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!
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!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.)
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.
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.
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!!!
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!!!
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!
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
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
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").
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"
).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.
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.
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
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
FIXED!!!
We fixed it !!! I am so greatful to all of you, who helped me here 
! Special thanks to Perry, I absolutely blown away by your knowledge in this area and your desire to help others. Andy, your comment about cross-ref parts was priceless, helped me a lot.
What it was:
DC on pins 9 & 10 was way too high (4.6V, there must be a difference in output transistors inside DBL494 and TL494CN I used). Subbed IRFZ44 with 75339 may became a little more "easy to open" too. I cutted the leads between pins 9 & 10 and bases of drivers, added potentiometers there. Then I found resistance, which will provide a voltage drop on bases just enough for robust switching, but not enough for too heavy duty cycle, leading to overheating. Replaced both potentiometers with 220 OHM resistors and it's working!!! FET's without a heatsing getting just a little warm. Assembled everything, it's playing for an hour now, no problems. I got too attached to this amp so I'll put it in my car for a while
We fixed it
!!! I am so greatful to all of you, who helped me here ! Special thanks to Perry, I absolutely blown away by your knowledge in this area and your desire to help others. Andy, your comment about cross-ref parts was priceless, helped me a lot.What it was:
DC on pins 9 & 10 was way too high (4.6V, there must be a difference in output transistors inside DBL494 and TL494CN I used). Subbed IRFZ44 with 75339 may became a little more "easy to open" too. I cutted the leads between pins 9 & 10 and bases of drivers, added potentiometers there. Then I found resistance, which will provide a voltage drop on bases just enough for robust switching, but not enough for too heavy duty cycle, leading to overheating. Replaced both potentiometers with 220 OHM resistors and it's working!!! FET's without a heatsing getting just a little warm. Assembled everything, it's playing for an hour now, no problems. I got too attached to this amp
so I'll put it in my car for a while Now that you have the amp up and running, you really need to check the drive signal. A reading of 4.6 volts is not out of line. At full duty cycle, you can typically expect ~1/2 of B+ on pins 9 and 10.
Adding the resistor in series with pins 9 and 10 didn't change the duty cycle. It changed the voltage of the waveform. It's likely that the FETs are not operating at high efficiency. They are probably not pulling as much current because they are not being turned on fully. If you run the amplifier hard and the power supply transistors run hot, the drive signal may not be good enough for the amp to be reliable.
With a b+ voltage of ~12 volts, the drive waveform should swing to at least 10 volts. The right edge of the drive waveform at the gate of the transistor should be vertical for the top 85-90% of the overall height. At the lower 10-15%, the waveform may be a slope or curve. It should drop to below ~1 volt before the other half of the FETs swich on. You can watch for overlap if you have a 2 channel scope.
I know that this probably isn't what you want to hear but it's better to know 100% that the amp is properly repaired. It's very rare that you have to add components to repair an amp.
Adding the resistor in series with pins 9 and 10 didn't change the duty cycle. It changed the voltage of the waveform. It's likely that the FETs are not operating at high efficiency. They are probably not pulling as much current because they are not being turned on fully. If you run the amplifier hard and the power supply transistors run hot, the drive signal may not be good enough for the amp to be reliable.
With a b+ voltage of ~12 volts, the drive waveform should swing to at least 10 volts. The right edge of the drive waveform at the gate of the transistor should be vertical for the top 85-90% of the overall height. At the lower 10-15%, the waveform may be a slope or curve. It should drop to below ~1 volt before the other half of the FETs swich on. You can watch for overlap if you have a 2 channel scope.
I know that this probably isn't what you want to hear but it's better to know 100% that the amp is properly repaired. It's very rare that you have to add components to repair an amp.
Yeah, if they are not turning on completely, they will die under load... I feel completely blind without scope. While I am still shopping for it, and it will not be here earlier then in a mid of next week, I'll do a little stress testing today 
. I hope there will be not too much stress for FET's, my neighbours and 
to handle In worst case, I have a set of new 75339's that is ready to go in. That's not exactly the smartest way to go, it just the weather is so good today. I will definitely check waveforms on gates. I really appreciate your help, Perry! I want it to be fixed right.
. I hope there will be not too much stress for FET's, my neighbours and to handle In worst case, I have a set of new 75339's that is ready to go in. That's not exactly the smartest way to go, it just the weather is so good today. I will definitely check waveforms on gates. I really appreciate your help, Perry! I want it to be fixed right.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
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
Hi all I am doing a repair on a Sony amp and it has the same issue on the -ve fets. I replaced all the FETs and used a 4.7r gate resistor and the FETs changed to the famous p55 .
hi perry thanks for the heads up. the challenge i am facing is similar to valterdraws issue , the difference is the number of power output fets used , and the model number of the amps . i will post photos .
i was thinking of making an external oscillator using sg3525 maybe it will drive the p55 comfortable. i am forced to use p55 fets because they are available and original .
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