Pye PE98U SW problem - UK Vintage Radio Repair and Restoration Discussion Forum

kestrelmusic · 27th May 2020, 5:34 pm

I'm working on a Pye PE98U and have encountered a very odd problem.

On LW and MW bands it works a treat. On SW it only works on frequencies above about 12 Mc/s. The local oscillator output diminishes gradually as the gang is meshed and ceases entirely at rather less than half mesh.

I've cleaned the wavechange switch which has made a slight improvement, and changed the UCH42 which made no difference at all, so I'm seriously puzzled.

Relevant bit of the circuit diagram attached.

Andrew2 · 27th May 2020, 5:44 pm

Hi.
Assuming the frequency changer valve is OK and the DC readings up to scratch, there's a strong possibility that the relevant osc coil has gone low-Q due to either damp or some degradation of the wax impregnation. I'm not sure if that model uses wax impregnated coils, but if it does it's a possibility.
The other possibility is slightly leaky caps, especially the low-value ones in the anode and grid circuits of the oscillator. Some of these oscillators were a bit marginal at the extreme low or high end and it didn't take much to stop them!

cathoderay57 · 27th May 2020, 9:25 pm

C14, C17 & C20 are probably silver mica and so are unlikely to be leaky. Although often wax coated if the coating has breaks or damage at the edges it can allow moisture in and oxidise the silver foils which does occasionally affect capacitance. This can be enough to affect oscillator performance. Worth checking the values. Also, assuming L5 is the SW osc coil, check C13 as well. Oh, and check the voltages on the UCH42 compared to the Service data (R5 might have gone high). Cheers, Jerry

27th May 2020, 5:34 pm	#1
kestrelmusic Hexode Join Date: Feb 2008 Location: Wrexham, North Wales, UK. Posts: 466	Pye PE98U SW problem I'm working on a Pye PE98U and have encountered a very odd problem. On LW and MW bands it works a treat. On SW it only works on frequencies above about 12 Mc/s. The local oscillator output diminishes gradually as the gang is meshed and ceases entirely at rather less than half mesh. I've cleaned the wavechange switch which has made a slight improvement, and changed the UCH42 which made no difference at all, so I'm seriously puzzled. Relevant bit of the circuit diagram attached. Attached Thumbnails

27th May 2020, 5:44 pm	#2
Andrew2 Nonode Join Date: Mar 2010 Location: Dukinfield, Cheshire, UK. Posts: 2,037	Re: SW problem Hi. Assuming the frequency changer valve is OK and the DC readings up to scratch, there's a strong possibility that the relevant osc coil has gone low-Q due to either damp or some degradation of the wax impregnation. I'm not sure if that model uses wax impregnated coils, but if it does it's a possibility. The other possibility is slightly leaky caps, especially the low-value ones in the anode and grid circuits of the oscillator. Some of these oscillators were a bit marginal at the extreme low or high end and it didn't take much to stop them! __________________ Andy G1HBE.

27th May 2020, 9:25 pm	#3
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,384	Re: Pye PE98U SW problem C14, C17 & C20 are probably silver mica and so are unlikely to be leaky. Although often wax coated if the coating has breaks or damage at the edges it can allow moisture in and oxidise the silver foils which does occasionally affect capacitance. This can be enough to affect oscillator performance. Worth checking the values. Also, assuming L5 is the SW osc coil, check C13 as well. Oh, and check the voltages on the UCH42 compared to the Service data (R5 might have gone high). Cheers, Jerry