UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post

cathoderay57 · 13th Mar 2019, 7:59 pm

Hello folks,
I've been working on a Marconiphone 855 using Trader Sheet 736. It was almost untouched although the electrolytics had been replaced a long time ago. Having mostly recapped it and realigned it, the radio works very nicely on all bands except 49m. It gets progressively more deaf towards the LF end of 41m and receives nothing at all, not even noise, on 49m. Ditto injecting a modulated signal from a signal generator. The tuning capacitor vanes are not shorting. I have changed the oscillator coupling capacitors C6 and C11, and the SW padder C7. The oscillator anode voltage measured very high (232v versus 178v on the Trader Sheet). I reduced this somewhat (to 195v) by changing R3 the anode feed resistor from 15k to 27k. The oscillator anode current on MW, no signal, is about 3.6mA. Now here's the thing - the frequency changer is an X63 which tests good on the VCM163 but replacing it with a 6K8G brings up 49m loud and clear. I also have a 6A8 which is a closer equivalent to X63 but I haven't tried it yet. Now, I have had trouble with X63 frequency changers in EMI sets before, notably a Marconiphone 851. On that set (which I no longer have), it was unstable on 16m band and also deaf on 49m. By juggling about with various components a sort of compromise could be reached but in the end I just ran it using a 6K8G. Anybody had similar issues and found a more elegant fix?
Cheers, Jerry

13th Mar 2019, 7:59 pm	#1
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,384	Marconiphone 855 Silent on 49m Hello folks, I've been working on a Marconiphone 855 using Trader Sheet 736. It was almost untouched although the electrolytics had been replaced a long time ago. Having mostly recapped it and realigned it, the radio works very nicely on all bands except 49m. It gets progressively more deaf towards the LF end of 41m and receives nothing at all, not even noise, on 49m. Ditto injecting a modulated signal from a signal generator. The tuning capacitor vanes are not shorting. I have changed the oscillator coupling capacitors C6 and C11, and the SW padder C7. The oscillator anode voltage measured very high (232v versus 178v on the Trader Sheet). I reduced this somewhat (to 195v) by changing R3 the anode feed resistor from 15k to 27k. The oscillator anode current on MW, no signal, is about 3.6mA. Now here's the thing - the frequency changer is an X63 which tests good on the VCM163 but replacing it with a 6K8G brings up 49m loud and clear. I also have a 6A8 which is a closer equivalent to X63 but I haven't tried it yet. Now, I have had trouble with X63 frequency changers in EMI sets before, notably a Marconiphone 851. On that set (which I no longer have), it was unstable on 16m band and also deaf on 49m. By juggling about with various components a sort of compromise could be reached but in the end I just ran it using a 6K8G. Anybody had similar issues and found a more elegant fix? Cheers, Jerry