Car amplifier(Polk Audio PA660) question

[tibimakai]

Re: Car amplifier(Polk Audio PA660) question

6 FETs not 4.

[tibimakai]

Re: Car amplifier(Polk Audio PA660) question

The PA660, it's working great after replacing the two blown final transistors.
Next is the PA880(nightmare) amp.
Need to measure every component and order what I need. I will go with the suggested final pair.
Any recommendation for the FETs?

[kaboom]

Re: Car amplifier(Polk Audio PA660) question

FDP55N06, here's a Mouser link:

http://www.mouser.com/ProductDetail/...252bCTnwymc%3d

[tibimakai]

Re: Car amplifier(Polk Audio PA660) question

Thank you for that.
I'm buying my parts from Digikey, they have an under $3 shipping option.
They have it too for $1.17/each.
I will ask a stupid question, if I will used these transistors and the other finals that you have recommended, the amp will have more power, or they can just take more punishment?

[kaboom]

Re: Car amplifier(Polk Audio PA660) question

I will ask a stupid question, if I will used these transistors and the other finals that you have recommended, the amp will have more power, or they can just take more punishment?

No, it's not stupid at all.


As to the question, the latter applies...

The output power is limited by the supply voltage to the power amp itself. This is set by the ratio of the toroidal transformer in the unit's power supply.

You may notice a few watts output (5-10) more than with original output transistors, since better ones maintain their beta better with increasing (output) current, dropping less themselves and keeping the voltage to the load (speakers) better/higher. IOW, better devices simply maximize what you should've been getting from the unit from the outset.

Depending on the RDS of the replacement MOSFETs in the power supply versus the originals, it is possible for them to run slightly cooler.

But mostly, these little upgrades will make it far more durable and "abuse tolerant," always good for these car amps.

There are 12 resistors associated with those MOSFETs. 6, 10K-47K, from the gates to GND, ensure that any leakage does not allow the MOSFETs to conduct. These are of such high value that a shorted device's backfeed will not destroy them.

There are another six, 4.7-22 ohms, from each gate to its driver. Check all of them for opens, since these will be damaged should a MOSFET backfeed.

Just to be sure, check, with a scope, the drive waveforms before putting the new MOSFETs in.

Replace the fuses with a single 2A fuse, and apply power through a 10 ohm, 20W or greater, resistor. You should see the amp's caps charge up. Normal voltage should be from +/- 65 to 75V. You'll have less than this with the resistor in the 12V supply, but you should still have those two supplies.

You'd want to check this without the 6 power transistors soldered in, for obvious reasons.

I'd also consider replacing those two diodes

[tibimakai]

Re: Car amplifier(Polk Audio PA660) question

Do you mean the double diodes, F182CAI and F182CCI? I can't find any info about them. One is common cathode and the other is common anode.
I have removed the first one and checks out OK. They show weird in circuitry.
I will check out those resistors, some of them(the blackened ones) are even broken off from the soldering. I just hope that I will be able to see there value.
I have started working on this amp now and I'm trying to find out which parts are needed to be changed, to be able to order them.
I have an opportunity to pick up the same set of amps locally(found on Ebay) for $60 both. I may jump on the offer, repair them and sell them.

[tibimakai]

Re: Car amplifier(Polk Audio PA660) question

I also see some ICs, they can't be blown?

[kaboom]

Re: Car amplifier(Polk Audio PA660) question

I also see some ICs, they can't be blown?

Which ones, in the power amp, preamp, or power supply?

[tibimakai]

Re: Car amplifier(Polk Audio PA660) question

Any one of them, specially the TL494.

[kaboom]

Re: Car amplifier(Polk Audio PA660) question

A scope is your friend...

Just to be sure, check, with a scope, the drive waveforms before putting the new MOSFETs in.

[tibimakai]

Re: Car amplifier(Polk Audio PA660) question

OK, I will do that. A TL494 is $0.73, I will just get one.
Thank you.

[tibimakai]

Re: Car amplifier(Polk Audio PA660) question

Everything looks pretty good. Two 100Ohm resistors legs are broken off at the through hole, but they all show good values(the ones around the burned FETs).
One suspect transistor I have found which I can't figure out if it's good or bad.
It's a J111 JFET transistor. Between the Drain and Source, I'm measuring 22 Ohms in circuit. I have never dealt with a JFET transistor before. How can I check it out?
8 pin IC is a TL072CN.

[tibimakai]

Re: Car amplifier(Polk Audio PA660) question

Do you think that NTE373/NTE374 is a good choice to replace these 2SC3421/2SA1358?

[tibimakai]

Re: Car amplifier(Polk Audio PA660) question

I have started working again on this amp.
The problem is that I don't know what I'm suppose to do.
I have replaced all the FETs including the JFET. Tested all the resistors arounf the FETs and they all have checked out good. I have also replaced those three 100 Ohm resistors next to the FETs, which looked black.
I have replaced the fuses with 2A once. I have tried with one and it blew it.
I haven't tried with a 10 Ohm/20W resistor though.
The amp started up and it didn't show any protection error.
The finals are not installed yet. Since the PCB is back in it's housing/heatsink I couldn't measure the voltage at the capacitors, but at the pre-finals I'm measuring 60V.
I'm lost with the scope measuring. What kind of signal should I use, where should I measure and on what scale?
Any help would be appreciated.

[tibimakai]

Re: Car amplifier(Polk Audio PA660) question

I have soldered in all the components and all seems fine except the JFET F\J111 which measures 22 Ohm between D and S.
That is normal, or should I investigate more?
Any help would be appreciated.
I'm about to turn it on and before that I would appreciate any input.

Thank you.

P.S. This is what they say in the datasheet: Static Drain−Source On Resistance at 0.1V, 30 Ohm max.
So it seems fine, no?

[tibimakai]

Re: Car amplifier(Polk Audio PA660) question

I have found a short between + and -.
Need to find out what is causing this short.

[tom66]

Re: Car amplifier(Polk Audio PA660) question

n-JFET has short when V(gs) = 0V. So that is probably OK.
JFET is a rare device to use nowadays. Only used for high speed or ultra low input current, or maybe to write "JFET Input Circuit" on packaging.

[tibimakai]

Re: Car amplifier(Polk Audio PA660) question

It was my mistake.
I have used the wrong screw, to mount the fuses to the front panel. It was to long, and it was touching the positive terminal of one of the fuses.
The amp is working!!
Thank you all, for any help offered.
I have my amps for my car now, I just have to install them.

[jakdedert]

Re: Car amplifier(Polk Audio PA660) question

Old thread, I know; but thanks all for the info and encouragement. I just picked a dozen of these...mixed PA660s and PA880s. I worked for a small salvage biz a couple of years ago, testing and reworking various electronic returns from a big-box electronic store. These were left over, and I just happened to run into the owner the other day and asked about them. I've only pulled one 660 apart, and it's got toasted outputs, of course. They got so hot some of the foil traces on the board lifted.

I'll do inventory (I don't even know how many of each I have yet) and testing over the next few weeks. Back when I was doing it, some of them worked, others failed in protection, others were totally DOA.

Should be fun....

[jakdedert]

Re: Car amplifier(Polk Audio PA660) question

Inventory shows nine PA660 and two PA880. I have a PA330 in my van bridged mono for subwoofer power. It's basically half of a 660, two channels with the same specs as the 660. It's been a very stable and solid amp for two years. I'm thinking about swapping that out for a 660, and the owner wants one of the 880s if I can fix them.

Next step is to set up a test rig and start checking them.