Hartke 3500 Mosfet TA

[buford]

There's one of these local for 35.00 with no output. The guy say's it's the output board. He said he's changed them before on it but doesn't want to be bothered with it again. I've read that these Mosfet boards are pretty much impossible to work on and they used to change them out for bipolar boards that probably are no longer available. Also that these older amps go up in flames. He say's it powers up with lights and fan but no output. Also said it didn't smoke when it quit. I doubt it's as simple as a fuse. I know there's some older threads here but I wonder if this amp nowadays is worth bothering with since it;s so old and theres so many new modern better amps out there for not much more if the parts for this get too pricey and maybe never works correctly if at all. Should I pass on this?

[J M Fahey]

It's the same as buying an old, broken down car.
If you are a good mechanic, have access to tools and proper parts, go ahead; if not ... it's iffy at best.
*Never* seen the Mos Power schematic anywhere, so maybe that's a big problem.

[buford]

It's the same as buying an old, broken down car.
If you are a good mechanic, have access to tools and proper parts, go ahead; if not ... it's iffy at best.
*Never* seen the Mos Power schematic anywhere, so maybe that's a big problem.

the only tools I have are a dvm and a tone generator No variac or scope.Hartke_3500_Mosfet.pdfI built when I went to school for electronics but that was many moons ago. I actually found a Mos schematic I'll try to post. Some of these forums don't take attachments.

[MarkusBass]

I've read that these Mosfet boards are pretty much impossible to work on and they used to change them out for bipolar boards that probably are no longer available. Also that these older amps go up in flames.

I think that most of such opinions are not true. I haven't seen this power amp but I suspect that board was changed due to difficulties in purchasing the MOSFETs used in the amp. What type are the MOSFETs transistors (I don't see it on the schematic)? Is it possible that thay are 2SK135 or similar? The schematic looks like the amp is using special type of MOSFETs - lateral MOSFETs - possible to buy but quite expensive.

Should I pass on this?

No, don't give up. Check what output transistors are used in the amp. Check with DMM whether they are shorted or not. Disconnect the power amp and check whether you can turn the amp on in such a case. If you can't fix it, give the amp to a tech - this is a good amp and it would be a pitty to throw it away.

Mark

[buford]

I think that most of such opinions are not true. I haven't seen this power amp but I suspect that board was changed due to difficulties in purchasing the MOSFETs used in the amp. What type are the MOSFETs transistors (I don't see it on the schematic)? Is it possible that thay are 2SK135 or similar? The schematic looks like the amp is using special type of MOSFETs - lateral MOSFETs - possible to buy but quite expensive.
No, don't give up. Check what output transistors are used in the amp. Check with DMM whether they are shorted or not. Disconnect the power amp and check whether you can turn the amp on in such a case. If you can't fix it, give the amp to a tech - this is a good amp and it would be a pitty to throw it away.

Mark

I haven't bought the amp yet without first running it by the forums. I think the mosfets are IRFP240A Manu:IR Encapsulation:TO-247,TRANSISTOR ,available on ebay for 3 dollars each but I doubt they are matched pairs which I guess is essential. I read about someone that worked at Hartke when they were making these and he said they would go up in flames because of a design flaw and they would swap the board with a bipolar retrofit. Thing is, Everyone raves about the mosfet ones, not the bipolar ones.

[MarkusBass]

For IRFP240 the amp is incorrectly designed (vertical MOSFETs), but it is correctly designed for BUZ901 (lateral MOSFETs). With the first ones it will not work to long (due to thermal runaway), but it will work very good with the later ones. Are you sure that IRFP240 were originally used? Do you see this symbol on the schematic?

Mark

[buford]

For IRFP240 the amp is incorrectly designed (vertical MOSFETs), but it is correctly designed for BUZ901 (lateral MOSFETs). With the first ones it will not work to long (due to thermal runaway), but it will work very good with the later ones. Are you sure that IRFP240 were originally used? Do you see this symbol on the schematic?

Mark

no Mark, I'm just going by another post I read claiming that those were the part #'s on it. I've also seen IRFP240 / IRFP9240 pairs on ebay but if you say they won't work then they must be another part. I've heard that they are no longer made so you're probably right.

[J M Fahey]

Thanks for posting the schematic.
1) it *is* repairable, as Markus said
Problem is those are lateral MosFets, very probably the 2SK/2SJ Toshiba types , which are no longer made, so any remaining ones are *very* expensive.
2) and yes, I see a gross problem in that board design, that amp will work very well (like all Lateral Mos types) into its rated load, but the protection circuit is suicidal. Not kidding.
3) If I had that amp on my bench, I might even mod it to accept bipolar transistors or even Vertical MosFets (IRFP240/9240) but the design effort is not justified for just one amp.
4) for $35 I would buy it anyway, and use it as a preamp.
I would pull out or at least disconnect the power module so it does not blow the main fuse.
5) maybe Samson (Hartke's owners) can sell you a replacement bipolar power module.
Ask them, who knows?