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Old 16th Sep 2006, 4:57 pm   #1
JoshWard
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Default Pye P75- Strange noises

The other day I got another P75. The seller said it was working and after cleaning it out I tried it. It made a lot of squealing noises and a loud hum (altered by volume control) and I could hear Radio 4 in the background.

I changed two resistors and various capacitors (including the 16 & 32uF main smoothers). I now get a lot of thumping and squealing noises all over the dial. The stations are weak and can barely be heard above the squealing. However, 5-Live comes in loud and clear with no noises at all.

Question is, how do I stop it squealing? I'll leave my other P75 (working, unrestored) in the workshop for now in case I need to compare anything. Any suggestions . Cheers, Josh.

Last edited by Darren-UK; 30th Jul 2007 at 10:42 pm. Reason: General tidying.
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Old 16th Sep 2006, 8:19 pm   #2
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Default Re: Pye P75- Strange noises

Hi Josh.

My guess is that it is suffering from IF instability. I would suggest you change the wax capacitors decoupling the HT supply and screen grid of the IF amp and mixer...in fact if it was me, I would change all the old waxies anyway. You'll probably find this will solve a lot of problems. As you have another working set, you can try subbing the mixer and IF amp valves just in case one of them is causing the problem.


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Old 16th Sep 2006, 8:22 pm   #3
stevekendal
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Default Re: Pye P75- Strange noises

Mine was exactly the same. Just needed the ht decoupling cap to the front end changing. Think it was a 2MFD electrolytic, but could be wrong Steve.
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Old 16th Sep 2006, 8:24 pm   #4
PJL
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Default Re: Pye P75- Strange noises

I guess you've already done the obvious and replaced the audio coupling capacitor and checked the HT voltage is OK against the trader sheet? Does it do the same on gramaphone input? (assuming it's got one!)
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Old 17th Sep 2006, 12:27 pm   #5
JoshWard
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Default Re: Pye P75- Strange noises

I have already changed the waxies and I've changed two resistors. I'll try all the valves in a minute. It is possible it could be the 2uf capacitor. I'll swap it with my other one. Thanks for all advice so far. Josh.
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Old 17th Sep 2006, 1:16 pm   #6
paulsherwin
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Default Re: Pye P75- Strange noises

Quote:
Originally Posted by JoshWard View Post
I'll try all the valves in a minute. It is possible it could be the 2uf capacitor. I'll swap it with my other one.
You can confirm or eliminate this by temporarily wiring a new electrolytic across it. The value isn't too important.

In addition to everything others have mentioned, have a good look round for dislodged wiring around the wavechange switching and coils, and make sure the wavechange switch is clean.

Good luck, Paul
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Old 17th Sep 2006, 4:46 pm   #7
yestertech
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Default Re: Pye P75- Strange noises

I had this problem on a P75 - It was down to the decoupler for the HF valve screen grids, which I think is the 2 mf cap mentioned. As it is shared across both valves, it seems that any inefficiencies due to leakage causes coupling between the stages and instability. Nice little sets when they're working.
Watch for poor contacts in the valve bases, they didn't appear of great quality in my set .... Andy
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Old 19th Sep 2006, 4:48 pm   #8
JoshWard
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Default Re: Pye P75- Strange noises

I temporarily fitted a 10uf 450v capacitor in and it worked fine. This is my second P75 and they are very good sets if you want a small fifties woodie. In fact they're good sets full stop! Josh.
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Old 20th Sep 2006, 9:08 am   #9
peter_sol
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Default Re: Pye P75- Strange noises

HI Josh.
The P75RG has a larger value cap in this position presumably to reduce hum with the larger speaker so you could leave your 10uf in if you are not worried about authenticity
Cheers
Peter.
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Old 20th Sep 2006, 10:05 am   #10
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Default Re: Pye P75- Strange noises

Josh

Good to hear that you fixed your problem.

I've come across a number of Pye P76 sets where I needed to replace the electrolytic that decouples the supply to the 1st AF amp anode, IF screen grids and local oscillator anode. Symptoms were similar to your set; instability, howling, heavy distortion on bass notes etc.
I believe that the P76 and P75 are similar if not identical in using this arrangement.

John
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