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A long time ago (1979-1980)
I made a transceiver for the 2 Meter band based on a set of integrated
circuit chips designed by Plessey Towcester. The idea for the
project came to me when the local rep happened to mention his
interest in amateur radio and offered me some samples of chips
suitable for making a 2 meter synthesised receiver. These had
been designed for the new UK Citizen's Band promised by the government,
but seemingly, at the 11th hour government advisors had recommended
switching from internationally recognised standards to a brand
new standard said to offer improvements for users. Gone would
be AM or SSB, and high power pirate transmissions, and in their
place would be low power FM. The killer change, as far as the
new Plessey chips were concerned was a switch to a brand new
series of 40 Channels.
So... armed with a few sets
of chips I set out to build a 2 meter transceiver... not too
difficult because the chief designer James Bryant G4CLF, had
incorporated, by a clever ruse, the option to use these CB chips
on the amateur 2 meter band. My new rig was duly designed and
built, and together with a high power amplifier, constructed
around a fancy Ferranti transistor (a free sample from another
rep) I managed lots of FM and SSB contacts. Reports were mixed
but after meeting a Swiss amateur during an opening, and receiving
complaints of strange modulation and splatter from locals, I
stood down the rig and moved back to the 80 meter band. The old
VHF rig has lain dormant gathering rust and mice droppings for
over 40 years, but recently I decided to investigate it. Hopefully,
now that I have some half decent test equipment, I can find out
the reason for the complaints. That's if (a) it's all there and
(b) I can get the thing powered up and working. As I recall I
"invented" some of the design techniques and for that
reason the RF output may have been slightly odd?
First some pictures of the rig..
as discovered (2nd May 2021) hidden away in the loft of my garage,
being dumped there after having been rescued from our damp and
decaying caravan. Feast your eyes on the thing below. Those rust/gold
things top centre are power supply regulators so I guess the
rig will operate from a single supply voltage?
The (vague) aim is to restore
the rig to a working state and, if successful, use modern test
gear to clean up its operation. |
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Apparently (I'd forgotten
this) I made printed circuits for the various modules, and below,
conveniently marked, is the identification of this, the largest
box, " SSB Generator 10.7MHz".What exactly is this
SSB generator and how did I arrive at its circuit diagram? If
it was a published thing it would probably date from the period
1977 to 1979. I can see what look like three audio transformers
which are most likely surplus from air defence system communications
interface equipment as that was what I was working on in 1979.
From external wiring evidence the upper area right carries microphone
input from a small transformer mounted near the loudspeaker,
possibly taken from a TR2002. Is it a "third method"
circuit (later I found that it's not)? It needs a 10.7MHz crystal
to drive the whole thing which is absent but possibly this is
located at the other end of that black coax lead (later I located
the separate 10.7MHz oscillator)? There are two black relays,
one of which might switch unmodulated 10.7MHz signal to FM circuits
from the input to the SSB generator. Centre left must be the
RF phasing circuits and is that slide-switch for swapping from
upper to lower sideband? |
SSB GENERATOR
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TRANSMIT MIXER
This loose module, from
the underside of the chassis, is marked "G3PIY 17/2/80 Mixer
135 + 10.7MHz" so the project must have begun in middle
to late 1979...
It should fit into the empty
diecast box visible under the chassis.. see later pictures.
Below is the main part of my
invention. It's a Variable-tuned Crystal Oscillator) VXO used
for driving the Plessey chips. As the chips produce fixed channels,
some way of tuning must be found so that, for example, you can
listen to SSB stations. By shifting the frequency of the reference
crystal used by the synthesiser you can tune across each channel.
The crystal is marked 10.245MHz. Although I say "invention",
a Plessey booklet published a few years later suggests this very
method of fine tuning between channels.
A drawback, from memory, was
a large amount of the 1250Hz audio reference signal, used by
the chips, which became superimposed on receive and transmit.
This was probably a by-product of using the VXO, and I had to
insert a double-balanced T-filter into the loop to remove it.
I also needed to modify the loop time constant to allow the VXO
to speed up locking when a channel was changed.
I also note a metal screening
plate fitted under the diode matrix board which I must have added
to help cure noise on the transmitted signal? |
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VXO MODULE 
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Below is what looks like
an RF amplifier. The transistor with the heatsink may be a 2N3866
or a slightly more exotic variant? Apparently it fits in the
aluminium box behind the loudspeaker and carries a label telling
me it's the 135MHz amplifier. |
TRANSMIT RF AMPLIFIER
|
Here's another module,
usefully labelled, "2N4416 RF Amp 144-146MHz". My guess
it's the front end of the 2 meter receiver but from all that
solder on the copper... is there a transistor missing? The 2N4416
is sitting over the screen a bit reminiscent of an early tetrode
RF valve used in receivers from the early 1930s.
Datasheet for 2N4416
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RECEIVE RF AMPLIER 
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Above is a partial view
of the key frequency-controlling feature, marked "Diode
Matrix", used for generating 2 meter channels, which from
memory are 20 x 100KHz. Each channel can be continuously tuned
up to either -50KHz or +50KHz from its nominal setting so for
example 145.500MHz at "0" on the dial allows tuning
down to 145.450 at "50" left or up to 145.550MHz at
"50" right using the VXO. Channel selection is via
a pair of rotary switches and a network of diodes.
To the left of the dial below
you can see three LEDs which I recall lit when the rig is tuned
to 144/145 or146MHz? The central dial feature is a red display
showing the channel number "0" to "9", within
the selected 1MHz segment of the band.
Turning over the chassis revealed
more parts and a view of the front panel. The heavily tarnished
flywheel tuning knob was made for me in the Liverpool Cheapside
factory as a special favour. Once cleaned it should restore to
a shiny brass finish.. in fact my first task. I'll need to try
and read those labels.. a quick look tells me I even got the
rig to work over repeaters! |
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Below a more comprehensive view
of the diode matrix and front panel switches (why did I remove
two of these from the front panel??)
A better option for the tangled
mess of the discrete diode array below would be to use an EPROM.
Maybe I'll consider this later if things go well? |
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Below.. more modules including
the empty outer case for the mixer module (which is dangling
by its wiring). Somewhere will reside the AM/FM and SSB receivers
and judging from the loudspeaker an audio amplifier? The box,
centre right has a label "10.7MHz Osc" so that explains
the location of the SSB generator input.
I seem to have chosen a 10-legged
Plessey device for the oscillator, and in the same box is a second
circuit board which also carries another 10-legged Plessey device.
Could that circuitry be for the FM transmitter?.
Further
down if you read on there's the original schematic. |
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Somewhere I should have
the original Plessey SL/SP manuals and hopefully a complete circuit
diagram for the transceiver.... Will I be able to resurrect the
thing??? I can identify no less than five more modules (including
one with two circuit boards) screwed to the chassis.. are these
labelled?
After searching high and low
I found lots of early my Plessey documents.. but not the one
I was looking for. However.. I did find specs for the main Plessey
chips. These can be seen by clicking the appropriate link below.
Much to my surprise I found a folder containing lots of sheets
of circuits and plans for the rig. I also found, on the Net,
a copy of the publication "1980 Plessey Frequency Synthesis
IC Handbook" which had most of the 2 meter circuitry I was
looking for. |
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FM RECEIVER
I had trouble finding a datasheet
for the SL664, which I used in the FM receiver but I did spot
this drawing showing enough details for my purpose.
Below is a picture of the FM
receiver pcb. I need to see what that device is at the top right. |
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An early task will be
to draw a block diagram of the rig and make sure nothing vital
is missing, but that wasn't the first job because I cleaned up
the brass tuning knob... now awaiting its final polishing.
When I built the rig I had very
little in the way of test gear and in order to check things as
I proceeded I designed the circuitry's internal signals to be
much higher in power than they needed to be, probably a bad thing
to do as this would introduce unwanted byproducts such as harmonics
and undesirable mixer products. A better technique would have
been to keep signals at a low level, incorporate filtering, and
to only amplify once a clean output signal had been generated.
However, this would have been possible only with decent test
gear in place of things like an absorption wavemeter and a GDO
which I used in the 1970s. From my notes I see I'd used a huge
storage scope which I'd bought for next to nothing from Plessey
and a Tektronix 545A bought from an MoD auction in 1970 (both
of which I devalved and scrapped). |
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PLL THEORETICAL CIRCUIT
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The key to the whole rig
is this phase lock loop using three Plessey chips. Basically
you have a VHF oscillator running at the desired transmit output
frequency minus a suitable IF and, as 10.7MHz is historically
the most convenient, the oscillator covers 133.3 to 135.3MHz.
This permits the use of a 10.7MHz
SSB circuit for transmit and standard 10.7MHz IF amplifier components.
The SP8921 and SP8922 were designed
to be used for the Citizens Band so the SP8621 divide by five
chip is used to bring the oscillator down to frequencies similar
to those used by CB where it can be locked according to the seven
program inputs to the SP8922.
Locking of the loop at precise
2 meter band channel frequencies is accomplished by comparing
the VHF VCO frequency with a crystal reference by choosing a
frequency common to both (in this case 1250Hz). |
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One of the mysteries to
resolve is the wiring of the diode matrix (picture shown after
the following table) that I used for selecting the tuneable channels.
To help with understanding this I've constructed a table below
which gives the states of the pins on the SP8922 needed to lock
the indicated frequencies. SP8922 programming pins are: G 16,
F 15, E 10, D 14, C 11, B 13, A 12 (slightly confusing!). I've
listed all the 25KHz channels although I only use whole numbers
of hundreds; and in fact only 20 of these rather than the 21
possibles (depending on the end channel requirements). "1"
means a positive voltage greater than 2.4 volts on the SP8922
pins and "0" means less than 0.5 volts above ground
potential. |
COMPLETE LIST OF 2M 25KHz CHANNEL SELECTION
CODES
|
FREQUENCY MHz |
F |
E |
D |
C |
B |
A |
G |
OCTAL |
USED |
|
144.000 |
0 |
1 |
0 |
1 |
1 |
0 |
0 |
54 |
OK |
|
144.025 |
0 |
1 |
0 |
1 |
1 |
0 |
1 |
55 |
- |
|
144.050 |
0 |
1 |
0 |
1 |
1 |
1 |
0 |
56 |
- |
|
144.075 |
0 |
1 |
0 |
1 |
1 |
1 |
1 |
57 |
- |
|
144.100 |
0 |
1 |
1 |
0 |
0 |
0 |
0 |
60 |
OK |
|
144.125 |
0 |
1 |
1 |
0 |
0 |
0 |
1 |
61 |
- |
|
144.150 |
0 |
1 |
1 |
0 |
0 |
1 |
0 |
62 |
- |
|
144.175 |
0 |
1 |
1 |
0 |
0 |
1 |
1 |
63 |
- |
|
144.200 |
0 |
1 |
1 |
0 |
1 |
0 |
0 |
64 |
OK |
|
144.225 |
0 |
1 |
1 |
0 |
1 |
0 |
1 |
65 |
- |
|
144.250 |
0 |
1 |
1 |
0 |
1 |
1 |
0 |
66 |
- |
|
144.275 |
0 |
1 |
1 |
0 |
1 |
1 |
1 |
67 |
- |
|
144.300 |
0 |
1 |
1 |
1 |
0 |
0 |
0 |
70 |
OK |
|
144.325 |
0 |
1 |
1 |
1 |
0 |
0 |
1 |
71 |
- |
|
144.350 |
0 |
1 |
1 |
1 |
0 |
1 |
0 |
72 |
- |
|
144.375 |
0 |
1 |
1 |
1 |
0 |
1 |
1 |
73 |
- |
|
144.400 |
0 |
1 |
1 |
1 |
1 |
0 |
0 |
74 |
OK |
|
144.425 |
0 |
1 |
1 |
1 |
1 |
0 |
1 |
75 |
- |
|
144.450 |
0 |
1 |
1 |
1 |
1 |
1 |
0 |
76 |
- |
|
144.475 |
0 |
1 |
1 |
1 |
1 |
1 |
1 |
77 |
- |
|
144.500 |
1 |
0 |
0 |
0 |
0 |
0 |
0 |
100 |
OK |
|
144.525 |
1 |
0 |
0 |
0 |
0 |
0 |
1 |
101 |
- |
|
144.550 |
1 |
0 |
0 |
0 |
0 |
1 |
0 |
102 |
- |
|
144.575 |
1 |
0 |
0 |
0 |
0 |
1 |
1 |
103 |
- |
|
144.600 |
1 |
0 |
0 |
0 |
1 |
0 |
0 |
104 |
OK |
|
144.625 |
1 |
0 |
0 |
0 |
1 |
0 |
1 |
105 |
- |
|
144.650 |
1 |
0 |
0 |
0 |
1 |
1 |
0 |
106 |
- |
|
144.675 |
1 |
0 |
0 |
0 |
1 |
1 |
1 |
107 |
- |
|
144.700 |
1 |
0 |
0 |
1 |
0 |
0 |
0 |
110 |
OK |
|
144.725 |
1 |
0 |
0 |
1 |
0 |
0 |
1 |
111 |
- |
|
144.750 |
1 |
0 |
0 |
1 |
0 |
1 |
0 |
112 |
- |
|
144.775 |
1 |
0 |
0 |
1 |
0 |
1 |
1 |
113 |
- |
|
144.800 |
1 |
0 |
0 |
1 |
1 |
0 |
0 |
114 |
OK |
|
144.825 |
1 |
0 |
0 |
1 |
1 |
0 |
1 |
115 |
- |
|
144.850 |
1 |
0 |
0 |
1 |
1 |
1 |
0 |
116 |
- |
|
144.875 |
1 |
0 |
0 |
1 |
1 |
1 |
1 |
117 |
- |
|
144.900 |
1 |
0 |
1 |
0 |
0 |
0 |
0 |
120 |
OK |
|
144.925 |
1 |
0 |
1 |
0 |
0 |
0 |
1 |
121 |
- |
|
144.950 |
1 |
0 |
1 |
0 |
0 |
1 |
0 |
122 |
- |
|
144.975 |
1 |
0 |
1 |
0 |
0 |
1 |
1 |
123 |
- |
|
145.000 |
1 |
0 |
1 |
0 |
1 |
0 |
0 |
124 |
OK |
|
|
FREQUENCY MHz |
F |
E |
D |
C |
B |
A |
G |
OCTAL |
USED |
|
145.000 |
1 |
0 |
1 |
0 |
1 |
0 |
0 |
124 |
OK |
|
145.025 |
1 |
0 |
1 |
0 |
1 |
0 |
1 |
125 |
- |
|
145.050 |
1 |
0 |
1 |
0 |
1 |
1 |
0 |
126 |
- |
|
145.075 |
1 |
0 |
1 |
0 |
1 |
1 |
1 |
127 |
- |
|
145.100 |
1 |
0 |
1 |
1 |
0 |
0 |
0 |
130 |
OK |
|
145.125 |
1 |
0 |
1 |
1 |
0 |
0 |
1 |
131 |
- |
|
145.150 |
1 |
0 |
1 |
1 |
0 |
1 |
0 |
132 |
- |
|
145.175 |
1 |
0 |
1 |
1 |
0 |
1 |
1 |
133 |
- |
|
145.200 |
1 |
0 |
1 |
1 |
1 |
0 |
0 |
134 |
OK |
|
145.225 |
1 |
0 |
1 |
1 |
1 |
0 |
1 |
135 |
- |
|
145.250 |
1 |
0 |
1 |
1 |
1 |
1 |
0 |
136 |
- |
|
145.275 |
1 |
0 |
1 |
1 |
1 |
1 |
1 |
137 |
- |
|
145.300 |
1 |
1 |
0 |
0 |
0 |
0 |
0 |
140 |
OK |
|
145.325 |
1 |
1 |
0 |
0 |
0 |
0 |
1 |
141 |
- |
|
145.350 |
1 |
1 |
0 |
0 |
0 |
1 |
0 |
142 |
- |
|
145.375 |
1 |
1 |
0 |
0 |
0 |
1 |
1 |
143 |
- |
|
145.400 |
1 |
1 |
0 |
0 |
1 |
0 |
0 |
144 |
OK |
|
145.425 |
1 |
1 |
0 |
0 |
1 |
0 |
1 |
145 |
- |
|
145.450 |
1 |
1 |
0 |
0 |
1 |
1 |
0 |
146 |
- |
|
145.475 |
1 |
1 |
0 |
0 |
1 |
1 |
1 |
147 |
- |
|
145.500 |
1 |
1 |
0 |
1 |
0 |
0 |
0 |
150 |
OK |
|
145.525 |
1 |
1 |
0 |
1 |
0 |
0 |
1 |
151 |
- |
|
145.550 |
1 |
1 |
0 |
1 |
0 |
1 |
0 |
152 |
- |
|
145.575 |
1 |
1 |
0 |
1 |
0 |
1 |
1 |
153 |
- |
|
145.600 |
1 |
1 |
0 |
1 |
0 |
0 |
0 |
154 |
OK |
|
145.625 |
1 |
1 |
0 |
1 |
1 |
0 |
1 |
155 |
- |
|
145.650 |
1 |
1 |
0 |
1 |
1 |
1 |
0 |
156 |
- |
|
145.675 |
1 |
1 |
0 |
1 |
1 |
1 |
1 |
157 |
- |
|
145.700 |
1 |
1 |
1 |
0 |
0 |
0 |
0 |
160 |
OK |
|
145.725 |
1 |
1 |
1 |
0 |
0 |
0 |
1 |
161 |
- |
|
145.750 |
1 |
1 |
1 |
0 |
0 |
1 |
0 |
162 |
- |
|
145.775 |
1 |
1 |
1 |
0 |
0 |
1 |
1 |
163 |
- |
|
145.800 |
1 |
1 |
1 |
0 |
1 |
0 |
0 |
164 |
OK |
|
145.825 |
1 |
1 |
1 |
0 |
1 |
0 |
1 |
165 |
- |
|
145.850 |
1 |
1 |
1 |
0 |
1 |
1 |
0 |
166 |
- |
|
145.875 |
1 |
1 |
1 |
0 |
1 |
1 |
1 |
167 |
- |
|
145.900 |
1 |
1 |
1 |
1 |
0 |
0 |
0 |
170 |
OK |
|
145.925 |
1 |
1 |
1 |
1 |
0 |
0 |
1 |
171 |
- |
|
145.950 |
1 |
1 |
1 |
1 |
0 |
1 |
0 |
172 |
- |
|
145.975 |
1 |
1 |
1 |
1 |
0 |
1 |
1 |
173 |
- |
|
146.000 |
1 |
1 |
1 |
1 |
1 |
0 |
0 |
174 |
OK |
|
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|
What exactly do these
tables of ones and noughts translate to in 1979 amateur radio
hardware? Below is the answer.. a forest of silicon diodes (bent
hither and thither over years in storage). Even a decade later
one would have needed pretty expensive test gear to program its
equivalent in ROM, but now, because I have a universal programmer,
I can swap all this for a small chip. However.. maybe I should
get the 1979 version going before making wholesale modifications?
Just to add to the complication of restoring this I have a slight
suspicion that when I added FM repeater operation to the rig
I made some changes to the switching circuits? Maybe that bunch
of diodes below the 220 ohm resistors? Channel selection is via
the two black rotary switches next to the diode matrix board. |
DIODE MATRIX
|
100KHz CHANNEL SELECTION CODES
|
FREQUENCY MHz |
F |
E |
D |
C |
B |
A |
G |
OCTAL |
|
144.000 |
0 |
1 |
0 |
1 |
1 |
0 |
0 |
54 |
|
144.100 |
0 |
1 |
1 |
0 |
0 |
0 |
0 |
60 |
|
144.200 |
0 |
1 |
1 |
0 |
1 |
0 |
0 |
64 |
|
144.300 |
0 |
1 |
1 |
1 |
0 |
0 |
0 |
70 |
|
144.400 |
0 |
1 |
1 |
1 |
1 |
0 |
0 |
74 |
|
144.500 |
1 |
0 |
0 |
0 |
0 |
0 |
0 |
100 |
|
144.600 |
1 |
0 |
0 |
0 |
1 |
0 |
0 |
104 |
|
144.700 |
1 |
0 |
0 |
1 |
0 |
0 |
0 |
110 |
|
144.800 |
1 |
0 |
0 |
1 |
1 |
0 |
0 |
114 |
|
144.900 |
1 |
0 |
1 |
0 |
0 |
0 |
0 |
120 |
|
145.000 |
1 |
0 |
1 |
0 |
1 |
0 |
0 |
124 |
|
|
FREQUENCY MHz |
F |
E |
D |
C |
B |
A |
G |
OCTAL |
|
145.000 |
1 |
0 |
1 |
0 |
1 |
0 |
0 |
124 |
|
145.100 |
1 |
0 |
1 |
1 |
0 |
0 |
0 |
130 |
|
145.200 |
1 |
0 |
1 |
1 |
1 |
0 |
0 |
134 |
|
145.300 |
1 |
1 |
0 |
0 |
0 |
0 |
0 |
140 |
|
145.400 |
1 |
1 |
0 |
0 |
1 |
0 |
0 |
144 |
|
145.500 |
1 |
1 |
0 |
1 |
0 |
0 |
0 |
150 |
|
145.600 |
1 |
1 |
0 |
1 |
0 |
0 |
0 |
154 |
|
145.700 |
1 |
1 |
1 |
0 |
0 |
0 |
0 |
160 |
|
145.800 |
1 |
1 |
1 |
0 |
1 |
0 |
0 |
164 |
|
145.900 |
1 |
1 |
1 |
1 |
0 |
0 |
0 |
170 |
|
146.000 |
1 |
1 |
1 |
1 |
1 |
0 |
0 |
174 |
|
|
|
Above are the codes I'm
expecting to find wired into the diode matrix. I see that Pins
A and G are not used (ie. grounded) leaving five pins to select
22 channels. The matrix has 11 vertical columns each of 22 rows
(including one spare row) so I may have used some of the patterns
for repeater operation as only the first 5 of the 11 columns
need to be wired to the PLL selection pins?
I now have the answer. I found
an ancient buff folder with lots of foolscap pages containing
sketches and notes. One page (see further down) has a drawing
showing 11 colums and 21 rows. The first 5 columns are headed
"Synth" and correspond precisely with columns FEDCB
in the table above. The next 4 columns are labelled "Digital
Display" and carry codes 0 to 40 octal then repeated. The
next two columns are marked "MHz LEDs" with the 10
row144MHz group as octal 0 and the next 10 row 145MHz group as
20 octal. Finally, row 21 has the code octal 1. So there it is..
Most of the diode matrix is used for providing drive to the channel
number digital display and the 144/145/146MHz LEDs mounted above
the tuning dial.
There's also additional notes
showing a further 16 diodes marked 11a-14a and 17a-20a which
relate to repeater offsets.
Below, the outputs from the
matrix board for pins FEDCB are carried by the blue connector
above the SP8922 chip. |
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In the decipherable pages
I unearthed is this circuit showing the phase locked loop and
the way it links via a filter (bottom left) into the VCO.
I still need to work out how
FM is produced. Two ways are possible. One is to wobble the VXO
(the box labelled VCXO top left) about its 10.245MHz (+/- its
pulling) or to wobble the VCO by interfering with the feedback
loop. Hopefully the wiring for this is still intact and I can
trace it.
From memory I had to make a
double bridge T filter tuned to precisely notch out the frequency
of the locking tone of 1250Hz. |
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PLL MODULE
For some reason I used the SP8620
chip (centre top) rather than the SP8621 shown in the PLL circuit
above. Maybe the Plessey rep from whom I got the samples gave
me this rather than the SP8621 or possibly the SP8621 was introduced
only after I got the samples?
Clearly, despite the power supply
difference (neg rather than pos, and dire warnings not to short
its output noted in its spec) it worked OK.
No less than four small pots
fitted here so I must have been experimenting with the filter
used to control the VCO?
Also.. I see several CV7047
germanium diodes in there...maybe for filter shaping or DC level
maintenance?? |
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Below is a second pass at the
overall block diagram of the 1980 2-meter rig. I'm not yet sure
how frequency modulation (or tone burst for FM repeaters) was
applied but I've shown below to the VCO for FM and for tone burst. |
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I've now found a few scruffy
pages of notes that should help with getting the diode matrix
restored back to working order. The first has leaked ink from
the reverse but the wiring is legible. The other pictures show
the lower half of the page and depicts the extra diodes and clues
to additional wiring. The biro had faded to a pale brown and
it was only when Photoshopped could detail be seen (hence the
yellow areas that appeared).
I see I had to drill 231 holes
plus another 55 for the repeater diodes. I should have invested
in a sheet of Veroboard instead of all that drilling and etching... |
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Two more important extracts
from the foolscap pages. Left shows the connections between the
diode matrix and the pair of rotary switches and above... I'm
not sure about this just yet?
The switches have 12 positions
and are wired to the 21 rows of the diode matrix. The second
row corresponds to 144.100MHz whilst row 11 is 145.000MHz and
row 21 146.000MHz. Inserted between rows 11 to 14 and 17 to 20
are the repeater diodes. These are shown below. Repeater selection
must be via one of the front panel switches. |
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Detail of repeater switching
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Below
is a sketch of one of the module circuits. Rather useful as it
clears up the 10.7MHz oscillator, receiver mixer and the audio
output circuitry in one hit. The original SN76005 spec can't
be found so I linked instead to the SN76002 which has the same
pinning.
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10.7MHz Crystal Oscillator, Product Detector
and Audio Output modules
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SSB GENERATOR
Now to look at the SSB generator again
after I found a couple of foolscap sheets with what looks like
the basic design details (shown below). |
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Above is the audio processing
circuit starting with a microphone amplifier and ending with
a pair of anti-phase audio outputs.
Below is the RF processing circuit
taking an output from the 10.7MHz crystal oscillator and ending
with SSB RF output. Audio inputs are marked A-B and C-D with
a reversing switch and a netting switch. The transformers are
a key component and if I recall correctly, these were samples
from a major manufacturer. Plessey was developing audio interface
equipment for air-ground-air comms in 1979 and the designers
ordered a selection of transformers to test in their prototypes.
I think these three were surplus to requirement and were liberated.
If they weren't properly sealed a winding or two might have gone
open circuit due to damp and that would put an end to this exercise. |
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VHF VCO (135MHz)
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This pcb is slightly puzzling.
I'd overlooked it in my earlier investigations. It had a copper
cover marked "135MHz VCO" which I'd removed, and later
the diode matrix board was obscuring it. I had to detach the
wooden side panel to move the diode pcb so I could figure out
what this board was for.
Two of its coax leads are disconnected
but I'm pretty sure the pcb has the VHF VCO plus buffered outputs.
I suspect the 135MHz level might be
too high necessitating a 135 notch filter in the final RF output?
I'm uncertain about how FM is generated
and some of this circuitry may be used for FM? |
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TRANSMIT MIXER
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Noted on this pcb is its
function and the date "Feb 1980" and as usual the schematic
doesn't show anything like the complete circuit which has a lot
more transistors. From the (simplified) schematic, SSB RF is
passed to the VHF mixer above at the pin marked "10".
The second input comes from the VHF VCO and the output goes to
the 2m RF amplifier.
I can see three stages of amplification
following the mixer with the final stage having a bias circuit
to provide decent linearity. It looks a bit messy with maybe
a couple of wires detached. |
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10.7MHz Oscillator
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Buried away in a heavy
metal box are two further pcb's. This one is the 10.7MHz crystal
oscillator which I see uses an old Plessey chip followed by a
buffer amplifier. |
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SSB Receiver, Mixer/IF
Amplifier
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Because of a mass of wiring getting
in the way I was only able to photograph this end of the second
pcb in the heavy metal box. This pcb doesn't carry a label but
the MD108 in the centre, suggests it's the receive mixer which
accepts the amplified 2m signal from the RF amplifier and the
VCO and produces a 10.7MHz IF. The signal must pass to a product
detector for SSB and to the SL664 FM receiver in another metal
box.
The extra parts in the centre suggests
I wasn't happy with the original pcb design. |
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Tone Burst module
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If it hadn't been for
the label, I'd have had no idea what this was. Clearly an afterthought
because it isn't a pcb.
Also, it doesn't have a home..
just dangling in the wiring.
Those familiar with 2m FM operation
will know that a repeater usually needs a tone at the start of
a transmission to open it for use. |
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Next, I looked at the front
panel copied below. There are three single-pole 12-way rotary
switches, one 12-pole 12 way switch, three potentiometers, four
jack sockets, a miniature single pole on/off switch, three slide
switches, the main tuning control and a meter.. but, although
most are labelled, nearly all the markings have vanished. Internal
connections are made predominantly in pink/orange twisted pairs
and all are harnessed making it an awkward job to figure out
the purpose of some of the controls. Using clues and guesswork
I've marked up the front panel below. Clearly, audio volume and
RF gain controls should figure and possibly a method of monitoring
voltages ( the three controls marked ???). The 146MHz LED might
be a Lock indicator? The mode switch might include FM, SSB, AM
and CW. When the picture was taken the two rightmost switches
hadn't been refitted. |
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Ready for checking and powering up
I reckon to have gone as far
as possible without properly checking wiring. The rig was fitted
into a 19 inch case which got used for something else ages ago
and then, because the chassis was unprotected, things got bashed
around especially the diode matrix board on which the thing rested
(for 40 years), bending all the diodes flat.
As with any chassis like this
with bits sticking out, and needing work, I made a wooden frame
so that it can be upended and moved around without damaging things.
Several coax cables are disconnected,
at least half a dozen diodes broken off the matrix and lots of
wires have come unsoldered.
Written on the side of the regulator
panel, just visible, appears to be "Output 11 volts"
which is a bit odd but, as both regulators are so rusty their
code numbers are obliterated. To see why it's 11 volts I'll need
to detach this panel and see which of the several wires from
the underside are input and which are output. At least initial
testing should be relatively easy because my power supply has
an adjustable current limit. Donkeys years ago it was usually
a case of switch on and look for smoke. |
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Power Supply
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I detached the power supply
carrying the two rusty regulators and found it was insulated
from the main chassis. Underneath was marked routings for various
leads (1 is SYN, 2 RX1, 3 TX1 OUT, 4 VCO, 5 SSB+TX and 6 MIC)
but as the ink on the labels on all the leads had disappeared
the numbering wasn't too useful. I guess the best bet is to fit
new labels after tracing the lead destinations. The wiring to
the power devices is shown below. My guess is the two TO3 devices
are 5 volt regulators and the smaller TO5 is marked 78M06 making
at least one rail 5+6 volts which matches the inked marking of
11 volts on the metalwork.
Another complication are two
large rusty relays connecting the power leads to the circuitry..
probably for transmit/receive? |
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Here's what I traced of
the power regulators. Far from ideal and clearly a junk-box design
because I still have a box full of those TO3 LM340K devices.
In fact that box has been joined by a second box full of TO3
12 volt regulators but many years too late for this job.
This design required the metal
heatsink on which the pair of larger regulators is mounted to
be insulated from surrounding metalwork.
You can see above, changes made
as power consumption increased during construction, when those
green ballast resistors were shorted out.
The right hand leg of the 78M06
connects to the TO3 regulators via the metal heatsink. Red is
the DC+ input, brown DC- and white 6 volt output with four pink/orange
pairs feeding 11 volts to the various transceiver modules. |
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I also noted lots of complicated
wiring to the LEDS.. in fact there's a 4th LED which I'd overlooked
and being red is going to be the PLL (un)lock indicator. I'd
imagined the 144/145/146 LEDs would be wired simply to the diode
matrix but no such luck as there are two transistors connected
into the wiring. I also noted that the pointer for the frequency
tuning had come adrift from its drive string. |
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The rear of the chassis
carries 6 connections. Two or more will be DC power, and as I
used a high power linear amplifier maybe a Tx/Rx control feed.
I'll also need to check the panel carrying the (rusty) regulators
to work out their input requirements.
Now that I've established that
the transceiver is more or less complete with no vital parts
missing I'll attempt to get it going, then perhaps make sufficient
improvements to have a 2 meter QSO or two. Below is an index
for the various pcb's and transceiver modules. |
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