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Old 17th May 2011, 06:50 PM   #1
luka is offline luka  Slovenia
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Default Ground Zero GZTA2.250mk2

Hey

So, I need help with repair of this amp, it had blown fuses, shorted output, shorted input.

Both output main transistors, and primary side power supply fets are out, there is no more short on input or output (output reads few 10k ohms)

Power supply part I don't need help, not yet anyway
I get gate signals for both sides, TL494 seems to work ok (don't have fets in)

Now, I have no experience with AB amps of this sort, how and what do I need to check, before I repower the smps (or can I do it even without output transistors?)

Thanks in advance
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Old 17th May 2011, 07:10 PM   #2
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I'd suggest replacing the power supply FETs and powering it up. I don't remember ever having an amp that had a problem powering up with the outputs out of the circuit. If it powers up OK, reinstall the outputs and power it up through a current limiter. If it draws more current than it should, you'll have to find which channel is causing the amp to draw excessive current. The outputs in the faulty channel will generally get hot within a couple of seconds. Don't leave it on for longer than that (even with a limiter) unless all transistors are clamped tightly to the heatsink. If the transistors are clamped down (safer), measure the DC voltage across the emitter/source resistors to determine which channel is drawing current. If the amp has no bias pots, it should have 0.000v DC across the resistors at idle.
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Old 17th May 2011, 07:19 PM   #3
luka is offline luka  Slovenia
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You know what, I might removed all this too soon
Since all fets and output transistors on their own read ok

One thing that did fail is diode on input (making the short)

I'm thinking user has reversed polarity on input

I might just go and put one set of fets in, to see if it will power the supply

Amp has no bais pots, if supply powers up, and after I put transistors on back, will check if it reads 0.0v at idle (Can I put just one PNP/NPN per side?)
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Old 18th May 2011, 08:22 AM   #4
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You can install one transistor of each type in each channel for testing.
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Old 18th May 2011, 09:32 AM   #5
luka is offline luka  Slovenia
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installed everything, both channels work, it was input diode
thanks for the help
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