would be ideal for adding
a medium-waveband. At least that would enable relatively noise-free
listening. |
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These drawings show (left) the section of the
R1475 circuitry dealing with the guard receiver and (b) the detail
of V4, the guard mixer in Module D. |
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Right.. the two versions of the R1475 guard
receiver plug-in module. |
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The difference between Types 131 and 132 are
minimal, being the coil sizes. The one in my box of bits is the
lower frequency version covering 2 to 4.2MHz so it may be possible
to shift the low end down to a section of the medium waveband
by merely adding a capacitor or two.
Note those wires soldered to the three preset variable
condensers to limit their range to 180 degrees. |
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Let's see if this is feasible in theory.. The
oscillator is driven by a crystal but its likely that substituting
a wire? (yes.. this was true) or a small capacitor for this may
suffice to get the 6K8 triode to oscillate. A quick check in
daylight on my SDR revealed the strongest station here is Smooth
Radio on 1557KHz. That would need the local oscillator to run
at 2157KHz. The oscillator is governed by L35 and C92/C93. The
mid tunable capacity is roughly 87pF allowing 25pF for strays
and the self capacitance of the coil. For 3000KHz + 600KHz an
oscillation of 3600KHz what will we need to add to give say 1557KHz+600KHz?
The calculated value of L35 will be 22.5uH and with an extra
fixed 155pF capacitor, this will give new tuning frequency of
1557KHz. The 25pF trimmer will allow higher and lower frequencies
to be received.
It's vital that the RF tuning coils L33/L34 are also
padded because otherwise the receiver will tune to the image
(= 2.6 to 4.8MHz). L33/L34 are about 32.35uH and 37.5uH so to
resonate at 1557Khz will require an additional 235pF and 200pF
respectively.
To cover a larger part of the medium waveband the
RF padders should be say 270pF and 220pF and for the oscillator
200pF.
The first thing I discovered was the mixer valve was
duff. The second was rather weird and might have been a message
from one of the two signatories listed previously (the messages
scratched on the casing)? I found a wire leading to the oscillator
coil was cut and tucked under the coil (apparently connected!).
Soldering this back got the oscillator running, but not much
signal from the aerial so I prodded a few more wires and quickly
discovered a wire to the outer RF coil was also snipped and wedged
to disguise the fact. I soldered this one back in place and was
rewarded by the ability to tune all three coils to hear an increase
in background noise level. I found I could now hear weak heterodynes
by twiddling the set of presets. A check with my TinySA revealed
I could tune from something like 1600 to 1500KHz but 1557KHz
was very weak suggesting the RF coils weren't yet matched to
this frequency. After trying various capacitors I finally got
optimum performance on 1557KHz but, alas the signal was too weak
on a random wire with the local ambient background noise so I
checked and noted the strongest signal was on 909KHz requiring
the local oscillator to be on 909KHz+600KHz = 1509KHz instead
of 2157KHz. A rough calculation revealed an extra 300pF across
the oscillator coil should do the trick.
Now the RF coils.. these worked on 1557KHz with the
addition of extra capacitors of 100pF and 47pF but I reckon around
500pF will be about right. In fact I was able to tune 909KHz
using 550pF by changing the cores slightly. The background noise
level messed up further testing as it resulted in almost complete
blocking of the signal with the magic eye overlapping. Using
the TinySA in sig gen mode I could just hear about -77dBm. Time
to test using my 80m dipole which gives me a signal strength
of -57dBm on 909KHz with a noise floor of -125dBm.
I moved the receiver from our conservatory where it
wasn't as cold as my workshop (and we needed the extra space
over Christmas) back to my workshop (=unheated converted remote
garage) and to my surprise found the guard receiver worked perfectly
pulling in the medium wave broadcast loud and clear with the
available 80m inverted-V aerial. |
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Only a few parts to put in place. The guard
unit outer case and legend panel (after rubbing a white crayon
over the surface to highlight the engravings). The outer cover
for the magic eye which proved awkward as the holes were 6BA
but the threads 4BA. I drilled the holes to fit 4BA then reorientated
the magic eye by drilling additional holes in the octal holder
as the originals skewed the display by 45 degrees (note that
the box of bits didn't include the magic eye bracketry and octal
holder). I ended up with the aperture at the top. Everything
seems to work but an annoying feature is that the reduced IF
bandwidth settings run with the BFO. I decided to use a pot with
a switch to ground the top cap of the BFO when ON.The pot is
for the guard receiver gain and so will not affect guard reception
as this will only be used in the broad IF setting. An alternative
could be the noise limiter switch. A final change was to fit
four 6 volt 500mA bulbs for the display with a 10 ohm resistor
in the lamp common 12-volt feed. |
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At this point I delayed completion of the R1475
as I'd decided to make a jig for holding the receiver so I can
align it and at the same time redraw the tuning line on the perspex
dial |
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Using the new jig I was able to carry on with
the final alignment. RF trimmers and coils are identified by
coloured blobs, whilst IF cores are accessed via those 10 holes
either side of the module edges. |
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For IF alignment I grounded the top cap of V8
(the BFO valve) otherwise you get the 600KHz signal superimposed
on the IF curves. the pictures show the vresponses Narrow, middle
and broad plus the BFO. |
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Narrowest bandwidth |
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Middle bandwidth |
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Widest bandwith
I need to check this as it seems too similar to the
middle setting. Switching between the three settings sounds exactly
as it should when listening say to 40m SSB signals. |
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BFO
All the measurements were made using 10:1 oscilloscope
probe and a 10k series resistor to V9 detector diode, with 600KHz
AM injected at the mixer top cap. |
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I then started to set the lowest and highest
dial markings to the received signals using my TinySA which you
can see in the picture of the receiver above. The receiver is
now remarkably sensitive on all wavebands plus Radio 5 Live at
good strength from the guard channel.
The next step was to erase the original dial markings.
These are on the inside surface of the perspex and I tried a
couple of chemicals but got nowhere so, remembering advice from
my friend from way back whose favourite school subject was biology
(he relished cutting up frogs), on cleaning chrome parts of my
teenage bicycle, I tried spit with a cotton bud. This worked
like magic and all traces of the original black line were soon
gone.
Each of the curves for the four wavebands should ideally
be a straight line but of course this depends on a couple of
things. One is the precise characteristic of the oscillator section
of the tuning condenser with its wiring and strays, another is
the oscillator coil and padder. Below are pictures of the latest
dial (box of bits) and the one from my complete receiver. |
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As you can see the calibration lines on the
two dials are completely different. Both are original, having
been made in the factory. The lower dial comes from the first
R1475 I bought and that is dated 1948. The upper dial is probably
ten years newer as the dates on some parts showed it was serviced
in 1963/64 and 1970. Both show that the main criterion must be
the tuning condenser because this is common to all four wavebands
and all the curves on each dial have the same general shape.
I marked the upper dial against my signal generator
and my first attempt produced the same general shape but more
pronounced. The only way I could see to make the new calibration
line was to mark the outside then later, when I was happy with
the results, trace this through the perspex to the inner surface.
Ordinarily one could tackle the departure from linearity by bending
the tabs on the outer plates of the tuning condenser, but because
of the way the tuning condenser assembly is constructed and instralled
in the receiver there's no way sensible way to correct the departure
from a straight line,
It's possible that early receivers were fitted with
tuning condensers that were checked and adjusted for linearity
before being fitted into their screening cases and installed
in the receiver. Later, perhaps for cost reasons, this exercise
was skipped? |
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The latest calibration (above) isn't ideal so
I'll make another attempt by lowering the curves closer to their
pointers and re-check the first curve whose double-hump is a
bit peculiar.
Lowering the curves might be possible by altering
the core/trimmer settings to realign the oscillators. Another
way might have been to move the tuning condenser end settings
to add or reduce the capacitances but as this is now a difficult
mechanical exercise involving the anti-backlash gears and the
endstop turns counter so I'm disregarding this method out of
hand. I'll tackle the third range first.
One interesting point you'll notice is that in both
original dials the calibration curves line up at their ends whilst
mine do not. Again, this may not be feasible in practical terms
because the left and right drums are fitted independently and
to modify their relationship means messing around with the anti-backlash
gears and that dratted endstop turns counter. No doubt the documentation
used by the factory assembly line took account of all of this.
One of the final things I need to do is to add a small
relay for grounding the top cap of the BFO so I can operate the
receiver on the narrower IF passbands on AM. An alternative might
be to wire the switch or relay into say the HT feed to the BFO.
A convenient position for the switch might be the vacant hole
to the left of the mode switch. The latter idea seems by far
to be the easier as if wiring needs to be done removing HT from
the BFO is straightforward. V9 doesn't need HT for its diodes
(used in circuits other than the BFO) to function.
Below, with the new BFO switch fitted to the left
of the mode switch and tuned to 40m SSB. Apart from a few metal
fittings and tightening varios screws, the receiver is complete
and working well. |
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