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Old 9th November 2010, 09:15 AM   #1
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Default Power supply buzz

I have recently rebuilt my SE 6DT5 into an old valve radio I bought on ebay, for my sisters birthday. In a late night beer fueled rush to get it finished in time, I rushed the wiring and there was a hurrendous buzz in the signal path a few seconds after turn on. I put this down to me being a fool, and bought her chocolate instead.

So over the last week I have rebuilt it to a much better standard, but the buzz remanes there.

Originally I thought that it was the new transformers I am using, mentioned here...

SMALL SIZE OUTPUT TRANSFORMERS

... cheap I know, but an upgrade from the hallogen light heaters I was using as OTs before! Ill fork out for decent parts when Im more experienced.

So the buzzing comes after a few seconds of turning on, but when touching a probe to the primary on the OT is cuts out and gets louder, and makes all sorts of horrible noises. Can anyone suggest what this might be?

Ive checked all my wiring, again, checked the diodes and the p/s caps, moved the transformers around, and have now reached the limits of my knowledge.

I was concerned that the transformers might not be up to the job, but from the other thread that doesnt seem the case.

I did spend all night searching the threads, but nothing really hit the mark.

Thanks for any help in advance.... still learning

Charlie
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Old 9th November 2010, 10:00 AM   #2
chrish is offline chrish  Australia
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Star Grounding
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Old 9th November 2010, 10:10 AM   #3
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Thanks for your help, but this was the first thing I checked

Charlie
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Old 9th November 2010, 10:15 AM   #4
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interesting read though chrish, thanks
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Old 9th November 2010, 12:32 PM   #5
korey is offline korey  United States
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Is it possible for you to post a pic? It sounds like a power supply filter capacitor issue, but don't want to guess. Maybe a fresh pair of eyes can see something that you may have missed.
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Old 9th November 2010, 12:39 PM   #6
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I just got in and fired it up again, and after a while the buzz slowed down to a clicking.

To an inexperienced ear it sounds very much like a low frequency oscillation.

There isnt much room inside the radio, and i tried to use exterior holes, but this placed the mains switch close to the speaker terminals. The change in 'buzz' was me moving the switch away from the terminals.

So I guess im looking at mains interference of some kind?
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Old 9th November 2010, 12:44 PM   #7
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ok, and touching the mains plug results in this oscillation to speed up again.

im certainly stumped
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Old 9th November 2010, 01:00 PM   #8
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ok.....

i hadnt realised that the volume pot was floating. Having seen my mistake, I thought to myself that this couldnt be the reason for all this horrible noise....

... how wrong I was. There is still a bit of hum, so im going to go back over everything.

But at least there is hope!!

Thanks to everyone for putting up with me

Charlie
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Old 9th November 2010, 02:39 PM   #9
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RFI? Try grid stoppers?

Contrary to popular belief, chassis grounding, done responsibly, has much better results. Particularly at RF, where you can't tolerate the huge ground return inductances that star grounding inevitably gives you. Radios and TVs are all chassis grounded, and with good reason: they need excellent RF performance!

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Old 9th November 2010, 03:27 PM   #10
Arnulf is offline Arnulf  Europe
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I wouldn't want my AF amplifier to have excellent RF performance Quite the contrary, I want it to suck at RF amplification.
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