Many years ago, I stumbled across the G3PDM Mark 1 receiver in some old RadComs…only to discover the Mark 2 version in the blue RSGB Handbook. It looked like an interesting design…but I found the coil manufacturers had ceased trading…bugger!
Before I go any further…if you’ve got, built, or are considering building a G3PDM receiver, please get in touch. Maybe you just have some internal pictures of an old set…anything that may save me (or you if you are building) considerable head scratching.
The G3PDM receiver is probably about a tenth of the weight of the G2DAF designs, popular in the 60’s and 70’s. It uses some really advanced (for the time) ideas so I set about locating one. Adverts in PW and RadCom turned up nothing and I think I missed one on eBay…but it is probably the one I eventually bought off eBay.
I was told it was an unfinished project…but with it came a note saying ‘I hope you manage to get rid of the howling and feedback in the IF strip’. Mmm…knowing now who it came from, I expected a challenge. Most of it was there…certainly all the important bits…but there are problems; some of the minor ones have been fixed.
From above, it looks pretty much like the original article in RadCom…although those trimmers look a bit vulnerable at the upper left.
Mine is a Mark 1 version by the way…although that might change as there were issues with the first version. Chances are mine will end up being a Mark 1.5.
Underneath, again it looks like the original article but…a mention is made of ‘extensive shielding under the chassis’…which there certainly isn’t here.
The main problems with this receiver were due to the very high gain of the IF strip, and of course the synthesiser. I think I may need some aluminium sheet and valve base screens.
The initial problems with this receiver were unsoldered connections; the valve heater feed being one of them.
Various diodes were found to be defective in the AGC circuit and negative voltage regulator.
But the howling from the IF strip was dreadful, combined with the loud hum from the speaker. I worked through the circuit and eventually found that the original builder hadn’t been aware of the way Electroniques made their IFT’s…see if you can spot the likely confusion.
It was a Eureka moment…dashed to pieces when I found it made no difference at all to the howling.
As things stand now, the synthesiser locks on most bands, the audio hum disappears when a magnetic shield is put over the 7360 product detector…and I can receive SSB signals rather well.
I’ll post updates under the G3PDM category as the project moves forward.