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Old 1st November 2013, 04:14 AM   #1
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Default radio like interference condition in SA-100

Anybody ever do the coupling caps on an amp and then wind up with radio like interference where something is acting like an antenna on the input tube side? Or does anybody no of this condition and how it occurs?
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Old 2nd November 2013, 02:12 PM   #2
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Basically what happens is when i attach the 47k input resistor to pin 7 it gets a nasty buzz/humm through it. I start measuring about .120 of ac volts. Where on the other channel I measure .004 ac volts.
schematic here-( http://www.quadesl.com/pdf/sa100_photofacts.pdf )
Any ideas would be great. I have given it about 8 hours of trying and I cant figure it out. I will try anything.
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Old 2nd November 2013, 02:21 PM   #3
Mooly is offline Mooly  United Kingdom
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What do you mean by "Anybody ever do the coupling caps on an amp....

Have you replaced caps and now find you have this problem where before you did not ?

Disclaimer... I don't normally do tooobs however caps, which are basically large areas of foil wrapped up in a can, act or can act as antennas. Were the original caps fitted in such a way that the "outer" of the foil was connected at the low impedance end of the cap location thus using the outer foil as shield. Or were they a construction that was immune to pickup.
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Old 2nd November 2013, 08:51 PM   #4
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Yes that is exactly what happened but it only happened on one channel. I double and triple checked making sure nothing was touching something it shouldnt be. As far as the new capacitors they are orange drops and i never had a problem before. hmmmm
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Old 3rd November 2013, 02:27 AM   #5
gootee is offline gootee  United States
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Which caps did you replace, exactly? And why did you have the 47k disconnected if you were replacing caps?
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Old 3rd November 2013, 11:12 AM   #6
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I replaced the four .047 coupling caps and the .022 cap on the input. I only removed the bypassed 47k to try and trouble shoot it. It is back on there right now. I gave this a couple of days to sit and see I could come back to it with a fresh head and the solution would present itself. I usually just rebuild things from the ground up when I run into problems, because I don't know enough. I would like to avoid that and learn something about tube amps this time.
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Old 3rd November 2013, 08:52 PM   #7
DF96 is offline DF96  England
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Blindly replacing components instead of fault-tracing is as likely to introduce new faults as fix old faults. As you have now found. Even when bulk replacement is appropriate, this should only be done a few components at a time with checks for correct operation between each set of changes. Then you know where you went wrong.

Something you did was wrong. You have to keep checking until you find it. Or hand it over to someone who can fault-trace - which will be difficult with so many changes.
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Old 3rd November 2013, 10:17 PM   #8
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Quote:
Originally Posted by DF96 View Post
Blindly replacing components instead of fault-tracing is as likely to introduce new faults as fix old faults. As you have now found. Even when bulk replacement is appropriate, this should only be done a few components at a time with checks for correct operation between each set of changes. Then you know where you went wrong.

Something you did was wrong. You have to keep checking until you find it. Or hand it over to someone who can fault-trace - which will be difficult with so many changes.
Thanks for the lecture
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Old 3rd November 2013, 10:59 PM   #9
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It might be helpful to take measurements with a DMM of the 'good' channel,and compare them to the 'bad' channel.

Sometimes it's hard to spot what you've goofed up,if indeed you did goof up. Some checks with an ohm meter might shed some light.
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Old 3rd November 2013, 11:28 PM   #10
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Quote:
Originally Posted by DigitalJunkie View Post
It might be helpful to take measurements with a DMM of the 'good' channel,and compare them to the 'bad' channel.

Sometimes it's hard to spot what you've goofed up,if indeed you did goof up. Some checks with an ohm meter might shed some light.
Thank you for being constructive. I know its nearly impossible to arm chair diagnose something like this. I could bring this to somebody to fix it, but it wouldn't be satisfying and I would miss the accomplishment of doing it myself. Not having any formal training in electronics, I have been taught everything I know by these forums. To the credit of folks around here I have had many success story's. Thanks for the good spirit.
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