Apologies to anyone who might have been looking for updated material. The whole site crashed from a high building (sort of) and I’ve been driving a friend nuts who’s been sweeping the bits up for me. So to speak. Anyway, to the Hartke.
It seems from my client that this fault has been popping up all over the place. The amp runs for a half hour or so, gets fed up, and cuts out. This is only when it is being used in anger. So what can we glean from that? First off, it’s not an Iq fault. (That doesn’t tell you how clever the amp is. Iq is the current that the amp’s output stage takes when it’s not doing anything.) If the amp takes more current than it should when it’s not working hard, then it will cut out, often within a few minutes of switch on.
As that wasn’t happening, it’s probably a temperature related problem, and that’s exactly what this amp showed up. The fan has a circuit associated with it, which increases the speed as the temperature increases. I’ve no idea why, other than to save a half a penny on your electricity bill. The voltage that supplies this circuit is 12 volts and this is dropped through a simple transistor/resistor arrangement which is controlled by a sensor on the heat sink. The one that I had in, fed a constant 5volts to the fan. The power transistor had blown, and the big dropper resistor that it shunts dropped the voltage to 5v.
This circuit is on a little seperate pcb mounted on a couple of screws behind the front fascia on the right hand side as you look at it from the front. The easiest repair is just to make the fan run continuously, which involves shorting out the metal oxide dropper resistor at the top of the pcb. Replacing the power transistor would be the correct way, and it would then function as designed. However the addition of a bit of heatsinking plate to that transistor would probably prevent it from happening again.
Hope this helps. Don’t put your fingers anywhere you shouldn’t. It hurts.