Car amplifier(Polk Audio PA660) question

[tibimakai]

I'm repairing an almost new car amplifier, which I'm planning to use in my own car. The amp, is a Polk Audio PA660.
It was entering in protection mode.
I have found a pair of final transistors shorted at one of the channels(4 channel amp). I haven't found any other component blown or shorted.
Voltage, has dropped from 12V to 8.4V after a couple of seconds after start up.
It is possible that the only bad components are these two final transistors?
They are 2SD718/2SB688, it seems like they can be replaced with NTE36/37 transistors.
I have seen some videos on Youtube where people are replacing these two transistors and the amp it's good to go.
How come in a home theater receiver I have to replace a bunch of components?
I have also purchased, a mono version of the same series amp, which seems like it is in protection mode too. I will get it in the next couple of days.
Need amps in my new car.

[kaboom]

Re: Car amplifier(Polk Audio PA660) question

Avoid NTE devices- overpriced for what they are. Also, don't fall for this "exact match" nonsense. Just parameter match, the same way transistors were subbed for years, using the datasheets.

I'm sure you've heard this before, just saying we have come a long way with audio transistors in the last 40 years...

The original devices are rather weak. Some of the weakest TO-247 "output devices" I've seen. I've done better with TO-220 transistors. Replace them with 21193/21194s, which are far more rugged.

I have seen some videos on Youtube where people are replacing these two transistors and the amp it's good to go.

Yep, it sure is. Don't need to check bias or offset, or even idle current draw on the 12V side...

How come in a home theater receiver I have to replace a bunch of components?

Because home theater stuff is generally junk... Direct connected driver and output stages, while being commonplace and understood since the mid '70's, have been cost reduced. So the usual phrase "if one device blows, they all blow" is even more substantiated today.

Actually, standard procedure when any amp loses its outputs is to check everything. I would check the drivers in that car amp for shorts.

Also check base isolating and emitter resistors for opens.

[tibimakai]

Re: Car amplifier(Polk Audio PA660) question

Thank you for your help.
I already have an extra set of NTE36/37 from a Yamaha receiver repair. I haven't finished with the Yamaha yet, so hopefully I won't blow them up again and then I will have an extra set.
I will consider that pair of transistors for future repairs, thank you. I have never heard about this pair yet. They are half the price, as the NTEs.
I have already checked the components around these two transistors, that is why I'm surprised that nothing else blew up. The Emitter resistor shows 0.3 Ohm, same as the other channels.
This is my first car amplifier repair. In a couple of days, I will have the mono version of the same amp to repair, the PA880.

[kaboom]

Re: Car amplifier(Polk Audio PA660) question

Ok- I didn't realize you already had the NTE pair. They're still better devices than the originals- don't hesitate to use them.

As for the PA880, from pix I've seen, you've got six output devices for that single channel. Three each of the NPNs and PNPs.

Q109/110 (BD139/140?) seem to be drivers, themselves driven by Q107/108? Looks like one of those "self balancing" differential amps, comprised of four transistors, Q102-105 and associated resistors.

These newer car amps no longer have an output relay, so you can't measure the output bus to check DC, or across the emitter resistors to check for excessive bias. Anytime they go into "protect," they put the smps controller (TL494, SG3525) to sleep, removing the B+/B- rails from the power amp.

If the output is stuck at one of the rails, it could be as simple as a bad joint on one of the rectifier(s)- if missing B-, it'll be stuck at B+; or as involved as one or more Q102-105s could be open or shorted. That could get interesting, since it is direct coupled. There will be two zener diodes supplying that diff amp, probably with either +/-12V or +/-15V.

[tibimakai]

Re: Car amplifier(Polk Audio PA660) question

Do you know from where can I get schematics for these amps?
I will get the PA880 in the next couple of days and I will check it out. Thanks for the pointers.

[kaboom]

Re: Car amplifier(Polk Audio PA660) question

I haven't had good luck finding schematics for anything recent, unless it was one of those "chip amps" with a TDA7370 or similar "multiwatt," ~20WPC/BTL/4ohms device.

The only car amps I have found schematics for would be out-of-production stuff.

And certain Pyle/Pyramid/Lanzar schematics are easy to find, but this doesn't help you. I think it's only because those models have been around forever, with upsized power supplies and... "updated (fudged)" power ratings.

It appears that these Polk amps are relatively new to the market, so schematic stingyness doesn't surprise me. All I've found are board pix on other forums so far.

You'll probably be better off taking good, clear pix of your '880 board, both solder and component side, in order to make sense of it. As if the output stage is compound pair or Darlington, how the drivers are connected, that 4-transistor differential/current mirror, etc.

[tibimakai]

Re: Car amplifier(Polk Audio PA660) question

For now, I will take pictures of the PA660 amp.
When the other one arrives, I will take some with that amp too.

[tibimakai]

Re: Car amplifier(Polk Audio PA660) question

OK, I have received the PA880 amp and checked it out in a hurry and there is extensive damage in there. The worst that I have ever seen.
Basically, everything what is mounted on heat-sink, is blown. And not just those.
I'm at work right now. When I have a little bit of time I will take those pictures that I have promised.
It's Christmas now and hard to find time.
I'm contemplating, if it's even worth repairing this amp. Cost will be pretty high.
I'm thinking of keeping this for my car.
Transistors are:
2SA1694/2SC4467 x 3 pairs
2SC3421/2SA1358 x 1 pair
power supply:
FQP65N06 X 6 transistors
Most likely, there is more that got damaged, I will have to measure a lot of parts.
All the power supply FETs are blown/cracked badly, there is a lot of ash on the PCB. Mica is brown behind them.

[tibimakai]

Re: Car amplifier(Polk Audio PA660) question

Found this Infinity Ref1300a SM. Uses the same transistors, but most likely they are not using the same schematic.

[tibimakai]

Re: Car amplifier(Polk Audio PA660) question

Wouldn't you have, some substitute transistors for these by any chance? I need to repair this as cheaply as possible. Since I need to replace way to many transistors, it wouldn't heart using some cheaper alternatives.
I can't even get two of each, because it's getting to expensive.
I will have only a one time go at it, if it blows then I will just put it away.

[kaboom]

Re: Car amplifier(Polk Audio PA660) question

Wouldn't you have, some substitute transistors for these by any chance? I need to repair this as cheaply as possible. Since I need to replace way to many transistors, it wouldn't heart using some cheaper alternatives.

Not/never a good idea. You need plenty of SOA for the output devices, since it's not uncommon for these amps to operate in high ambient temps. Also, they're often "abused"- previous owner ran too low an impedance, further stressing output devices, as well as MOSFETs in power supply.

I always use larger/better rated finals, as I described in an earlier post.

The MOSFETs in the power supply should also be upsized, possibly the output rectifiers as well.

I can't even get two of each, because it's getting to expensive.
I will have only a one time go at it, if it blows then I will just put it away.

You're not kidding. If it blows up again, that board will probably be burned beyond repair, if it isn't already. I'll need some pix of it.

What "two of each" can't you get?

I'd be better able to advise you of your options w/ this amp if I had, again, some good pix. If the board is still mostly solid, the amp can be repaired to better-than-new condition.

[tibimakai]

Re: Car amplifier(Polk Audio PA660) question

I will take tbose pictures later on.
It's not worth getting duplicate parts, it's getting to expensive.

[kaboom]

Re: Car amplifier(Polk Audio PA660) question

I will take tbose pictures later on.
It's not worth getting duplicate parts, it's getting to expensive.

Then stay away from NTE!

[tibimakai]

Re: Car amplifier(Polk Audio PA660) question

OK, I have some pictures, the lighting that I have in the garage, it's not the best though.
If it's not good enough, then I will take some other ones tomorrow outside.

[kaboom]

Re: Car amplifier(Polk Audio PA660) question

What value are the fuses?

All MOSFETs in that power supply have blown apart. The diodes could even be bad.

Either a cascade failure of finals-> then power supply, or running too-low a load impedance for far too long.

[tibimakai]

Re: Car amplifier(Polk Audio PA660) question

There are two 30A fuses.

[kaboom]

Re: Car amplifier(Polk Audio PA660) question

Ok. They're two different colors, thought there was a 20 and a 30.

FWIW, avoid cheap "harbor freight" blade fuses. They've been known to be jumpers, "protected" by your amplifier...

[tibimakai]

Re: Car amplifier(Polk Audio PA660) question

The fuses are good! Can you imagine, all that damage and the fuses are good?

[tibimakai]

Re: Car amplifier(Polk Audio PA660) question

6 way protection my axx.

[tibimakai]

Re: Car amplifier(Polk Audio PA660) question

You were right, the two fuses have slightly different green tint. I guess, somebody has changed at least one of them.
I should get some 10A fuses, for testing?
I still have to go through all the components to see which one is shorted.
I know for sure that the 6 x finals, 1 pair 2SC3421/2SA1358, 4x FETs at the PS are blown for sure. Emitter resistors seem fine.
This is how far I have checked so far.
I have a TV that I have to repair right now, which should be a quick fix. Gamma IC replacement on a T-con board. Another Yamaha receiver I have to finish.
These bring money, the amps are for myself.