Please help! Behringer KX1200 master volume stops past 4 and no lines work just fx return

[Padredelaverde]

Mostly newb here. Looking to fix this amp. Don't know what more to add besides the title. The fan isn't hooked up but doesn't seem to effect playing bass at low levels. Any help is appreciated.

[Jazz P Bass]

The FX return should go directly to the power amplifier.
So you have a dead preamp.
I would suggest that you check the preamp supply voltages.

[Padredelaverde]

I'm a little confused. I have my bass guitar plugged into the fx return and it plays. I don't have a preamp.

[g1]

Please be more specific about the master volume. If you use the regular input, everything works fine til you get to 4 on the master?

[Padredelaverde]

Line 3 is the input for bass guitar but none of the lines work besides fx return. I plug the bass into fx return and I can play up to the master volume level 4. After I turn the master volume knob past 4 I get a mild static sounds from the amp but no sound from the bass. Turn it back below 4 and sound comes back. Don't know if the fan not being connected is an issue but doesn't effect play below 4. Please Lemmy know if I should clarify anything further.

[g1]

It's possible the master pot is bad as well as the other problem.
Was the amp working good for you previously or is it something that you received in this condition?
Do you have basic electronics tools and experience?

[Padredelaverde]

I received in this condition. Yes I have basic electronic tools and experience. I have a bit of trouble reading schematics and applying that to electronics, but if monkey see, monkey can do. If someone says, turn your voltmeter to this, put here and here on master pot, and that will tell you if it's bad or not. Like the fan reconnect looks like (I could be wrong) 2px1 but I don't know which terminal is the positive and which is negative, but if some shows me where to solder I can do it.

[Delta362]

To help clarify the JPB comment, by having it work when connecting to the FX return, that means the power amp section of the Behringer is working correctly. The preamp section of the Behringer is where the problem lies. He does not mean a separate preamp unit.

[DrGonz78]

The FX return should go directly to the power amplifier.
So you have a dead preamp.
I would suggest that you check the preamp supply voltages.

Yup I had one of these units in for repair back in 2016 and it was one small shorted cap on the preamp board. So yes, verify preamp voltages and see if there is a problem. Perhaps those small electros on these amps had some cheapo caps that short out.

To help clarify the JPB comment, by having it work when connecting to the FX return, that means the power amp section of the Behringer is working correctly. The preamp section of the Behringer is where the problem lies. He does not mean a separate preamp unit.

This could be the case but just realize that if there is a fault in the low voltage supplies it could come the power board or the preamp board. I remember finding a failed preamp voltage on the amp and then I think I disconnected the power supply board from the preamp board. At that point I saw my voltages restored fed from the power board to the preamp board. So connecting the preamp board caused the short. Then I just tested all the small voltage supply caps on the preamp board and found one that shorted.

[g1]

IC1 and IC2 are 7815 and 7915 voltage regulators. Verify that you have +16V at pin3 of IC1 and -16V at pin3 of IC2.

[Padredelaverde]

IC1 came in at +15V but IC2 came +16V. So should I replace IC1 or IC2 or both?

[g1]

Did you mean -16V at IC2? It should be negative not positive. If so, I think those voltages are close enough. Now you need to find out if those voltages are getting to the IC's.
Check that IC6 and IC10 have +15V at pin4 and -16V at pin11.

[Padredelaverde]

I did not mean -16V at IC2. I do mean +16V. Is that where the problem is then? IC10 pin4 was+15V and pin11 was -15.6V. IC6 however had +15V at pin4 and +8v at pin11.

[Padredelaverde]

IC7 looked a little odd like there might have been a loose attachment. I touched it and this happened. So I think I found where at least one source of where the issue is. Does this mean I need to repair the pcb, seeing as how it's now missing the 2,3,4,and 5 input spots, in order to properly reattach IC7? The whole area looks questionable at best.

[Padredelaverde]

I really appreciate all the help. I hate revealing my massive wealth of ignorance on topics usually but I suppose it's the only way I'll learn. Can someone tell me where to solder the fan to? It looks like x12p but I could be wrong. I also don't know which side is positive and negative. I'm trying to upload an attachment but it appears I'll have to size it down since it will exceed my upload quota.