Hartke 3500-Output transistors

[Teleman55]

I was doing some tests on this amp and shorted one of the Output transistors.Now the amp blows fuses unless I unplug the output section.How can I determine a suitable replacement transistor?

[Peter Clothier]

Are there any markings on the transistor at all?

[Teleman55]

Yes there are,this is what I found-
IFRP 240
5H4U 9036-there are four of these.

[Enzo]

Well, IRFP240 is the transistor type. If Mouser doesn't have them, try Digikey. If only one is bad, remove it, the rest of the amp ought to work without it at least for testing

[Teleman55]

Thanks Enzo I,ll give it a try.

[Enzo]

Oh and for the record, I think you don't have a 3500, you have a 3500 MOSFET. They made them with both MOSFET and with bipolar output modules. You could replace the MOSFET module with a bipolar one by unplugging the cables and undoing some screws.

[Teleman55]

I called their service dept. and ordered transistors,and they emailed me a schematic.I am not very knowledgeable with SS and have no idea what voltages these transistors should be getting,I hope that my short didn't damage other components.

[Enzo]

OK, if you have the schematic, there are the 8 output xstrs in the power amp section of the drawing. My copy is old and hard to read, yours is likely better than mine. To the left of the outputs are four diodes, check them. To the left of them are the drivers, I think Q608, 610, test them for sure. Each power xstrs has a 470 ohm resistor at the base and a 0.47 ohm 5W ballast resistor at the source. Test them all for open condition. Immediately to the left of the four diodes and in parallel with them is a resistor (R619? 1.?k?) Make sure THAT is not open.

There are four 470 ohm on the positive side only fromthe source leads over to the right to Q621. MAke sure it is not shorted, and the resistors OK. And the two small xstrs to the right of the diodes, Q611,612, make sure they are not shorted.

Aside from ther MOSFETs, there are about 14 xstrs and a handful of diodes. Set your meter to diode test function and check the junction drops of all of them. Ten minutes tops. A lot faster if you are used to it.

If you don't do all this, who knows if the new FETs will be burnt out immediately? ANY part that connects to a burnt out part is immediately a suspect.

[Teleman55]

Thanks Enzo I don't know how you know all this but your a gift and I appreciate the help.I am basically a lowly parts mechanic when it come to SS devices.