Motorboating P75A
 

Hi,

Have a rather sorry looking Pye P75A on the bench right now. The main problem with it is that the set starts to motorboat (breaks into self oscillation) when the volume pot is advanced more than a few degrees. The problem is made worse if an incoming RF signal is particularly strong. I have already started to investigate and C25 in the feedback circuit did look a bit ify. However, the cap checks out at 0.1uF on my meter OK. Also the set is full of rather dodgy looking old electrolytics, including C30/31 which is used to decouple the grid on V4 an smooth the rectified mains from V5. Should I go on a capacitor (sorry condenser) witch hunt, or has anyone else come across a similar problem this this set?

Many thanks in advance,
Andy

Re: Motorboating P75A
 

Hi Andy, This sounds like a classic case of dried up caps or caps that have gone high impedance at RF with age. Providede you have change the AF grid coupling capacitor, a quick way to check the others ids to bridge across them one at a time with a modern 0.1uF. This should help you find the suspect.
One the ones I restore I al;sways fit an X class across the mains, even if one was not there originally; I often use a 0.22 across the main smoothers as well, especially if they have not been replaced only reformed.

HTH Ed

Re: Motorboating P75A
 

Andy, I had this fault on a P75 - The chief culprit was the common cap used to decouple the screen grids of both the RF valves. I think its something like 2 microF from memory ?
Andy

Re: Motorboating P75A
 

Capacitors only need to develop a small impedence and you can get motorboating. Use Ed's method to find the faulty capacitor but personaly I would change the lot to ensure reliability.


Rich.

Re: Motorboating P75A
 

Chaps,

Thanks for your advice. I pulled both C28 (the dual screen grid decoupling condenser mentioned) and C30/31 out of the chassis. Both were puffed out and spewing dialectric so I suspect this might be the cause. I will also replace C29 as suggested. Interestingly C30/31 was marked 'Oct 54' so I guess 52 years of service isn't that bad!

Regards,
Andy