Hartke HA-3500 static noise. Traced to output board thus far.

[Helmholtz]

SVR 301 adjusts the bias of the input transistor and thus the symmetry/balance of the amp..

SVR 302 sets the C-E voltage of Q308 which determines output bias.

Measure and compare the C-E voltage of Q308.

[Indyryder02]

SVR 301 adjusts the bias of the input transistor and thus the symmetry/balance of the amp..

SVR 302 sets the C-E voltage of Q308 which determines output bias.

Measure and compare the C-E voltage of Q308.

So I had to replace the SVR 302 on the “good” amp as it fell apart when I tried to adjust it.. should have left it alone in hindsight.. it’s running now after replacement but I can’t seem to adjust its bias either now. When the pot broke it it started to overheat pretty quickly so I shut it down then. May have strained and caused something else to fail…

so between c-e on q308 of both amps I have 0v.

im
surprised they’re both running and passing sound.
very strange. Neither is overheating.
they’re essentially both working.. and yet I can’t seem to adjust or measure the bias at all. Really strange.

[Helmholtz]

So both are running in class B now and should have crossover distortion.

Connect a dummy load, feed a signal and look for a kink around zero-crossing.

[Indyryder02]

So both are running in class B now and should have crossover distortion.

Connect a dummy load, feed a signal and look for a kink around zero-crossing.

Don’t see any at peak on either if I’’m understanding you correctly

[Helmholtz]

Is this scoped with a dummy load?

Check with a small output signal.

Crossover distortion shows most at low output.

[Indyryder02]

Is this scoped with a dummy load?

Check with a small output signal.

Crossover distortion shows most at low output.

It is with a dummy load, yes.

As to a small output signal. Not sure I understand. I’ve always had to take things to peaking to see any crossover distortion .

I don’t see any on the scope at any level. Just a nice clean sine wave up and down.

it’s all very strange… it’s like it has a fail safe mode!

I’ve double checked where I’m probing for the test points.
Just doesn’t add up. They shouldn’t even be working.

[Helmholtz]

As to a small output signal. Not sure I understand. I’ve always had to take things to peaking to see any crossover distortion .

Crossover distortion is a low level effect (below 1Vp) and does not increase with output.
So the larger the output the less it's visible.

[Indyryder02]

Crossover distortion is a low level effect (below 1Vp) and does not increase with output.
So the larger the output the less it's visible.

I’m only familiar with seeing crossover distortion as a notch or kink in the middle of a sine wave when it’s at peak level, clipping.
Like the attached pics.
would I be looking for something different than this then?

[Helmholtz]

Picture is hard to read but looks correct.
Doesn't say anything about peak level or clipping.
Middle of a sine means zero volt crossing.

The notch might be as small as 0.2V, so hard to see with a 50V signal.

After all the amp seems to be a good and forgiving design.

But what's the problem with repairing the simple bias circuit around Q308?
Also look for a ruptured track.

[Indyryder02]

Picture is hard to read but looks correct.
Doesn't say anything about peak level or clipping.
Middle of a sine means zero volt crossing.

The notch might be as small as 0.2V, so hard to see with a 50V signal.

After all the amp seems to be a good and forgiving design.

yes! Extremely forgiving!

But what's the problem with repairing the simple bias circuit around Q308?
Also look for a ruptured trace.

I have no clue what’s wrong with the bias circuits or what could even cause them to act this way. On either of them. The good amp was ok before the pot failed. Now I can’t read or adjust it either it seems. Plus there’s the fact I have more time in the amp than both of them could ever have been worth! Haha

[Helmholtz]

I have no clue what’s wrong with the bias circuits or what could even cause them to act this way.

Well without looking and testing you won't know.

The C-E voltage of Q308 must not be zero. Find out why it is.

[Indyryder02]

Well without looking and testing you won't know.

The C-E voltage of Q308 must not be zero. Find out why it is.

Yes. I know. This amp has really beat me up.. I think they only sell for like $200 U.S. this has just been me being stubborn and not wanting to give up. I haven’t found anything in the local bias circuit amiss yet in my initial testing with this new issue.
I’m still just really surprised they’re both working like this. I’d think they’d be horribly off. It’s definitely forgiving!
I know on the good amp. Before the pot failed.
it was reading around 5mv initially which seemed a bit low. I wanted to move it up to around 25mv as I read these are fine with that. It was extremely touchy and bounced to right over a volt. The pot disintegrated.. found corrosion in it.

then I had to shut it off as it was trying to overheat of course.
so I found a similar 1k pot to replace it. That calmed it down again. So it was no longer trying to runaway and overheat.

but then now I can’t measure anything between the test points just like on the bad amp. So the new pot stopped the hot bias but still isn’t right. And I’m baffled by how they’re both working fine in this condition. Definitely strange

[Helmholtz]

Did you remove SVR302 from the board and test it?

Could it be that the C-E voltage of Q308 is more like 0.2V?

How fresh is your meter battery?

[Indyryder02]

Did you remove SVR302 from the board and test it?

Could it be that the C-E voltage of Q308 is more like 0.2V?

How fresh is your meter battery?

Ok.. I went back to check the C-E voltages of Q308 again.
Not sure what happened. Maybe meter was in the wrong mode.. it’s a bench top Fluke so wasn’t the battery.

anyway…

C-E of Q308 is currently 1.93v on the bad amp and 2.33v on the good one.

[Helmholtz]

Now that looks better but likely somewhat low (by 0.5V or so).