So I'm working on a AA270 silverface SR and my buddy says the bass will drop out after playing it for a while. I tried it and sure enough after playing it for 5-10 minutes the bass dropped out. I've done the usual check capacitor values, looked for bad resistors, and I'm coming up with nothing. Has anybody experienced this that can offer some insight or point me in the right direction to find the culprit? Sounds like a capacitor issue but everything checks out.
Thanks in advance for any help.
BTW the amp is pretty stock although someone has gone through it and swapped out the electrolytic caps and all appear to be done with correct values and all fall within spec
74 Silverface Super Reverb problem
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kanderson70
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Re: 74 Silverface Super Reverb problem
Same in both channels?
With the exception of the overall bass level, do all controls still work as normal?
Any other changes go along with the reduction in bass?
A voltage survey in normal and fault modes may be beneficial.
With the exception of the overall bass level, do all controls still work as normal?
Any other changes go along with the reduction in bass?
A voltage survey in normal and fault modes may be beneficial.
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kanderson70
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Re: 74 Silverface Super Reverb problem
I haven't checked both channels. Only the reverb channel. All pots function normally.
I'll give the first channel a listen tonight.
I'll give the first channel a listen tonight.
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Stevem
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Re: 74 Silverface Super Reverb problem
I had this condition once and one of the speakers was cutting out.
It could be a bad solder connection on a preamp cathode bypass, or just a failing bypass cap when it get hot.
If the normal channel is ok then jump it’s second gain stage output ( after the plate output cap ) to the same cap on the effects channel and see if it still acts up.
Even new parts can be / go defective!
In doing this your eliminating the whole tone stack of the effects channel.
If both channels do it then it could be that crappy all of 2 cents worth ciramic coulping cap feeding the PI input.
It could be a bad solder connection on a preamp cathode bypass, or just a failing bypass cap when it get hot.
If the normal channel is ok then jump it’s second gain stage output ( after the plate output cap ) to the same cap on the effects channel and see if it still acts up.
Even new parts can be / go defective!
In doing this your eliminating the whole tone stack of the effects channel.
If both channels do it then it could be that crappy all of 2 cents worth ciramic coulping cap feeding the PI input.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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kanderson70
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Re: 74 Silverface Super Reverb problem
Interesting on the speaker cutting out.... makes me wonder.Stevem wrote: ↑Wed Apr 27, 2022 11:23 pm I had this condition once and one of the speakers was cutting out.
It could be a bad solder connection on a preamp cathode bypass, or just a failing bypass cap when it get hot.
If the normal channel is ok then jump it’s second gain stage output ( after the plate output cap ) to the same cap on the effects channel and see if it still acts up.
Even new parts can be / go defective!
In doing this your eliminating the whole tone stack of the effects channel.
If both channels do it then it could be that crappy all of 2 cents worth ciramic coulping cap feeding the PI input.
Funny you should mention the ceramic coupling caps. I just replaced all the ceramics in it last night (even though they all measured to spec). I hadn't fired it up again until today (and posting this thread). I played the first channel for about 30 minutes.... no bass drop. Then switched over to the other channel for another 30 minutes or more... no bass drop. I wonder if it had to do with those crappy little ceramic caps... dunno. Im going to keep it over the weekend and see if happens again. Thanks for all your advice and help! This was driving me insane in the membrane. I just hope one of those things fixed it.
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Stevem
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Re: 74 Silverface Super Reverb problem
My money's on that PI input cap, in which case both channels would have shown the same drop out issue!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!