DC Speaker pops Fender/Sunn Bassman 1200

[nradina]

Hello everyone!

First time posting.

I have a Fender Bassman 1200 on my bench that is exhibiting two issues:

1- When turned up "loud", I'm getting transient DC looking "POPS" from the speaker on loud and low notes
2- Customer also complaining that the overall output is reduced.

I've isolated it to the power amp section AND/OR power supply section.

All test points seem to jive except I'm getting a very low VAC at the output. (TP25)

Thinking it may be:

1- Leaky filter caps?
2- bad coupling cap?

I've attached a schematic.
I'm new at this and I'm always trying to learn faster and more accurate trouble shooting.

Thanks in advance for any insight or help!

[Enzo]

leaky filter caps would cause hum.

I assume you mean TP52? What low voltage do you see there, with what signal put into the amp?

How did you eliminate the preamp from the suspect list?

This power amp uses two rails, 94v and 47v. Are both rails up to voltage and clean on both polarities?

Drive the power amp from the FX return, use a sine wave, scope the output. Where does the output waveform clip and when? Now into a resistive load. Can it still push full out? What AC voltage across the load at onset of clipping?

Apply a steady tone, listen to it, ball up your fist and whack the top of the amp. Did the output waver or interrupt? If you are a sissy about whacking, use a rubber mallet.

[nradina]

Thank you Enzo!

I'm writing this before I get back to the shop, I'll do my best with these questions for now.

I assume you mean TP52? What low voltage do you see there, with what signal put into the amp?
Yes, you are correct! Test conditions were 25.8mv, 420hz, all controls at 5.5

How did you eliminate the preamp from the suspect list?
I used the "Power Amp Pass Through", output. Which looks to tap the signal post-master volume, before the power amp stage. (I've attached another page of the schematic with this output (far right)

This power amp uses two rails, 94v and 47v. Are both rails up to voltage and clean on both polarities?
Both rails read a bit high. I believe the 94v rail read 101v, yet I can't remember the 47v rail exactly, but it was over 47v. I honestly did not check both polarities. Yet I will when I get in.

Drive the power amp from the FX return, use a sine wave, scope the output. Where does the output waveform clip and when? Tried this and I couldn't get a strong signal at all. Very low volume, as if the IC on the return was not functioning

Now into a resistive load. Can it still push full out? What AC voltage across the load at onset of clipping?
I will check this also.

Apply a steady tone, listen to it, ball up your fist and whack the top of the amp. Did the output waver or interrupt?
I'll give this a try also!

Thanks again my friend!

nicholas

[Enzo]

I am not concerned the 94 might be 101, what I don;t want to see is +94 and -75 or missing -94 but both 47s present. If the highs are roughly twice the lows and both sides more or less equal, we're OK. So 94 and -94 is great, and 101 and -101 is great. 84 and -105 maybe not so much.

The FX return feeds directly into U11B, displacing the preamp signal there. That feeds the power amp but also feeds the passthrough driver U12A. SO if in doubt, connect a test signal to the FX return jack and the passthrough jack to som other amp. Does the test signal now come out the passthrough OK?


The fact you can;t drive the output from the FX return jack should be a large clue.

[nradina]

Thanks again for your help.

I worked on the unit yesterday.
I was able to put a tone into the return and get a proper output (the day before, it seemed I had a issue with my tone generator).

I will try the return to passthrough jack.
What I did notice on the scope is the DC spike seems to appear @ PIN 1 of U14 but not @ PIN7 of U14.

Still trying...

[Enzo]

If you can feed the return and get good output, then the passthrough test is no longer needed. It was to test that FX return function.

So then, driving the power amp from the return jack, you can or cannot get full output level withput load and full power into a load?