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

[Indyryder02]

Hello. So far I’ve been able to trace this static noise to the collectors of Q304 & Q305.. and some of the adjoined components. What can I do to further isolate it.. I’m suspect of those transistors.. I don’t see any noise before them.

I’m still trying to better understand about how and what can be grounded or shorted across to further narrow things down.

This amp isn’t worth a whole lot, this is really to better my understanding of ways to troubleshoot between individual components once traced to a small area of parts. I have a better idea of how to with tube amps.

I know Enzo and others are very good at this type of troubleshooting. Just trying to learn all I can.

Thanks in advance!

[Jon Snell]

Whatever happens on Q304 will inversly happen on Q305 ... don't get confused. To check if it is from before, use a double beam scope and see if the collector signals are inverted.
I would lift one leg of C302 to isolate the input side of the main amplifier.
Both Q304 & 305 faulty ... not very likely in my limited 50 years of experience.

[Indyryder02]

From what I can tell with my subpar scope I currently have… the collectors do not appear to be inverted..
And lifting a leg of c302 would take a lot of work on this amp to get that board out etc… is there any other way?

[Jon Snell]

There is a three pin signal connector between the preamp board and the main amplifier input, locate it and unplug it. (arrow).
If still noisy, I would take a close look at Q301, the unmarked dual transistor on the schematic. Q301 is only two transistors in the same can that are thermally connected to each other.
They don't usually go bad.

[Indyryder02]

I should have mentioned that I did unplug that previously but it makes a huge amount of static then that overcomes the original noise.. so I can’t tell if it’s taking it away or not..
it looks like two of the three pins are to ground. I thought maybe losing that was causing it. I tried connecting the two pins to ground but no change.

[Jon Snell]

What happens if you connect all three pins together.
Does the noise stop?

Edit, more added to question for clarification.

[Indyryder02]

Ok. That worked. That killed the overlaying noise. I’m still trying to learn where and when I can short things across or to ground etc to isolate the circuit without causing damage… can this always be done with the audio signal? Anyways…

So the noise is definitely after that connection.
that confirms my earlier thoughts.. and it exists already at Q304 and 305. So has to be between those points.
And the dual transistor is this Q301 and the Q067 next to it I presume. (Picture attached)
I can’t detect the noise on either of them with my audio probe or scope like I can on Q304 and 305. Unless it’s too small?
usually I can find these static noises easily with freeze spray.. but no luck on this one.. unless maybe it’s not a noisy transistor. Which is just usually what I look for with a static noise like this… unless it’s old carbon comp plate resistors etc or bad tubes…. Which we can rule out on this one haha.

[Jon Snell]

"Ok. That worked. That killed the overlaying noise."

Does that mean it stopped the noise? If it does the fault is BEFORE that point.
If it does, there is not much left to check.
VR6, maser volume I presume. Turn it down does the noise stop?
If it doesn't check the part between the vol control and the connector. IC202 will be in the middle!

[Indyryder02]

No, not the noise I’m tracing.. I apologize for not being more clear. Remember when I pulled the connector.. it made an overwhelming amount of noise.. that was much louder than the noise I’m trying to trace.. so I couldn’t tell if it was before or after that point for sure. Connecting the three pins together with it unplugged took away the overwhelming noise of being unplugged and allowed me to see the original static I’m tracing for still at the output of the amp.
so thus it is indeed on the output board and before Q304 and 305.

sorry for the confusion.. too many noises! Haha

[Indyryder02]

Is there a way I can trace any further? Would you also suspect q304 or 305?

[g1]

R318 feeds back any noise from the output to the base of 'Q no-name dual transistor package' (I guess it's Q301 ?).
Put a 0.1 or 0.47uF cap from that base to ground to see if the noise is coming in through R318.

[Indyryder02]

R318 feeds back any noise from the output to the base of 'Q no-name dual transistor package' (I guess it's Q301 ?).
Put a 0.1 or 0.47uF cap from that base to ground to see if the noise is coming in through R318.

That’s what I’m guessing as well. Looks like q301 and next to it is q067.. strange. Two separate 2sc3200 tried the cap to the bases… no change on 301 , And on 067, it increased the noise.

[Indyryder02]

Anymore thoughts on this one? Seems I must be close

[g1]

Need to clarify which transistor is which. Which one connects to R318 ?

[Jon Snell]

Ok. That worked. That killed the overlaying noise. I’m still trying to learn where and when I can short things across or to ground etc to isolate the circuit without causing damage… can this always be done with the audio signal? Anyways…

So the noise is definitely after that connection.
that confirms my earlier thoughts.. and it exists already at Q304 and 305. So has to be between those points.
And the dual transistor is this Q301 and the Q067 next to it I presume. (Picture attached)
I can’t detect the noise on either of them with my audio probe or scope like I can on Q304 and 305. Unless it’s too small?
usually I can find these static noises easily with freeze spray.. but no luck on this one.. unless maybe it’s not a noisy transistor. Which is just usually what I look for with a static noise like this… unless it’s old carbon comp plate resistors etc or bad tubes…. Which we can rule out on this one haha.

DO NOT connect a capacitor between R318 and ground. It will take off and become unstable. I have seen them destroy the power transistors. The noise, will pass through R318 and Q301 will cancel it, if Q301 is working correctly.

Shorting all three pins together gives you no noise? Please confirm that shorting the pins stops the problem you have or just stops the static noise.

If it does;
That means the noise is coming in BEFORE that point, nothing tp do with the power amplifier at all !!!!