Hey Guys,

I am an amateur electronics hobbyist working on a repair to this bass amp. The problem when I initially received the amp was the output sounding from the speaker was very distorted. Shortly after playing on it to diagnose the sound, the output to the speaker completely cutout. My first thought was possible blown speaker. After physical inspection and checking the impedance the speaker aced the test. So then, I figured there surely is something wrong with the preamp section because I know that failing op-amps can cause the signal to distortion or regardless encounter more of less THD. To rule out if the preamp was failing, figuring if its not making it to the Speaker it surely won’t make it’s way to the output connectors. Well I ran both the TR/S and XLR direct outputs into my mixer and seen signal a clean as a whistle. Next I check the headphone amp, Heard the input 100% you would expect it. At this point I felt the preamp board may not have the issue that causing the speaker not to generate the output signal. On to the power amp board!

The power amp board indicated they’re have been previous repairs. I see some heat marks (mainly discolor to the board and more melted flux coating). There also is a copper strip that must have incinerated off the PCB which there has been a coated 28 gauge wire routed and installed in its place. After surface testing components, I rule out that 4 power diodes (D9, 10, 11, 12) and the 2 power MOSFET’s (T2’s controlling the +40V, and T3’s controlling the -40V) all have shorted out. I have replaced all of these components with like for like parts. After replacement, I now went to hook up my scope to see if I can see signal going to the terminal leads for the speaker before prior to hooking it all back up. It was right here when I messed up and cause a nasty short on a 1kΩ resistor(R16) and two SMD diodes(D14, 15). I replaced all of them.

Finally I turn the unit back on, and without any fireworks or dramatic displayed entrance it maintained normal operating behavior! (Eureka! minus the speaker because it is not hooked up) I test the preamp section and I hear everything perfect in the headphone. Now for the test with the speaker......(drumroll please) I got nothing. No noise whats so ever its like it was before I cracked it open. 

Here is what I am concerned about:

Obviously I didn’t successfully route out the issue, I am also scared I may have made more work for myself from the short I caused. I am confident that truly evaluated everything but I need some guidance and support. Im going crazy. Help.

P.S. I have a copy of the schematics in .pdf form here: Behringer BXL1800A Ultrabass Schematics

I repair this type of equipment full time. But, I have a list of brands I don't, Behringer is on that list. BTDT. The parts support is terrible. I'll stop there about the company that produced it. You would be better off to buy another brand of amp. But if you are wanting to learn, regardless of cost, you really need a scope, signal generator at minimum, I'm assuming you have a DVM. If they had that power amp pc board availble, that would be the most cost efficient way to finish the repair. They would sell that board for less than $30, if they had it. The PDF link above does not work. I get an XML error. I probably have the schematic at work anyway. Do you have a signal gen & scope ?

Yes I have an old scope and I use a signal generator app on my phone. The preamp section works fine but for some reason the power amp board is drawing way more current than it should. (2) 150ohm resistors and power ic's getting way to hot

The two 150 ohm resistors, R4, R5 are feeding the 7915, and 7815 regulators, maybe what you are calling power IC's. If the resistors are actually hot enough to smoke or burn, not good. Barely warm, maybe acceptable.  So I assume they are cooking. You have shorted regulators, or else something shorted on output of regulators. It's possible a 4580 op amp IC has shorted. If the output FET's were shorted and replaced, I would check R2, R3  47 ohm for value. If either is open or way off value, I'd also check MJE340 and MJE350 driver transistors, and the MPSA06 bias transistor that sits between them. If any of them check shorted or leaky, IC4 likely took damage also. If you can get rid of the problem with your 150 ohm resistors and you can measure plus and minus 15 volts output at the respective regulators, before you put signal into the amp, check the bias voltage measurement.  Meter leads over R1, should be 5 millivolts. Ajust the trimpot to lower it to 5mV within first few minutes of turning it on when it's cold. If you got the FETs swapped in backwards, 9140 in the 140's place & vice versa, that would also cause issues. The 9140 should have about 2 to 3.5 volts on it's gate when bias circuit is working OK. The 140 should have about -2 to -3.5 volts. As you adjust the bias trim pot, these voltages should change respectively. The 4 zener diodes near the FETs should be checked, since the FET's shorted. If you don't know how or aren't sure, you could just replace them. D1,D2,D3,D4. They limit the voltage swing of signal at the gates. They are probably OK. But this is a behringer, and you are a beginner :) 

When I measure the input of 7815 (Positive voltage Regulator) it's only getting 15.5VDC and the output is 14.2VDC. It's negative counter part 7915 is getting -24.6VDC on the input and outputting -15.3VDC. Deffinately a current draw which explains why the R4 and R5 are cooking and IC4580 is also heating up. I also hooked my DVM up to the speaker terminals and read -42.5VDC which told me that MOSFET T3(140) is being turned on when it should not be. Tracing my steps back I found out that T5's(350) emitter is also cooking. So im asusming that T5 is causing all my trouble. Im waiting for replacement parts. Please weigh in if you think I am on the right track or something I am missing. Thanks

Sounds like progress. IC4 could have been damaged in domino fashion. Output transistor fails, which takes out the driver, which takes out IC4. Be sure to check value of the resistors directly connected to any shorted transistor you replace.

Checked all the resistors, noticed that R7 is supposed to be 220 ohms but there was 200 ohm in its place. I dont think that would of caused a major problem. I replaced T5 (350) and to no avail I still have -42V going right through the speaker terminals. I am at such a loss. I dont know what other options I have