Ampeg BA115HPT distortion help

[Bishop]

I have an Ampeg BA115HPT that has distortion on the output.

It does not distort from the headphone jack so I went from there and it seems to start very soon after, but my mind is playing tricks on me a little. What I do know at this point that strikes me as weird is that U7:A has signal on pin 1, but none on pin 2. There is also signal at U4:A pin 1 (located at E-3 on page 3 of the schematic). R104 and R85 measure correctly. So should I consider U7 as the prime suspect (it was changed a few years ago)? I thought U7 was solely for the headphone out, but I am not sure. Should there be signal there and could that be affecting the speaker output?

Thanks a lot!

[Jazz P Bass]

If signal is at U4 A Pin 1, then move on to U3 A Pin 1 & U3 B Pin 7.
If the signal is not distorted at either, then you have a power amp issue.

And yes, U7 is the Headphone driver.
Sometimes it is not possible to see a signal at the input of an opamp.
A 'Virtual Ground' thing.

[Bishop]

If signal is at U4 A Pin 1, then move on to U3 A Pin 1 & U3 B Pin 7.
If the signal is not distorted at either, then you have a power amp issue.

And yes, U7 is the Headphone driver.
Sometimes it is not possible to see a signal at the input of an opamp.
A 'Virtual Ground' thing.

Cool thanks for clarifying that about the OpAmp.

I did check the other outputs and now I am confused again because I am getting good signal at U4:B, distortion at U4:A, but then good signal again at the headphone jack. It seems like the headphone signal is fed from U4:A so I'm not sure how that is possible.

[g1]

That is confusing. If the signal is distorted at U4A, it should also be distorted at U7A and U7B.
But you said it sounds ok at the headphones, so move on.
TP5 appears to be where the signal is taken for the power amp. Try looking further along the line. How about U3A pin1 (TP12) of the power amp (like Jazz asked about above)?
Also, they have AC voltage (RMS) readings for some of the test points, use their signal conditions and check them.
Signal conditions are 170mV 1khz signal into 0db input jack. Volume and level at 10, tones at 5.

[Bishop]

That is confusing. If the signal is distorted at U4A, it should also be distorted at U7A and U7B.
But you said it sounds ok at the headphones, so move on.
TP5 appears to be where the signal is taken for the power amp. Try looking further along the line. How about U3A pin1 (TP12) of the power amp?
Also, they have AC voltage (RMS) readings for some of the test points, use their signal conditions and check them.
Signal conditions are 170mV 1khz signal into 0db input jack. Volume and level at 10, tones at 5.

U3 was distorted as well. I even double checked everything and it was still the case, bad signal at U4:A, and all the spots I checked past it, EXCEPT the stuff towards U7. I've been using clip leads into a DI while playing bass through it to test this. I did have it on a O-scope with a 1khz signal previously and the output actually 'looked' good, so I went with the audio scope method, didn't measure AC voltages though.

[J M Fahey]

The audio tracer is the poor manīs scope, but a real one is much better.
If you hear distortion you must see it too.
And it allows you to differentiate it.
For example, a strong oscillation above the audio range will not be directly heard but will dirty signal a lot.
So try to get distortion and then clip your scope leads to the same place and look.
No way a distorted signal looks "normal".

[Bishop]

The audio tracer is the poor manīs scope, but a real one is much better.
If you hear distortion you must see it too.
And it allows you to differentiate it.
For example, a strong oscillation above the audio range will not be directly heard but will dirty signal a lot.
So try to get distortion and then clip your scope leads to the same place and look.
No way a distorted signal looks "normal".

Thanks! I was just about to do this, but unfortunately I had this on my bench for a few hours, not even playing at the time, and it blew a fuse outta nowhere. Gonna hook it up to the light bulb limiter and see where I'm at, but I had to step away for a few days.

As I remember now, when I had it on the scope the waveform looked smooth but another waveform of seemingly opposite phase would be flickering in and out. I just wasn't sure if it was just a bad clip connection, or what.

[J M Fahey]

another waveform of seemingly opposite phase would be flickering in and out.

That by itself is not an amplifier fault but usually poor sync triggering at the scope itself.
No big deal.

[Bishop]

OK, I replaced the fuse and hooked it up to a bulb limiter. I first had it running with no load and it was all good. I hooked up a 1kHz signal and a scope and it looked really smooth at the output.

Then I hooked it up to a load, as I turn the signal all the way up the bulb starts glowing and I can see the signal distort. It actually was cycling from normal glow to strong glow as I watch the scope go from smooth signal to distorted signal and back at a constant rate. I measured the 50V and 16V rails and they were low and also changing at the same rate as the bulb was cycling. As I turn the signal down the voltage rails return to normal.

After poking around a bit, the up and down thing stops and remains constant, but the bulb glows and voltages are still low when there is full signal (around 28V and 10V).

So with a load and a strong signal something is pulling too much current... Does this add up?

[J M Fahey]

OK, I replaced the fuse and hooked it up to a bulb limiter. I first had it running with no load and it was all good. I hooked up a 1kHz signal and a scope and it looked really smooth at the output.

Then I hooked it up to a load, as I turn the signal all the way up the bulb starts glowing and I can see the signal distort. It actually was cycling from normal glow to strong glow as I watch the scope go from smooth signal to distorted signal and back at a constant rate. I measured the 50V and 16V rails and they were low and also changing at the same rate as the bulb was cycling. As I turn the signal down the voltage rails return to normal.

After poking around a bit, the up and down thing stops and remains constant, but the bulb glows and voltages are still low when there is full signal (around 28V and 10V).

So with a load and a strong signal something is pulling too much current... Does this add up?

Well, as described, that is normal.

When you raise output without a load, drop through bulb limiter is small, because SS amps idle consumption is low (no filaments or high idle current power tubes present) and you are creating output voltage but no current (no load).

Is this a useless test then?

Not at all, it shows us a ton of things, such as the signal going all the way from input to speaker jack, tone and volume controls, whether we have hum/hiss/oscillation , that we have both halves of the signal, that voltages are present, output transistors not shorted, etc. ... and all without blowing fuses.
Could write a whole page

Now *after* that , we apply a real load and drive the amp to a couple Watts output, we check for stability, biasing, etc.
Of course NOW we have a load and current increases dramatically.

Where does such power/current come from?
You guessed it: from the wall outlet ... through the bulb limiter ... which will shine brighter, drop more voltage, etc.

If amp seems to work fine now, you can plug it straight into the wall and recheck.

99% of the cases, itīs cured now

[Bishop]

Well, as described, that is normal.

When you raise output without a load, drop through bulb limiter is small, because SS amps idle consumption is low (no filaments or high idle current power tubes present) and you are creating output voltage but no current (no load).

Is this a useless test then?

Not at all, it shows us a ton of things, such as the signal going all the way from input to speaker jack, tone and volume controls, whether we have hum/hiss/oscillation , that we have both halves of the signal, that voltages are present, output transistors not shorted, etc. ... and all without blowing fuses.
Could write a whole page

Now *after* that , we apply a real load and drive the amp to a couple Watts output, we check for stability, biasing, etc.
Of course NOW we have a load and current increases dramatically.

Where does such power/current come from?
You guessed it: from the wall outlet ... through the bulb limiter ... which will shine brighter, drop more voltage, etc.

If amp seems to work fine now, you can plug it straight into the wall and recheck.

99% of the cases, itīs cured now

I'll be damned, I plugged it into the speaker and viola, clean sound again. I forgot to mention that after the fuse blew I reflowed a few solder joints and that must have been it. I would have still been racking my brain if it weren't for your help... Many thanks!