Ampeg BA-115HPT issues... guidance needed

[Bishop]

I have an Ampeg BA115HPT that is having some issues. It basically has a buzzy distortion behind the notes, it is pretty prominent, but cuts off like a gate as the note trails off an gets quieter.

Now, a few times I have turned the amp on and got 'nothing' (some noise, but no bass) intermittently, until I turn it off an on again. One of these times was when I had the amp going into a load and was playing bass through it and it caused R48 (page 1 top left) to start smoking. I replaced that with resistor. Since then, I only got 'nothing' once more, but this time, while I was unplugging the headphones from the headphone jack.

That's the other thing, the sound from the headphones seems normal.... no buzzing.

The initial voltage readings I get the +50v supply is pretty high around 70-75V at R68 (page 2 middle right) the power resistor that drops the +50 (70 in this case) to +16, which also gets extremely hot (A LOT hotter than it's negative counterpart R66, which reads normal voltage), although the +16 supply at the other and of the resistor is normal. So I guess this seems to be the most likely culprit, but I don't want to start stabbing in the dark as to why it's high, seeing as this is an SLM amp I would like to solder as little as possible.

Also, R48 seems to be after the headphone jack, but what would have caused it to burn up? The diodes tested in circuit near that resistor tested normal, but I'm not actually sure what that little circuit there.

I will begin further testing soon, but I guess what I am asking here is if anyone has any input as I am a little green on the solid state stuff...? Also, if there happens to be any common problems with these amps that I am unaware of.


Bass Amplifier PCB Schematics (697SCH_0).pdf

[Bishop]

I should add that I tested the headphone jack by pushing on the contacts, with no effect. Also replaced the tube for the hell of it.

[Bishop]

OK, so I guess my previous posts were not very helpful, but I did some further testing and now the amp is not making sound at all and remains consistent.

I decided to try the line out and for a few minutes it was sounding normal through there as well just like the headphones (J4), but from pulling the XLR chord out (I did have phantom power on) and checking the headphones again, then going back to the XLR, the amp has now stopped making sound completely. The light is off, but I am getting power from the transformer.

Now, when I measure at one end of each of the power resistors R66 and R68 I get about 60vdc (wall voltage is pretty high), but the other end is only 1.84vdc. They both get equally very hot. So my 16 volt supply has gone out seemingly (hence the power light goes off I suppose)...

Should I assume something related the the headphone jack and the line out like I am? Could a bad U7 cause these problems? Is there a good way to test a NE5532 in circuit, or is replacing it the only way to find out?

[52 Bill]

Is there a good way to test a NE5532 in circuit, or is replacing it the only way to find out?

Temperature and voltage are the two simple tests. Is either one getting hot? If the low voltage supply is being pulled down, there has to be a short somewhere. Read the voltages on the output pins and see if there is any dc there, but my guess would be that the heat will be apparent.

[Bishop]

Thanks for the response.

I tested U7's outputs and pin 1 is at about 300mv DC and pin 7 is about 150mv DC. (This is at E-3 on page 3 on the schematic for reference).

I also checked U3, which is the other NE5532 in the amp (B-2 on page 1 of schematic) and it measures much lower one both outputs. 6mv on one and a little higher (I think it was 15mv) on the other.

I also noticed that U7 does get slightly hotter to the touch than U3, but I wouldn't say really hot. I have to keep my finger on it for a second to notice.

Does that seem abnormal? Also, would a bad U7 NOT effect the headphone and line out signals for sure?

[52 Bill]

Does that seem abnormal? Also, would a bad U7 NOT effect the headphone and line out signals for sure?

That may be abnormal, but if your low voltage supplies are not right it may screw up your readings. U7 only drives the headphone and line outs.

Pull it and see if the supply voltage comes back to normal. If not keep looking to find the cause of the power supply problem.

[Bishop]

That may be abnormal, but if your low voltage supplies are not right it may screw up your readings. U7 only drives the headphone and line outs.

Pull it and see if the supply voltage comes back to normal. If not keep looking to find the cause of the power supply problem.

I'll be damned if that wasn't it. Works like a charm now. I guess that op-amp does have the ability to effect the output of the amp. Thank you for your help.

[cdwooley]

I just got hosed on a Craigslist deal. This amp has the same gated fuzz sound and low output. I'm going to socket u7 and get a replacement chip. I hope it works. I've retained some electronic knowledge, but rusty on a lot. I just had to sign up and say thanks for giving me hope. I appreciate you posting.

[52 Bill]

I just got hosed on a Craigslist deal. This amp has the same gated fuzz sound and low output. I'm going to socket u7 and get a replacement chip. I hope it works. I've retained some electronic knowledge, but rusty on a lot. I just had to sign up and say thanks for giving me hope. I appreciate you posting.

Welcome to the place. Sorry to hear about your problem with the amp. Please understand that your amp may not have the same problem that the earlier poster had, and changing U7 may not fix your amp.

It would be better if you were to take a more systematic approach to finding and fixing your amp.

[cdwooley]

Welcome to the place. Sorry to hear about your problem with the amp. Please understand that your amp may not have the same problem that the earlier poster had, and changing U7 may not fix your amp.

It would be better if you were to take a more systematic approach to finding and fixing your amp.

Thanks for the reply. I understand I'm shooting in the dark, so I've downloaded the schematics. All I have is a multimeter, so I'll check what I can with it and see if I find any other bad parts. I'm comfortable with a soldering iron and I have the parts on hand, so that's the reason I was going to try replacing u7 first. I've tried the chopstick test and visually inspected everything. I also pulled the pcb, so I can clean all the pots and contacts good. Nothing is burnt or smoking and the amp powers up. It's just low output volume and gated fuzz trailing the notes when played, which sounds very similar to the OP problem. If replacing u7 doesn't fix the problem, I'll start a new thread to hopefully find the problem...I'm sure some guidance will be needed at that point.