On trying to use a Drake SP75 Speech Processor with the TR4310 Communications Transceiver.
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I'm pleased to report that the TR4310 has been working flawlessly since it's recent major surgery. I have been using it on 160, 80 and 40 with a Shure 444 microphone, which I find always produces great results with any TX or TRx from the same era. I've replaced the Drake FA-7 110 Volt AC cooling fan with a 12 Volt DC computer-type one - about 5 Volt DC gets it spinning nicely without screaming. I don't like the US practice of routing AC primary mains into radios for on/off switching, or fans, especially where the radio is otherwise fully low-voltage DC powered.
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The job lot of Drake I bought some while ago included an SP75 Speech Processor. The SP75 is a proper RF Clipper, with a compression stage on the front. I'm a big fan of the Datong 'ASP' Automatic RF Speech Processor units, made in the UK many years ago. The SP75 uses the filter method (at 459 kHz) to create it's SSB, while the Datong uses the phasing method. I thought the SP75 would make a nice companion for my TR7 / TR4310 and free-up a Datong for use with other vintage radio here.
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On plugging the Drake processor into the TR4310, it behaved rather strangely. The 'on' LED would only illuminate when the microphone PTT was pressed. It passed audio when it did that sounded not too bad. When the case of the processor touched the case of the transceiver, the transceiver went into transmit!?!
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I pulled the processor apart and checked it's jumper settings, microphone socket and microphone plug wiring against the schematic. All in agreement.
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Next I put the manuals for the TR7 and TR4310 side by side. Surely Drake would not wire the microphone sockets differently between amateur and commercial versions of, largely, the same product, would they. The cynic in me thought they just might, in order to add a barrier to commercial users trying to cheap-out and use accessories from the amateur line. No! I banished my cynical thoughts - the wiring was specified as identical.
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That left only one possible cause: The wiring of the microphone socket in the TR4310 was wrong / not as designed. I flipped the TR4310 over and put it side-by-side with the flipped-over TR7 that had been supporting it all these weeks. The microphone socket in the '4310 was mirror image vertically of that in the TR7. So, what should have been ground on the connector was wired as PTT and vice versa. What should have been hi-level mic audio was wired as lo-level and vice versa. The soldering did not look too bad, but less good than I would have expected ex-factory. I re-wired it to be correct against the schematic and concluded that the wiring error was introduced when someone had the front panel off previously. I can see that the movable photo interrupter, part of the tuning dial, has been changed. If the fixed one has been changed too, that would have necessitated front-panel removal.
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Now to try the processor to see how it performs!
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Regards,
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Mark, G4FPH.
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