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Old 23rd Nov 2020, 7:02 pm   #1
Whaam68
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Default Understanding transistor tester readings

Well specifically are these devices testing as good or not?
First the background...around 2012 I built a diy class A kit amp the Nelson pass Amp Camp....one of the first amps I built. I brought it down from its 10 year resting place in a box in the loft on Saturday to use it in the spare bedroom (now my office). All was well and it sounded great. Stupidly I moved a speaker with the amp on and heard a bit of static and the channel driving that speaker died and the heating cooled rapidly. I removed the board from the duff channel and de soldered the two output mosfets and the other mosfet and transistor. I had recently picked up a cheap transistor checker and it is recognising the n channel mosfets. If anyone can confirm the readings seem ok or not it would be appreciated as a start in my figuring out what went wrong. The outputs are IRFP240 the “driver?” Is a 2sk170 jfet and there is a ztx450 transistor.
Thanks
Mike
Mods please move if this is in the wrong section! Thx
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Last edited by Whaam68; 23rd Nov 2020 at 7:19 pm. Reason: Wrong section?
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Old 23rd Nov 2020, 8:58 pm   #2
MotorBikeLes
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Default Re: Understanding transistor tester readings

If they are in the order you listed them, first two probably OK (somebody may disagree), the third looks OK, a MOSFET with a diode across source to drain, the final one looks like two diodes (which a transistor CAN look like), but it rather looks like a dead transistor to me. Not sure without getting a book out, but I think the ZTX450 may well be a pnp transistor. Get one and see what that shows on the tester, different I am sure, with a proper identification given.
Les.
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Old 23rd Nov 2020, 9:03 pm   #3
paulsherwin
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Default Re: Understanding transistor tester readings

ZTX450 is a medium power NPN transistor - pretty standard bipolar driver in a power amp.
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Old 23rd Nov 2020, 9:08 pm   #4
buggies
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Default Re: Understanding transistor tester readings

Certainly should not be Emitter - Collector 0.08 Ohms.
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Old 23rd Nov 2020, 10:07 pm   #5
Whaam68
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Default Re: Understanding transistor tester readings

Thanks all and for clarity pic 1 is the 2sk170 jfet
Pics 2&3 are the same IRFP240 MOSFET (haven’t se soldered the other yet)
Pic 4 is the ztx450...the cheapest part by far to replace.

De soldering the mosfets was a pig as the board is high quality which is why I’m keen to figure out their condition before re fitting.

The full circuit if anyone is interested is on page 5 of this short paper.

http://www.firstwatt.com/pdf/art_amp_camp_1.pdf

At the mo the consensus seems to be the ztx might be iffy. At least it’s by far the cheapest part to replace not that any of these are that pricey...I just ordered some spares.
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Old 5th Dec 2020, 9:25 pm   #6
Whaam68
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Default Re: Understanding transistor tester readings

Just to thank everyone for the guidance. The ztx transistor on the one channel was correctly ID ‘d as suspect but turned out to be the only thing blown. I de soldered all including the outputs checking with a multi meter after finding a very thorough YouTube tutorial. I fitted a replacement last week and the amp is singing perfectly again.

Last edited by Whaam68; 5th Dec 2020 at 9:47 pm.
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