Hartke HA1200 Kickback Burnt

[AtomicMassUnit]

I've got a HA1200 Hartke bass amp, and it had some parts burn out (literally). There's a small transistor that attaches to the heat sink via a couple wires flown off the board. These wires are torched, and the mini pot on the board (bias adjust I'm guessing) is torched, along with a couple other components.

I've been researching this amp, and apparently they had some heat problems, and there are many threads here on other Hartke models talking about that transistor mounted to the heat sink, etc. Is there anything I should watch out for when replacing the bad parts? Any better way to keep it cool? I haven't begun testing it, but I'm figuring on replacing the outputs, drivers, bias, and pretty much everything there that might have taken some heat.

Here's the schematic I have.

[soundguruman]

The "transistor" connected with 2 wires from the heat sink is a thermistor. Not a transistor.
There are actually three of them:
1. The fan control thermistor, speeds up the fan when the heat sink gets hot. BUT it is located in the wrong place. It is mounted at the coolest part of the heat sink, where the fan is. Therefore it does not speed up until the entire heat sink is hot. This should be located to the opposite end of the heat sink, so that the fan remains fast, even when one end (the fan end) of the heat sink is cold.
This amp has a problem, the last transistor on the heat sink gets the hottest. This thermistor should be located where the hottest transistor is mounted. Then the fan keeps going fast, as long as that last transistor is still hot.
2. The Bias thermistor: is supposed to crank up the bias when the heat sink gets hot. But it can fail, and the bias does not crank up, as expected. The amp is biased way too hot to start with, and this makes it worse when it fails.
3. The overheat thermistor: Is supposed to shut off the output relay when the amp is overheating.

The pots on the board: supposed to be two pots, one to adjust DC offset, and the other to adjust bias.

Let me warn you, this amp has a lot of quirks, and is challenging to repair it so it works correctly. There is a lack of original replacement transistors, or at least LOUD technology claims there are no original replacements.
The Driver transistors blow, and this takes out many other previous stages. Lots of parts to replace.

Even when it does work correctly, it runs very hot. I question the design of this thing, for sure. There are several tweaks to make it run cooler, but overall, it's a poor design, with very poor thermal management.

"apparently had some heat problems" OK I think you understand it now. It gets so hot that it destroys itself, that's a given.
I honestly think that you are better off junking it. But if you insist on making it work again, I can share whatever I have to offer. I have some parts substitutes lists, and other notes. I went over and over the thing for a while, trying to come up with tweaks that could help make it last longer.
But you are in for a job, if you are determined to fix it, I mean a real job.

[Dave Curtis, dB AudioTech]

Fan? What fan? Mine doesn't have a fan.

Also (last I knew), Harman owns Hartke; are you saying Loud owns Hartke now?

[Jazz P Bass]

Uhmm.
Hartke is supported by Samson Technologies, not Loud.
And I don't think the Kickback has a fan.
That transistor on the heatsink is most probably Q108, the bias adjust transistor.
If you had a severe output device failure & it took out the drivers, the bias transistor cannot hold back that current & poof.
The bias pot is probably bad too.
You may want to inspect all of the ballast resistors on the output devices, for opens.

[AtomicMassUnit]

It's the bias pot that burnt. Looks like the offset pot is ok. Also R214 and R218 (pretty sure I'm reading that right, the board is scorched where the label is). And you guys are right... no fan on the amp at all. Should I move the location of that thermistor when I replace it? It's right in the center of the heat sink.

[J M Fahey]

The "transistor" connected with 2 wires from the heat sink is a thermistor. Not a transistor.
There are actually three of them:
1. The fan control thermistor, speeds up the fan when the heat sink gets hot. BUT it is located in the wrong place. It is mounted at the coolest part of the heat sink, where the fan is. Therefore it does not speed up until the entire heat sink is hot. This should be located to the opposite end of the heat sink, so that the fan remains fast, even when one end (the fan end) of the heat sink is cold.
This amp has a problem, the last transistor on the heat sink gets the hottest. This thermistor should be located where the hottest transistor is mounted. Then the fan keeps going fast, as long as that last transistor is still hot.
2. The Bias thermistor: is supposed to crank up the bias when the heat sink gets hot. But it can fail, and the bias does not crank up, as expected. The amp is biased way too hot to start with, and this makes it worse when it fails.
3. The overheat thermistor: Is supposed to shut off the output relay when the amp is overheating.

Sorry:
No thermistors
No fan
No output relay

And when temperature rises, Bias circuits are supposed to crank DOWN the output devices.

[Jazz P Bass]

It's the bias pot that burnt. Looks like the offset pot is ok. Also R214 and R218 (pretty sure I'm reading that right, the board is scorched where the label is). And you guys are right... no fan on the amp at all. Should I move the location of that thermistor when I replace it? It's right in the center of the heat sink.

It really sounds to me that you had a 'bad' failure.
(is there a good one?)
The wrong parts went at the wrong time.
Replace the bad parts & check it out real good.
No need to re-engineer this amp.

[Jazz P Bass]

And when temperature rises, Bias circuits are supposed to crank DOWN the output devices.

Well, properly designed ones, that is.
And this one is designed to do just that.
Thermal Tracking I think they call it.

[Enzo]

Just fix it, no need to move anything around.

If the bias control burnt, verify all those resistors associated with it, R154,155, and D103,104. Q108 is likely shot.

Something caused all that, so not only check ALL the outputs and their resistors, check Q107, Q116 and that resistor between them, R163.