Mark III series Bass Combo low output

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Fictitiousfreedom
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Mark III series Bass Combo low output

Post by Fictitiousfreedom » Wed May 11, 2022 6:29 pm

I couldnt find the answer anywhere so sorry if its been discussed.
I have an old Mark III combo with a 15” speaker in it. The entire time I've had it it has a had a problem. After playing it for around 15 mins the output drops and the input and output gain need cranked really high to get a usable level. I usually stop playing it because i assume something is overheating. I took it apart today to see if there is an obvious component issue but everything looks fine. I have some experience with repairing gear but not the most. I make pedals and some pro audio stuff, ive even made a small SS guitar amp. I figured i would ask and see if there is a common problem like this or if one of you guys with experience could help me out. Thanks!!!!

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Enzo
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Re: Mark III series Bass Combo low output

Post by Enzo » Thu May 12, 2022 7:08 pm

Amps with unstable output have been around as long as amps have been around.

If it were overheating, the rear panel would be very hot.

Gota photo of front and rear panels?

First suspects are any FX loop jacks, any insert jacks, and any preamp out and power amp in jacks. When the output drops, poke a plug into and out of each of those jacks a couple strokes. Any of them wake it up? Take your guitar or other test signal and plug into the power amp IN jack or the FX return. Does that come through? Or is that also still diminished?

Fictitiousfreedom
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Re: Mark III series Bass Combo low output

Post by Fictitiousfreedom » Thu Jun 16, 2022 9:07 pm

sorry it Took me so long to respond. I really appreciate your help. Below are some pictures of the panels. I obviously could only check the two input Jacks and the pre-amp out Jack because i believe the other two Jacks are for external speakers and it looks like they are parallel to the internal speaker wiring. Ive never found the exact manual though so i may be wrong. It no longer drops the output, it is now always too low.

I doubt this is related because it has been this way since i received the amp but i believe the limiter is non functioning. I cant hear any compression whatsoever and the light has never lit. I did recently replace the LED just to be sure and nothing has changed. I also cleaned all of the pots and connections.

If it would be helpful ill open the amp up again and snap some pictures. I saw nothing that looked problematic but that obviously isn’t a very good way to test components lol i have been in the market for an oscilloscope, once i get one maybe that can help but learning how to use it and what to look for will require a fair amount of time. Thanks again for your help its much appreciated. I paid next to nothing for this amp and i don’t even use it much. It is more important and enjoyable for me to learn how to fix it honestly, that is my real goal with this.
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Enzo
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Re: Mark III series Bass Combo low output

Post by Enzo » Fri Jun 17, 2022 3:30 pm

Don't worry about the compression for now. It is really a clipping eliminator, it won't generally be an audible effect.

On the power amp board is one 8-leg IC number 87478. Note which way it is in the socket, and remove it. Now doe the amp sound strong?

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