Power supply FET's overheats in seconds - no load, why?

[valterdaw]

I am debugging a huge amp (Audiobahn A2200HCX) and it is almost working, but power supply FET's (they were blown IRFZ44, replaced with 75339) overheating in a couple of seconds without heatsink. With heatsink it takes around minute for them to die. There are 16 of them (8+8 in parallel). This minute amp is working terrific This am had blown 494 too, replaced it.
I disconnected power supply from everything, 494 generating fine (no scope though), if I replace timing capacitor with bigger one, I hear loud sound from both transformers.
Removed output capacitors, in case they can be shorted inside - same thing.
Please, give me some ideas where to look at. I am lost Thanks!

[Stocker]

you need to look at an oscilloscope. I don't know anything about the amplifier or the topology you are dealing with, but FETs going bang with "no" signal seems like 80% likely oscillation, 20% something truly strange going on in the circuit ( bias currents or voltages, strange transients etc.)

[Perry Babin]

If there are two transformers, the FETs are likely in four groups (two groups per transformer). Are all of the FET groups getting equally hot? Does one group fail repeatedly?

Was anything else blown in the amp (output transistors, rectifiers...)?

Did you check the PNP gate drivers for the FETs?

If you pull the rectifiers, do the FETs overheat?

Do you have an amp meter on your power supply that shows how much current it's drawing before the FETs fail?

You need to use a current limiter in the B+ line. It allows you to troubleshoot problems with less of a chance that you'll damage the new parts.

It seems like you're going to be repairing amplifiers regularly. Buy a scope. For ~$200 you can get a good used scope. Look at the Tektronix 465 and 465b.

[valterdaw]

Thanks for replies!!! Looks like I reopened mine 20 - year ago hobby for myself, so I'll be shopping for scope today...

And here is answers to your questions, Perry:

- nothing else is blown to my knowledge, If I pull 494 or just two legs - 9 & 10 - nothing is getting hot.
- drivers seems to be fine
- I pulled rectifiers - same situation, FET's are hot
- 25 AMP at first startup (triggers my power supply protection), if you restart it within couple of seconds, around 20 AMP

May be I need different FET's there?

Something is not right here

I will be digging my garage for a headlight bulb - one should be there.

Thanks a lot!!!

[RetroAudio]

Don't be discouraged Valter, just hang tough. Many of us go through the grinder trying to figure this stuff out that just doesn't making sense. I'm going through the same on a couple pieces of gear that just about have me at wit's end, having pulled almost every component that tests just fine. But when it's over you'll have added to your bag of tricks and be the better man for it. Really, they just don't teach this in school!

[Jack Crow]

Valterdaw,
Got a question for you.

Are you the type who returns paper back books?

If so, I can loan you a 'real' power supply that will help in your trouble shooting.

It's a big old HP that's good for CV or CC up to 10 amps.

Home is in Herndon, so it 's no big deal to head out to Leesburge.

I fly home tonight for 20 some odd days of R and R before going back to the war.

What you said about 20 amps of steady draw is a warning that's somethings wrong, that's a lot of power for an amp that's doing nothing.

Give me a call on the second at 703 863 4574

Much luck

Jack Crow in Kuwait for another few hours

[valterdaw]

Thanks! And thank you for your offer, Jack. As I rediscovered my interest in fixing stuff, it will be better for me to invest in nice scope and power supply, then troubling you. And I really don't remeber when last time I had loaned a book Probably because internet displaced a lot of reading for me.

I missed a couple of questions:
- All FET's are getting equally HOT, all resistors involved are fine
- FET's failing randomly without any pattern

When supply is working it produces correct voltage and amp is working great ("hits a sub hard as hell" ).

[valterdaw]

Can any of 4 huge zener's can be bad? Desoldered they looked like a diode for ohmmeter, but that's not a good test for zener.

[Perry Babin]

Check continuity between the ground lug of the amp and the ground pin of the TL494 (pin 7). If the ground is open or intermittant, it will cause this sort of problem.

With pins 9 and 10 pulled, check both DC and AC voltage on the pins. DC should be ~1/2 B+. AC should be above 1v. The actual reading depends on the way your meter's frequency response. A true RMS meter with a wide bandwidth will read ~1/2 B+.

If you post the DC voltage for each pin of the 494, that may help troubleshooting. Use the chassis ground for reference (black meter lead on ground terminal of the amp).

If the zeners check OK with the standard diode check, they are likely OK.

[30bandgeek]

i dont use any cross refrence parts ever on any switching component. I will crossrefreance logic components and passives all day long and will even up grade parts i.e use mylar or poly instead of electrolytic on caps on some circuts where it would be benifical,

Well to sum it up. even tho one manufactures data sheet may be same as other manufacture i still use olny the same or known good parts from manufacture. I mean i could get some gerber data and some circut programming software and figure out what parts are accetptable, but it is a lot easier to just put in what was in there before so eliminate any issues befor you can really debug.

who knows, but i am just a ammuture, honestley.

you say they get hot fast even with no load.....?????
What do you mean no load. do you mean no speakers hooked up to output?
if so what rail are you talking about, in put or output?
The input side of the amp (rectifiers, regulatiors...ect) or output side
you said there are numberious amount them, you said they are IRF,
( sounds like a motorolla device ) you replaced with a-nother?
IRF devices are commonly used in output stage...
Do you have any pics you can post?

love to help more but need more info

Andy