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?
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Hartke 3500 Mosfet TA
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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.Juan Manuel Fahey
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Originally posted by J M Fahey View PostIt'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.
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Originally posted by buford View PostI'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.
Originally posted by buford View PostShould I pass on this?
Mark
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Originally posted by MarkusBass View PostI 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
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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
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Originally posted by MarkusBass View PostFor 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
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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?Juan Manuel Fahey
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