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

[Helmholtz]

Bad amp:

As said, you can't have different voltages at the 2 bases (i.e if connections are good, no broken trace etc.).
Seems pins are still wrong.

It's always a good idea to directly measure base to emitter voltage.
An npn transistor can only work if the base is around 0.6V more positive than the emitter.

[Indyryder02]

Bad amp:

As said, you can't have different voltages at the 2 bases (i.e if connections are good, no broken trace etc.).
Seems pins are still wrong.

It's always a good idea to directly measure base to emitter voltage.
An npn transistor can only work if the base is around 0.6V more positive than the emitter.

Ugh. Yes I had the C and B notations backwards. I’ve corrected it in the post. Bases are all the same

and good tip on measuring base to emitter.

[The Dude]

When measuring/posting transistor voltages, it's important to measure at least to the first decimal place. a half a volt here or there makes a big difference and tells us if the transistor should be turned on/conducting or not. Transistor bias is only about .6V. This is quite unlike most tube circuits where a volt or two makes little difference.

[Helmholtz]

When measuring/posting transistor voltages, it's important to measure at least to the first decimal place. a half a volt here or there makes a big difference and tells us if the transistor should be turned on/conducting or not. Transistor bias is only about .6V. This is quite unlike most tube circuits where a volt or two makes little difference.

True, but mains voltage fluctuation between successive measurements may fool you when the difference is is only a few percents.
Always best to directly measure base-to-emitter voltage.

[The Dude]

I don't disagree. I was just pointing out the difference between transistor bias and tube bias. The point is that a half a volt in a transistor circuit can be the difference between the circuit working or not. In a tube circuit, it will make little difference at all.

[g1]

The Q302 collector voltage is probably different due to the diff. transistor (Q067 that is not labelled on schem.) trying to correct for the DC offset at the output.
Check and compare TP301 and TP302 for the two amps.

[Indyryder02]

I don't disagree. I was just pointing out the difference between transistor bias and tube bias. The point is that a half a volt in a transistor circuit can be the difference between the circuit working or not. In a tube circuit, it will make little difference at all.

Makes sense.

[Indyryder02]

True, but mains voltage fluctuation between successive measurements may fool you when the difference is is only a few percents.
Always best to directly measure base-to-emitter voltage.

Thanks. Makes sense

[Indyryder02]

Ok.. some new readings.

the bad amp shows
Q302
E 49.3.
C -2.15
B -48.8
B to E .583

Q303
E -49.4.
C -48.7
B -48.7
B to E .618

TP301 -.71
TP302 -.70

output transistors q313 etc all have B-E of around -.34



good amp as follows

Q302
E -47.8
C -.78
B -47.1
B-E .589

Q303
E -47.6
B -47.1
C -47.1
B-E .621

TP301 1-2mv
TP301. 1-2mv.

B-E on outputs around .530

hope I did that right. Easy to get confused posting all the various numbers etc.

[g1]

Do you mean TP301 and 302 in bad amp are 0.7V? That would be 700mV compared to 1 or 2 mV in good amp.
If you are measuring TP's to ground, try measuring across the resistors instead (R324, R325)

[Helmholtz]

I think TP301/302 are provided to adjust the bias for a certain voltage BETWEEN these points via SVR302.
For reference see good amp.
Output transistors having a B-E voltage lower than 0.5V are underbiased and won't conduct properly.

Apart from that it might make sense to check/replace all the small ecaps in the power amp.

[Indyryder02]

So.. the dc on the output wound up being the Q301 pair.. painfully as that was one of the first things I suspected from doing hifi receivers with this issue. Usually I can get them to make the noise go away when I freeze spray them… I finally tried again while monitoring, and these ones were causing the dc to go up. So I replaced the pair. No more dc on the output.
I tried to match the bias to the good amp.. for some reason I’m not getting anything across the test points like I am in the good amp. Yet the amp is working. The bias control seems to do nothing on the bad (now sort of good ) amp. Everything is working fine but I can’t seem to read the bias (what could cause this?)… the wave looks good with no crossover. Idk. I could definitely see and adjust it on the good amp.
it’s not overheating or running away… so I may leave it be. I’m beyond over this amp!

[The Dude]

Bias not adjusting is a clue that something is still not right with the amp. I would certainly take the time to figure out why it's not adjusting or you may see the amp again and have to start all over from scratch. Is the bias pot itself good?

[Indyryder02]

Bias not adjusting is a clue that something is still not right with the amp. I would certainly take the time to figure out why it's not adjusting or you may see the amp again and have to start all over from scratch. Is the bias pot itself good?

Ugh. Idk. I’m just so sick of this amp. It’s been so far past a repair job and just more of an unwillingness to give up. And every time I work on it. I just lose more time. The DC is gone from the output. It’s working..: has a nice clean sine wave. And it’s not overheating or running away. I have no clue why I can’t see the bias at the test points like I should be able to. How is it even working? Ugh. Really sick of solid state! I fixed bunches of em. But man when I lose I lose big on them.
id have to assume as well that maybe the pot is bad.
really doesn’t make any sense. I checked my leads etc. I just couldn’t believe I couldn’t read anything. The other one reads and is super sensitive.. so much that trying to adjust it to a nominal bias of around 20mv or so. It jumped past and quickly got too hot. And that bias pot actually did fail! It fell apart. Not much to them. Now it’s far too much and I can’t adjust it lower. What a mess! It appears there was glue under the pot which impeded it I think and caused its failure when trying to adjust it.
I’ll check the bias pot on the other one and see if it also failed.
I’d still think i would see something at the test points. Idk.
should have tossed it in the bin long ago! Haha

[Indyryder02]

The pot tests ok. I really can’t figure out. How after changing the pair of transistors for q301 that the dc on the output is gone. I can adjust it to 0mv just about with the first trim pot.
it passes sound. A nice clean sine wave.
yes I can’t see the bias or adjust it.. the associated resistors and diodes near the test points are all ok.
the base - emitter voltages of the outputs are around .375v
so that doesn’t seem right. This amp is el Diablo haha