PPI AMP - BAD RECTIFIER OR TRAFFO?

[JHONNYICE420]

Hello all! I am somewhat new to the site, but have been reading posts for a while now. I have a prob with an amp I repaired; long story short I am a mechanic. I had a customer come in with a 65 mustang restoration for us to do the powertrain and electrical work on, and I offered to do the stereo install because the local stereo shops around here......uh....kinda suck. The customer had 2 pair of MB Quart 5.25 components, a pair of JL 10's and a PPI A400 amp to run it all. I told him to get me an amp to run the subs with and we'll run the A400 at 4 ohm stereo x 4ch for better sound quality. He brings me another PPI amp-this one's an AX400 - only difference is built-in crossovers. So I use the A400 as a slave to run the subs and the AX400 as the master on the mids&highs and it sounds great!

Now the fun part: give the car back and 2 months later car comes back with a dead batt - the AX400 from ebay had a draw... no biggie... tear it out and open it up and the rectifier is hurt. A couple of the IRFZ34's had corrosion on the legs and telltale soot and smell are present. This is easy to fix-8 transistors and 8 resistors later the draw is gone and the amp is back in service....for a week. Battery goes dead again, car comes back again, and I tear it out and open it up and the rectifier is REALLY fragged now. almost all of the transistors are blown. I put IRFZ44E in the rectifier because I could not get IRFZ34's and I read on here that they would work. I don't see why not, it's just a 5v operated switching transistor.

So what do you guys think? wrong transistors? bad transformer? there's no soot around the transformers, but I think I'll pull and check them for shorting. Can you check them in the circuit? Do you think the board will live through me removing them for testing? Should I have used IRFZ44N instead?

[Perry Babin]

The Z34s and Z44s are FETs, not rectifiers. The rectifiers have two legs in this amp.

Do you have a 12v power supply with an amp meter on it?

What value are the gate resistors (connected to the first leg of the power supply FETs)?

[JHONNYICE420]

THANK YOU SO MUCH FOR THE SUPER-QUICK REPLY
Sorry, I meant power supply when I said rectifier. the p/s was damaged (2 fets out of 8 burnt the first time around) replaced 8 z34's with z44's and replaced the gate resistors w/new 1/2 w 100 ohm units - now I got a BIG prob in the p/s section -- 6 fets burnt and gate resistors for 4 of the 6 are now burnt... and I tested this amp for 2 hours straight at very near full power and not a pop, whistle, or any funny smells.

On a side note: only one of the gate resistors was burnt when I made this repair, and now almost all of them are, so I'm thinking this amp was mounted down to a sub box before my customer bought it and possibly the transformer was vibrated untill the enamel coating wore off, but I don't see any soot around the transformer, and from what i've heard transformers smoke A LOT when they short. What I'm looking for is any other things that will cause a p/s to frag like this.

P.S. I have multiple 12v supplies, but only my batt charger has an ammeter built in. I can always wire a dmm in series on the power wire to monitor amperage, but I don't have a current limited power supply if that's what you're wondering

[Perry Babin]

It's common for multiple gate resistors to burn. In some amps, they all burn when the FETs fail.

It's the power supply FETs that smoke when the power supply fails, not the transformer. Transformers RARELY smoke when they short. More commonly, the windings short where the terminal windings meet the rest of the windings. This rarely is evident. If it's a dead short, the amp will draw excessive current. In some amps a primary to secondary short will cause DC to appear on the RCA shields (I don't know if that's possible on this amp). If it's an intermittent short, it can cause the power supply to fail for no apparent reason.

To test for a shorted transformer, I generally power up the amp through a 2 ohm current limiting resistor and twist the transformer to see if there is any change in the current draw. You can buy 8 ohm 20 watt resistors at radio shack. Wiring four of them in parallel will give you a 2 ohm current limiting resistor. Wiring the resistor in series with the B+ line and measuring/monitoring the voltage across the resistor will tell you if the current draw changes (higher voltage across the resistor = higher current).

[JHONNYICE420]

so what you're saying is get 8 more transistors, 8 more resistors, install them then power it up using a current limited source and do a "wiggle test".....damn....I was hoping I would be re-repairing an amp that will stay fixed this time.....guess I better call the shack and order the 2ohm resistors and get to work. I haven't even gotten to the worst part: I spent about 40-50 hours dressed in medical scrubs so I wouldn't have to worry about scratching a $10,000 paint job while I installed this system..... FOR FREE! The owner of this car has spent 12 years building this car and when it was "offically" finished late last summer, he was diagnosed with a terminal illness and this yaer will be the only one he has to show this car and now the trunk is un-showable untill I get this amp fixed AGAIN.
Man this sucks...if I had the cash and one was available I'd just buy another, but they're all used so it's just as big a risk and these amps aren't cheap.

[Perry Babin]

Radio shack stocks the 8 ohm 20 watt resistors. You don't have to order them.

When you do this, you must closely monitor the temperature of the unclamped FETs. When they're not clamped to the heatsink, they can overheat and fail within seconds when the amp is drawing excessive current (or when there is a problem with the drive circuit).

If you want to make clamps, the ones in the attached are very simple and quick to make.

[JHONNYICE420]

thank you for the clamp tip. It's one of the things that kept me from testing an opened amp - I like to test them on the bench and use an infared thermometer and my hands to look for excessive heat while running...it's been the best diagnostic tool I own for amps that work but don't work properly. I may screw it down to a big sub box and shake the amp while I test it...for like a week.
just gotta get it right this time

[JHONNYICE420]

New update: I removed the shorted fet's (2 per side) and now it powers up and runs fine. no draw when remote lead is un-powered. The thing actually sounds damn good for having half of a power supply. I checked current draw while twisting traffo and no change, so the traffo must be good....wich leaves me only one option: the fet's weren't matched closely enough?
the only strange thing I found is current draw cold is 1.5 amps w/ no signal on the rca leads and draw is 1.0-1.1 amps after an hour or two of medium work (4-10 amps draw while playing) not full volume, but at least 35%-50% output power average.
I am quite tempted to give this thing back in the condition it's in -- if it blows, I'm not out any more parts AND from what I read the z44's are capable of more current than the 34's so what are the thoughts of the experts? bad idea? won't be my first. any advice appreciated and thank you perry for the personal service you have given me FOR FREE.

[Perry Babin]

The Z44s are less than $1 each (~$0.50 at arrownac.com). I'd suggest that you replace them.

[ppia600]

...all of them with new, ftw