On 10/3/2020 4:33 PM, Phil Allison wrote:

David Farber wrote:

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** That is bad sign of other problems in the amp.

Those are high power IGBTs and they do not blow without a reason.


Next (but still without a new set of IGBT's installed) I slowly powered
the unit on with the Variac. Relay K1 now energizes right after power up
and the internal green LED glows normally. I will now place an order for
the IGBT's and NTC thermistor. Phil brought up the point, what would
cause them to blow in the first place? After much checking, I could not
find any other faults in the circuitry. I'll update you after the new
parts arrive and are installed.

** I have you have an isolation transformer or a scope that can be safety floated above earth. You need to inspect the drive waves to each IGBT.

The parts finally arrived. I'm a bit confused as to how the schematic of
the IGBT in the datasheet compares to the resistance readings on the
component itself.
http://datasheet.octopart.com/STGW30...t-10835944.pdf

Placing the positive probe on pin 2 and the negative probe on pin 3
reads open circuit. If I then put the positive probe on pin 1 and the
negative probe on pin 3 it reads open circuit. Then going back to my
original test of positive on pin 2 and negative on pin 3, I get a
reading of a good diode check around 475 ohms. Subsequently reversing
the probes from pins 1 and 3 and then going back to pins 2 and 3, that
will revert to an open circuit. These readings are what I expect when
testing an FET. According to the datasheet, there is a diode across pins
2 and 3. That would mean if I put positive on pin 3 and negative on pin
2, I should get a good diode check reading but I get an open circuit.
Can you make sense out of these tests?

Thanks for your reply.
--
David Farber
Los Osos, CA