Hartke 3500 MOSFET Output Transistors

[Teleman55]

I was given this amp because it was dead.I did some testing and found 2 dead Output Transistors,in another thread someone suggested (Enzo I think)disconnecting the bad components and trying the amp,it worked.The transistors are labeled-KTA1943 and KTC5200.At MOUSER I found Fairchild Semiconductor-2SA1943/FJL4215 and 2SC5200/FJL4315 are these suitable replacements? I also tested the power supply with my O-Scope (Hitachi V212 20 MHz) and it looks clean,no spikes.Should I replace just the bad components or all the Output transistors?

[Enzo]

When an output transistor shorts emitter to collector, it shunts the power rail to ground. Not only does this not stress the other outputs on the same side, it actually protects them by removing the voltage across them.

A short on one polarity does stres the far side, but one of those quickly shorts and then everybody else is OK. Just replace the bad pair.

A short from base to collector is another matter. That usually blows out the whole row.

Yes, 2SC5200 and 2SA1943. Faichild makes them too? I expect Toshiba or someone.

[Teleman55]

Enzo-Thanks for the reply,I found the original transistor specs on line and the Fairchild units are a match.I have no idea of Brand quality,does that matter? I checked the .47 ohm resistors and they seem ok.My MM is of medium quality.The last 3500 I repaired had the transistors mounted with a mica film,these seem to use a foam tape,is that because they are not MOSFET's? I had to look under bipolar transistors to find them.By the way I found an archived thread in which you gave extensive testing info,do all transistors have to be removed from circuit to test? I have an older incircuit transistor tester but have not tried it.

[Enzo]

It was not a quality brand issue, I was simply not aware Fairchild was making asian transistor types.

These are bipolars, not MOSFETs. Hartke also made a MOSFET version of this amp. If teh amp says 3500MOSFET on the front panel, it should originally have had the MOSFET power module. But of that blew up, someone may have replaced the whole module with a bipolar version. I have done that in the past myself.

Power semis need often to be insulated from the heat sink. There are sheets of mica and there are the silicone squares. "Silpads" by Bergquist is one brand. They are just alternative products that do the same job. There may be subtle differences in their thermal conductivity, but it is not an isue in this equipment. The choice has nothing to do with the type transistor.

If a transistor measures shorted in circuit, it probably is. Pulling the part out of circuit allows you to test only that transistor. With four parallel transistors, how tould you determine if one were open while still in circuit?

You had two dead ones and the rest of the amp works. I'd be willing to bet that that is indeed the case and no hidden matters await.