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DC Offset with Cap Coupled Amp
DC Offset with Cap Coupled Amp
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Old 12th October 2021, 11:51 PM   #51
dbxdx5 is offline dbxdx5
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Some more information, fwiw.

I just completed recapping both amp boards--no change.
I noticed that the DC I'm measuring at the speaker terminals (8 ohm dummy load) with my DMM's goes up by several hundred millivolts when I turn the input selector to a different source. Nothing is connected to the inputs.

I swear this thing is possessed.
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Old 13th October 2021, 08:32 AM   #52
OldDIY is offline OldDIY
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In your circuit, the load is connected through an electrolytic capacitor C109 to the ground rail. A small DC comes to the load through the GNFB chain. If С109 is serviceable, there is no significant DC at the output. It is not dangerous for the speakers. Artifacts (clicks) may occur when switching inputs.
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Old 14th October 2021, 12:33 AM   #53
dbxdx5 is offline dbxdx5
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Quote:
Originally Posted by OldDIY View Post
In your circuit, the load is connected through an electrolytic capacitor C109 to the ground rail. A small DC comes to the load through the GNFB chain. If С109 is serviceable, there is no significant DC at the output. It is not dangerous for the speakers. Artifacts (clicks) may occur when switching inputs.

Thanks. I think I was getting too caught up in DC blips that my meter was measuring, rather than just confirming that it wasn't audible. As for the AC noise I was seeing on my scope earlier, that seems to have been taken care of by recapping the power supply, amp boards, and preamp boards.
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Old 16th October 2021, 05:55 PM   #54
dbxdx5 is offline dbxdx5
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I thought the AC noise shown in my scope YouTube videos earlier in the thread had been solved by the recap. No luck. It's audible through my test speakers as intermittent rumbles (for lack of a better word), though I don't hear anything when music is playing or between songs.

So still suspecting the bass pot area, I finally just replaced the four .068uF 250V film caps. No change. With the caps eliminated as the cause, I did what I should have done a while back: I kept the speakers connected and hooked my scope to one of the bass pot solder lugs while pushing on and slightly wiggling the shaft of the pot. Finally, I saw on the scope and heard through the speakers a cause and effect from touching the pot. I'll be keeping my eye out for a good used replacement, though I'm sure it'll be a while. Anyway, just wanted to update the thread and thank those who weighed in.
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Old 16th October 2021, 05:58 PM   #55
rayma is online now rayma  United States
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Try freeze spray to locate the component. It's most likely to be a transistor or carbon resistor.
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Old 16th October 2021, 08:36 PM   #56
dbxdx5 is offline dbxdx5
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It's the pot.
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Old 16th October 2021, 08:49 PM   #57
OldDIY is offline OldDIY
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This can be mechanical wear, oxidation of contacts, violation of the soldering of the leads. Sometimes cleaning spray and lubrication of the potentiometer track helps. For diagnosis, you can use a knock-knock.
This can be a time consuming procedure involving dismantling, disassembling, revising and replacing the pot.

Last edited by OldDIY; 16th October 2021 at 08:54 PM.
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Old 16th October 2021, 09:10 PM   #58
dbxdx5 is offline dbxdx5
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I did try cleaning it a few times with Faderlube F5. I'm tempted to try flushing it with CRC QD, but I think the issue is mechanical wear. For example, I can feel that there's noticeably more slop in the tolerance between the inner shaft and the bushing, compared to the treble pot. A replacement is in order, I suspect.
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