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  #21  
Old 21st May 2019, 08:34 PM
Tom How Tom How is offline
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Join Date: Oct 2013
Location: Curdridge
Posts: 44
Default Re: K5881 hum / buzz

Tonight's report

Only one change - swapped in the new PSU capacitor. Whilst doing this I "unearthed" another potential (potentially fatal) gremlin. The DC side of one of the rectifier diodes was running VERY close to the tag board mounting screwhead. I can't believe it was actually arcing - that would have blown something - most likely me. I've been checking the case with a multimeter several times lately. At the same time it looked like it was making some kind of connection. The stupid things we did 20 years ago

It has survived and is making music once more. Less buzz but it is still there. and some hum also. Initial view is it sounds better! I'll reserve judgement until I've completed all the activities over the next few days.

Still lots left on the list, but one thing at a time.
Diodes next methinks. Expecting jokes about the ones down the my left side
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  #22  
Old 22nd May 2019, 06:48 AM
snowman_al snowman_al is offline
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Join Date: Nov 2016
Location: West Wales
Posts: 47
Default Re: K5881 hum / buzz

Tom,
Trying not to butt in here, but maybe posting a few clear pictures of the build would help?
I know you have changed a lot since the initial 'rat's nest' pictures, but things like twisting the heater wires and general routing, etc. all improve hum and buzz problems. It might help the helpers?
Alan
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  #23  
Old 22nd May 2019, 04:54 PM
John Caswell John Caswell is offline
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Join Date: Dec 2005
Location: Wokingham, Berkshire
Posts: 1,471
Default Re: K5881 hum / buzz

Hi all,
we did have some problems with this model's output stages oscillating due to:-
Long anode leads to the o/p tx
People strapping anode and grid leads together to make it look "tidy" and the like. This is where a 'scope is most useful.
It might be worth looking at this just in case.

John
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  #24  
Old 30th May 2019, 07:29 PM
Tom How Tom How is offline
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Join Date: Oct 2013
Location: Curdridge
Posts: 44
Default Re: K5881 hum / buzz

A few links of the innards right now
http://astro.neutral.org/tmp/5881/IMG_1053.jpg
http://astro.neutral.org/tmp/5881/IMG_1054.jpg
http://astro.neutral.org/tmp/5881/IMG_1055.jpg

I think most of the suggested changes have been made - although could still use some tidying up. The star earth post (top right near the op xformer) needs work.

I think hum is louder than buzz now.
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