Old Car Amplifier

Thread Starter

jester84

Joined Aug 19, 2011
8
I have an old car audio amplifier, PPI PCX2400 class AB switching, and am attempting to repair myself. I have 10 years of industrial controls experience, but never have gotten too much into circuit board troubleshooting. I have printed datasheets on some of the components, MOSFETS, H Bridge, and PWM-SG 3525A.


So far here is what I have determined:

There are 12 RFP70N06 (mosfets) on each channel (2 channels). All 24 (total), were destroyed. I have shorts from the Source to the Drain on all of them. Each side is of MOSFETS appears to be controlled by an HIP4081 High Freq Full Bridge Fet Driver. All the gate resistors seem intact and read 22 Ohms. The output transistors (?) A1303, all seem to be fine there is 70 Kohms between the drain and source, and 56 ohms from source to gate.

There are 2 resistors that were visibly damaged. I can't see the stripping anymore and ohm'd them out but don't know if this is accurate given their condition.
One of the resistors was 10 ohm and was on terminal 15 and 16 of the HIP4081A. This then went to the drain of one of the mosfets. I believe this was switching the positive side?
The other resistor was 3.3 ohm, and was connected between terminal 15 and 16 of the other HIP4081A and was connected to the B- terminal where the power is supplied.

I know I have to replace the MOSFETS, but I'd like to find the source of the problem before I fry $50 in transistors....again.

I know this is a long shot, but any help would be appreciated.

I do not have a schematic, but am going to attempt to make one :(
 

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wayneh

Joined Sep 9, 2010
16,390
Sad to say, but I think most here may think your cause is hopeless. Your labor, if it's worth anything, plus parts will likely far exceed the value of the radio.

I'm more pessimistic and less skilled than others here, but "a man's got to know his limitations" (Dirty Harry)
 

Kermit2

Joined Feb 5, 2010
4,162
Perhaps you know this, but if not...

The reading of a 'short' across those components will only be valid for individual devices. Reading them while still in the circuit board will show a short for all parallel devices even if only one of them is actually shorted out.
 

Thread Starter

jester84

Joined Aug 19, 2011
8
The amplifier that I'm working in is an old work horse beast. They still sell for over $650 used, no package, and a cross your fingers warranty. A broke one can sell for over $150-200?

The parts to this point are less the $45, as I can get the MOSFETS for under $2. Labor, free, and it's worth enough to make this amp work again.

I have already removed all the transistors and tested them. All of the transistors before the transformers (power side) were shorted. The only other components visually damaged were a resistor between the terminal 15 and 16 of the two H Bridges. Terminal 15 and 16 are the Vcc Vdd terminals according to my datasheet on both Bridges, and both the resistors were blown.
 

wayneh

Joined Sep 9, 2010
16,390
How exactly did you test your transistors? A MOSFET has a body diode that conducts in the direction of source to drain. Given the picture, I don't doubt that most or all are blown, but it is a straw to grab at.
 

Jimmeh30

Joined Mar 4, 2011
22
Blown isn't really a strong enough adjective for those FETS :p

If it were me trying that, first thing I'd be doing is removing the fets and powering it up with a limiting resistor, then checking to make sure the power suppl is still in good order and that your signal is making it through the front end of the amp.

Do you have a scope or a small amplifier to test with?
 

radiohead

Joined May 28, 2009
508
Did you test the transistors out of circuit or while still attached to the CCA? If you do your measurements in-circuit, you might get false/misleading readings. Is the underside of the CCA intact? Any damage to the traces? Corrosion? I ask because something made those things explode. I don't think it was the 50Hz hip hop thumppity thump. I could be wrong...
 

Thread Starter

jester84

Joined Aug 19, 2011
8
Yes, I tested all of the transistors off of the board. I'd say roughly half of them were in pieces (the ones that were on when it blew I guess). The other half still tested as shorted out. I decided it would be best to get new ones anyways just to be sure.

All of the traces seem to be good, no burn through, nicks or shorts that I have found.

My problem remains in what would cause the over-voltage/current situation that caused it to fry.

And this am doesn't see bumpy bumpy, death metal only.
 
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