wayne
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Post by wayne on Nov 14, 2021 16:00:39 GMT 12
Hi all. Ive just completed restoring a Pye PZ71 table model. I replaced all the capacitors and it's working well with a nice sound apart from a noticable hum which is there with the volume turned right down and is constant regardless of the volume selected. I understand that valve radios can be disturbed by LED and other appliances but I tried it in the garage which has its own supply from the main switch board ( so I've been told ) and the hum is still there.
If I remove the ECL80 valve the hum is gone and the same with the EZ40
Any ideas or suggestions much appreciated or is it one of those things just live with it ?
Thanks in advance
Cheers
Wayne
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peter
Tinkerer
Retired, collector of vintage radios and test equipment for restoration and repair.
Posts: 97
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Post by peter on Nov 14, 2021 20:44:51 GMT 12
Check the mains filter caps and the smoothing choke (if there is one - google 'pi filter' if you want a description) - usually mains hum is due to these caps being leaky, but it does depend on the design. The other place to check is any coupling capacitors to audio valve grids in the audio section. Again if these go leaky you can get hum bleeding through BUT these going leaky usually stuff the audio valves in short order as they are overdriven as a rsult. Good luck.
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wayne
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Post by wayne on Nov 15, 2021 22:07:06 GMT 12
Thanks Peter. All of the tubular caps and electrolytic caps had been replaced before I tried it. I've had a close look at the chassis and the schematic and I haven't found any that I missed. I can't see anything to me that looks like a smoothing choke so so far no joy.
Cheers
Wayne
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wayne
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Post by wayne on Nov 16, 2021 15:46:01 GMT 12
In trying to learn more I noticed that the electrolytic capacitor that comes off the ECL80 valve base does from the positive end goes directly to the back of the power transformer
then to a resistor then to a capacitor then to an earth. Would it be better for it to go straight to a tag strip to earth as the other one does or am I showing my ignorance. Any advice appreciated
Thanks
Wayne
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wayne
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Post by wayne on Nov 16, 2021 21:43:42 GMT 12
Please ignore my Last post I've just re read it and it makes no sense
Sorry
Wayne
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Post by Philconut on Nov 18, 2021 12:13:19 GMT 12
Check that none of the unallocated pins of the EZ40, 3,4,& 5, is used as a tag point. I had a similar problem with an Ultimate radio that used a 6X4 rectifier. One of the unallocated pins was used as a tag but inside the valve, the wire approached very closely one of the anode wires. It was close enough to give sufficient capacitive coupling to induce a hum in the pin used as a tag.
The network you mention is to provide negative bias voltage for the output valve.
Good luck.
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Post by Philconut on Nov 18, 2021 13:01:56 GMT 12
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wayne
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Post by wayne on Nov 18, 2021 18:36:42 GMT 12
Thanks for that.Ive had a look and the way I see it is going from a clock wise direction after the location pin no 3 has 2 resistors coming of it going the ELC80 valve and a capacitor back to the transformer earth. Nos 4 5 nothing. no 6 positive of electrolytic cap. No 7 to earth
Thanks again
Wayne
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6A8G.
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Hard working kiwi soundmixer.
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Post by 6A8G. on Nov 21, 2021 11:25:58 GMT 12
Have you tried replacing the ECL80? Just thinking of a partial heater-cathode short....
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wayne
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Post by wayne on Nov 22, 2021 16:55:43 GMT 12
I have tried some things and results as follows. The schematic says both filter capacitors as 8uf when I got the radio the capacitors fitted were a 40 and a 50 both of which in my novice opinion looked original. I replaced both with 20uf. Now working on the hum if I increase the capacitor fitted to the speaker transformer ? and direct to earth the hum increases but the higher I go with the other one which comes off the ECL80 the hum decreases.
Another issue is a very a random very distorted reception but by moving the EBF80 around the distortion goes away giving a nice sound.
Thanks for all your help
Regards
Wayne.
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Post by Philconut on Nov 22, 2021 18:16:28 GMT 12
One thing at a time!
1. Try shorting grid 1 (pin 9) of the ECL80 pentode to ground. If the hum disappears, the problem is before the ECL80, if not, it is around the ECL80.
2. I would suggest that the EBF80 distortion problem in either a broken solder joint on the valve socket or a dirty valve pin. Clean the valve pin up with some fine emery paper.
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Post by Philconut on Nov 22, 2021 18:18:08 GMT 12
Use a 0.1uf capacitor to short the grid to ground as the bias voltage is applied to the grid.
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wayne
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Post by wayne on Nov 22, 2021 18:33:31 GMT 12
Thanks for that and agreed 😊 I only asked in case it had anything to do with the hum. I will try that and let you know
Thanks again appreciated
Wayne
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wayne
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Post by wayne on Nov 22, 2021 21:01:15 GMT 12
I didn't know which was pin 9 so I searched on line for the valve info found it and worked it out and yes with the 0.1 cap to earth the hum goes away. Now my opologies as the filter capacitor doesn't come off the ECL80 as I previously said it comes off the EZ40
I had already cleaned all of the valve pins and bases
Thanks
Wayne
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roy51
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If it ain't broke, fix it anyway
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Post by roy51 on Nov 24, 2021 17:14:26 GMT 12
Does the radio have the original output transformer ?
This transformer forms part of the “Pi” filter system and could be the source of your hum (the tap on the transformer gives it away)
Cheers
Roy
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