Ampeg B3 (3158) problem

[spy]

I've replaced IC3 and IC1 but the problem is still there!

[spy]

I'll go crazy with this amp! I've replaced almost every part on the preamp section, film caps, diodes, zeners,, ICs, el. caps and the annoying crackles are still there! I don't know what else to do. I don't think the problem is at the output as it doesn't depend on the master volume. It's the same if the volume is closed or not and of course it gets louder if you turn on the volume but not different.

I don't know why it's visible at the oscilloscope only after R37 but I discovered that it's also audible if I use an external preamp at the Return input of the effects loop! It's not audible in low settings only when the input is high when the signal is starting to distort.

[Enzo]

It may not be a bad part, it could be a bad connection. DOn;t focus exclusively on parts to replace.

If coming in through a return from an external preamp still has the symptom, that pretty much says your preamp is not involved.

[spy]

Thank you Enzo for the answer! I'll try to check every connection. I'm pretty sure that it's somewhere in the send/return circuit. The good thing is that the amp will be brand new when I'll find the problem as all the parts now are new plus el. caps in the PSU!

By the way, I need a source for the EQ slide pots, if it's a European source it'll be better.

[spy]

I checked and refreshed a lot of connections without any luck, especially around the effects loop, the problem still exists. But... when I take off R7 = 1M, the input resistor at IC1 at the Return Input the crackles disappear! I hear only smooth distortion. I replaced it with a new one but nothing changed when I take it off the circuit, no crackles. Any idea?

[Enzo]

The amp works well without it? Leave it out then.

[g1]

Did you replace D8 & D9? If they are noisy I'm wondering if removing R7 is masking a problem with them.

[spy]

The amp works well without it? Leave it out then.

It was my first thought but I'm curious about the problem. Why the amp behaves well without it and not with it?

Did you replace D8 & D9? If they are noisy I'm wondering if removing R7 is masking a problem with them.

No, they measure just fine but I'll give it a try.

[spy]

Enzo, do you have any idea why it works well without it?

Today I'll replace D8 & D9 to check it out.

[Steve Conner]

R7 is needed to provide a path for IC1's bias current. The amp isn't guaranteed to work at all without it.

Did you clean the contacts on the FX loop jacks? Also, you could try replacing IC1 and C10 as well as the diodes. A likely scenario is that someone plugged the speaker output of another amp into the FX return, and fried some parts here.

Does the clipping indicator work? (all the diodes spread around the circuit that connect to "P") Tried disconnecting it, or replacing C54?

[spy]

Yes I have cleaned the FX loops and I have replaced IC1, C10 and C54. The clipping indicator works fine, also I have replaced some diodes around this circuit. I haven't replaced D8 & D9, yet. Hope it's there.

[spy]

Ok, I replaced D8 & D9 and the problem still exists! Again, without R7 sounds fine with R7 it crackles. Any other idea?

[spy]

Steve, what if I live the amp without R7?

[MarkusBass]

Spy, you mentioned the preamp first but now you are refering to the power amp, right? Both the preamp and the power amp have IC1 and R7. It looks like you are checking IC1 from the power amp. IC1 in the power amp is a simple voltage follower - on the output there should be the same voltage as on the input. You can simply test it by plugging a signal generator into the RETURN jack and checking the signal on the output (TP2). The R7 resistor shouldn't be removed. The diodes protect the input - the can be temporarily removed. Maybe the integrated circuit is oscillating. And it makes sense to find out the problem. The circuit that you need to check consists of just few parts: R6, C10, R7, IC1, +/- power supplies and possibly two ceramic capacitors on the supply rails. You can even assemble such a circuit outside the amp and test how it works. But this is just a voltage follower: V(out) = V(in). If it does not work in this way something is wrong with one of the components. But I wouldn't remove R7. I rather think that maybe the IC is oscillating. Check decoupling capacitors.

Mark

[spy]

I have replaced all of these parts, R6, C10, R7, IC1, both diodes with no luck. Good idea about the decoupling caps but there are only 4, two are close to the big electrolytics and the other two close to the output transistors, I replaced them just in case with no luck. This thing is driving me crazy!