OK - I know this isn't strictly a boatanchor. But it did come from a boat! This marine transmitter/receiver was acquired at one of many "container" sales organised by VMARS (Vintage and Military Amateur Radio Society) to dispose of the lifelong radio collection of the late G3TFC. John had amassed an incredible amaount of equipment over the years and VMARS took on the sad duty of ensuring that as many items as possible went to a good home. As can be seen, the set appears in reasonable condition (at least all it's knobs were present) and I was confident that, with a bit of elbow grease, it could be cosmetically restored to it's former glory!
Taking it out of it's case, however, it soon became clear that bringing it back to operational condition would be more of a challenge. There appeared to be a large chunk missing!
In it's former life, this equipment had been designed to run from a 12V DC supply. The receiver was fully transistorised as were the early stages of the transmitter and the modulator. The PA stage, however, used a pair of TT22 valves to achieve a modest 50W output and these were designed to work with a supply voltage of 700V. This was obtained using a solid state inverter and this was the chunk that had been removed by a previous owner. In some ways, I was relieved to discover this. Initially, not having any documentation, I had thought that it was the modulator that had gone awol. I could construct a high voltage power supply but the loss of the modulation transformer would probably have stopped the project in it's tracks.
Looking under the chassis, however, I eventually discovered that what I had thought was the inverter transformer was, in fact, the mod transformer. Although various other bits had been removed (as evidenced by the cut off loom stubs), it looked as if the modulator was all there.
Thanks to VMARS members, I managed to get hold of circuit diagrams of the transmitter and receiver. This confirmed the above deduction and, much to my relief, indicated that the other missing parts were not essential for amateur radio operation. I couldn't see much call for a warbling alarm signal!
I tackled the receiver first. Actually it didn't require much fettling. At first I thought there was a serious problem in the IF but it turned out to be a sticking relay which was permanently connecting a narrow audio filter for CW reception. The only other problem was non-operation of the RF gain control which was fixed by replacing an open circuit Germanium diode. The forward AGC doesn't quite manage to handle very strong MW signals on a long aerial but I don't consider that to be a real problem.
Now the early stages of the TX are supposed to be powered from an isolated 20V (?) supply from the inverter. I didn't plan to have this supply so I simply used the 12V supply and tweaked the drive preset to give a bit more gain. This did mean that I had to configure the whole thing as positive earth, however, so I would have to be a bit careful not to short things out. A couple of 12V relays were added to handle the R/T changeover as these had originally been located in the inverter.
I didn't have any TT22's to put in the PA so I used a pair of 6146's. This just required a swapping a couple of pin connections and arranging a 6.3V heater supply. As various other modifications had already been done to the set, I considered this to be acceptable. Eventually, a kind VMARS member let me have a couple of TT21's and so the valve bases where restored to their previous connections. The above chassis view looks a much better with "proper" sized valves!
A high voltage power supply was quickly lashed up and, with the help of a 12V battery, the rig was given a tentative trial run on the 3615kHz VMARS Saturday morning net. Signal reports were fairly encouraging although the modulation was apparently rather bassy. This was soon fixed by significantly reducing the value of a coupling capacitor in the microphone preamplifier stage and subsequent reports have been very good.
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