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Hartke A35 Repair problems

Started by Jesse, March 07, 2024, 10:38:03 AM

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Jesse

Greetings, I have been attempting to repair a Hartke A35 amplifier.
I am using a Variac, and an 8ohm Dummy Load during testing.
When I received the amp, Q8 and R51 were clearly damaged. I have replaced Q7, Q8, Q9, R50, R51, D13 & D14.
After replacing these parts I slowly powered it up again, this time causing Q4 to fail (a short from Emitter to Collector), and R97 is starting to burn up.
I then replaced Q2, Q3, Q4, Q5, Q6, as well as C23, C61, & C64, (R42 measures ok).
Something is still drawing too much current through R97 and causing it to overheat. (R98 seems ok)
With R97 lifted I have + 33 & - 33 volts on the V+ & V-
Does anyone have any ideas of what else to check, or what could be wrong?
Thank you for any assistance!

     -Jesse

g1

schematic attached.
Disconnect your load.  Check for DC voltage at the output.  Do not connect a load until you get the amp to the point where there is no longer any DC at the output.

Jesse


Jesse

At this point I have tested and/or replaced most components in the output section. With the input shorted, I am seeing about -1v DC at the output when the VARIAC is at about 45VAC.Any thoughts on where to look next for issues?

g1

If you do not have a load connected, continue increasing voltage from variac and monitoring DC at output.
It would help if you could monitor AC current from the variac as well, does the variac have a current meter?  Do you have a dim bulb tester?

Jesse

#5
I am measuring current as well, so far it stays very low. I'll try upping the AC voltage today. I'm just worried about R97 burning up, or Q4 shorting again. What do you think I should look for next?
Also, what I am experiencing is almost identical to this post: https://www.ssguitar.com/index.php?topic=4235.msg42173#msg42173
Thanks again for the help :)

g1

The other thread ended with Q8 catching on fire and no resolution.  It's better if you just tell us what is going on with your amp.

You can monitor the voltage across R97 (one probe at each end).  If you see more than 5VDC, stop.  As you move the variac voltages may jump around so give it a couple seconds to stabilize.
Also keep checking for DC at amp output.
If you are seeing low current on the variac meter, you should be ok to raise the voltage.

saturated

I was gonna suggest perhaps C61 is shorted to ground but it says you already replaced it.

Maybe start probing around and try to find where that portion of the circuit has continuity to earth.

 :P

Anyway I'm out I shouldn't be "contributing" to these fix it threads  :P
I ask stupid questions
and make stupid mistakes

criticism, critique, derision, flaming, verbal abuse welcome

Jesse

Thanks g1 and saturated. When the VARIAC is at about 28-30VAC there is 5VDC across R97, and -1VDC at the output. Current draw is still negligible.

g1

#9
Can you verifiy that R97 is still 100 ohms?
Also measure voltage across R98 when you have the 5V across R97.
And also voltage across R41.

Jesse

R97 is 100 ohms
When voltage across R98 is 5VDC then R97 is 0VDC and the voltage across R41 is 7.4VDC

g1

I think you got those R97 and R98 voltages reversed, but voltage across R41 seems normal, and 0V across R98 would be good.
So there seems to be about 50mA current through Q4, which is too much.
Is it possible Q9 is no longer insulated from the heatsink?

Jesse

ah!, you are correct, I swapped R97 and R98 while typing.
Q9 has fresh thermal paste and seems to be tight to the heatsink.

g1

Quote from: Jesse on May 07, 2024, 04:01:33 PMQ9 has fresh thermal paste and seems to be tight to the heatsink.
What about it's insulator?  Don't it and the output transistors that are mounted on the heatsink have mica insulators or similar?

Jesse

#14
The heatsink is a large aluminum fin style with Q7, Q8, and Q9 attached. When I received the amplifier for repair it had what looked like white lithium grease between the transistors and the heatsink and no mica insulators. I cleaned and replaced the grease with Arctic MX-6 Thermal Compound when I replaced the transistors. Could that be the problem? Does Q9 need to be isolated?
Small update: The version of the Q9 D600K that I used has a metal backplate that is connected to pin-2 the Collector (the original did not have an exposed backplate). I'm going to make an insulating wafer and will update soon.