Hartke 3500 MOSFET output transistor

[conway]

I have a Hartke 3500 with a shorted output transistor. I ordered a new one from Samson. The bad transistor has a part number IRFP240 2U4T 9222. The new transistor I received has a part number IRFP240A 747. The invoice also says 7000 MOSFET. Is this the part I want or did I receive the wrong part? Also, I noticed when I took the old transistor off that between it and the heatsink was a white pasty substance and a thin piece of what looked like plastic. I lost the plastic piece and have no clue what the white stuff is. Do I need this buffer between the transistor and the heat sink?

[Steve L]

Hartke Part

Yes, you received the correct part, however it is a mosfet, not a transistor.

The "plastic" piece is an insulator and you will need it. The mounting hole on it is likely insulated, however the case is probably not. Don't install it without an insulator.
The white "paste" is thermal heat-sink compound and you need to replace that as well.

In addition, are you sure that the one mosfet that you intend to replace is all that is bad? Usually when they fail, those Hartke power amps tend to fail big.

Steve

[conway]

Thanks Steve! I'm pretty sure that is the only faulty component. As soon as I turned the amp on it would instantly blow the fuse. I removed the shorted MOSFET and the amp worked. Where can I purchase the plastic insulating peice and the white thermal heat-sink compound? Thanks again for your help!

-conway

[jrfrond]

I used to work at Samson, and did the initial setup of their parts department. The invoice says "7000" because the same MOSFET power amps were used in the 7000 and 3500 bass amps.

If you do, in fact, have a single bad MOSFET and it works, consider yourself lucky. I wouldn't bet on it though. Those MOSFET power amps were problem-ridden and USUALLY went up in flames (literally). Consequently, bipolar transistor retrofit amp modules are now used.

[Steve L]

Hartke Part

You can try Digi-Key, Mouser, MCM, etc. That is not a common semiconductor case style (TO-247), however.

Steve

[conway]

Can I purchase the retro-fit module from Samson? Any idea what kinda price that would be? The amp is probably at least 15 years old. It's been a good, reliable amp. I just don't want to dump a bunch of money into it. But I also don't like the thought of it going up in flames. Was hoping for a quick, cheap fix.

-conway

[Steve L]

Hartke Part

Samson will probably sell you the module, however I doubt it will be cheap.

The insulator and heatsink compound will not cost very much. If the amp worked for you without the missing mosfet you may be OK.

Check the associated source resistor (large, white, 5-Watt resistor near the mosfet you removed) to make sure it is not open. If it is open, order one of those as well. They are not expensive either.

If the power amp fails after that, then I would go for a new module.

Steve

[conway]

I'll try that. Thanks for the help!

[conway]

Should I add some fresh thermal heat sink compound to the other MOSFET's as well? In the troubleshooting process I removed them from the heat sink a couple times. Also, is the compound I want silicon based or non-silicon based?

[Enzo]

Silicone heat sink grease. SOme variation of those words will be the name of the product, depending upon brand. I don;t know if Radio Shack still sells it

If all you need is one mica washer - the clear plastic thing - try asking at any electronics repair shop if they have one they could spare or sell.

[Mars Amp Repair]

one word of caution,
The Mosfets ususally are matched sets. If you are only going to replace one of them, you might get lucky & have it be a match or it could either draw little current & let the others do most of the work or it could draw boo coo current & try to do most of the work.
Either way you could have a disaster on your hands. I have not worked with Samson parts dept, but if you can procure the part from them it will most likely be matched to their others (much like Loud does with the mosfets for the Ampeg series).

glen

[conway]

Any ideas where I can find a mica insulator? I found some, but nothing for a TO-247. I searched on mouser, digikey, mcm, etc.

[Enzo]

Did you try asking locally? I mean they are used on transistor anywhere, not just guitar amps. TV sets, stereos, lots of things use TO247 transistors.

[conway]

I didn't know that. I will ask around. Thanks!

[jrfrond]

I have not worked with Samson parts dept, but if you can procure the part from them it will most likely be matched to their others (much like Loud does with the mosfets for the Ampeg series).

glen

I have worked with and for them, and I wouldn't bet on matching, or for that matter, even having the part!