Panasonic VSX-453 Repair

[tip120]

Pioneer VSX-453 Repair

I have a pioneer VSX-453 amplifier I'm working on for a friend. The amp left channel has a lot of static at lower frequencies, while the right channel is fine. I pulled it apart and hooked my scope up to it with it playing a 70 hz sinewave, and found that for the left channel, the waveform is fine up to the power amp, and past the power amp, the top of the waveform is 'chopped off', see below:

For that image, the upper channel is connected to the left channel input to the power amp board, and the lower channel is connected to the left channel output from the power amp.

This is a Class AB push pull amp, that uses a 2SA1302/2SC3281 PNP/NPN pair. Measuring the output of each transistor with the scope yields the following results:

PNP (2SA1302): The output waveform is the same as the lower channel in the image above.
NPN (2SC3281): The output waveform is a very shallow almost sawtooth looking wave.

To me it looks like the NPN output transistor on the left channel is bad, is this a fair assessment or do you guys think something else may be to blame?

 

[(*steve*)]

It's not totally unreasonable. However, since you have an oscilloscope, you might try tracing the signal back and confirm that the signal up to the final stage looks OK.

If it is a failure of one of the output transistors, it is possible that the stage prior to it may have the signal loaded down and affected, but prior to this it should look as you might expect.

 

[tip120]

It's not totally unreasonable. However, since you have an oscilloscope, you might try tracing the signal back and confirm that the signal up to the final stage looks OK.

If it is a failure of one of the output transistors, it is possible that the stage prior to it may have the signal loaded down and affected, but prior to this it should look as you might expect.

I wanted to do that but with the way the power amp board is designed, it's going to be hell to access. The traced side of the board is about half a centimeter from the heatsink with the transistors and has no holes to unscrew the transistors from the heatsink, have to desolder them first. I guess I'll have to take it apart and solder small wires to each test point and put it back together and test it that way.

 

[(*steve*)]

Show us a picture.

Unless the other components are surface mount, or have their leads totally obscured, you should be able to get a probe in somewhere.

 

[tip120]

I can get a probe in there without a problem, the issue is actually being able to see the traces on the bottom of the board without de-soldering the output transistors and taking the board off, drawing out a schematic/marking test points and then putting it all back.

The four transistors on the top are the outputs for the center channel, rear left and rear right which are all fine. There two output pairs on the bottom are for front left (the problematic one) and front right.

Also, it's a pioneer, not a panasonic, dunno why I keep mistaking it every time I mention it to someone.

 

[(*steve*)]

In general terms, the input is going to be in one place, the output in another and the signal path will move from one to the other. If it's stereo, then there are 2 identical channels, although the may not be laid out exactly the same.

From that you can generally make out which transistors belong to each channel -- and that's easier if you apply a signal to just one channel at a time.

You can probe the signal on the three pins of each transistor with respect to earth.

At some point you'll probably see a transistor that has a good signal on some pin(s) and a bad one on the other(s). Depending how the stages are coupled, and the actual fault, this may be the stage with the fault or the preceding stage.

 

[tip120]

In general terms, the input is going to be in one place, the output in another and the signal path will move from one to the other. If it's stereo, then there are 2 identical channels, although the may not be laid out exactly the same.

From that you can generally make out which transistors belong to each channel -- and that's easier if you apply a signal to just one channel at a time.

You can probe the signal on the three pins of each transistor with respect to earth.

At some point you'll probably see a transistor that has a good signal on some pin(s) and a bad one on the other(s). Depending how the stages are coupled, and the actual fault, this may be the stage with the fault or the preceding stage.

*Nods* I'll just have to poke around and see then, I know the board layout it a little strange since it's a 5 channel receiver, I wish I could find a schematic online for it that doesn't cost $20.

 

[JB@TRS]

Same problem but right side, similar waveform, possibly same parts.....driver and final. Bought 27 pieces from a community yard sale. bad fuses, cold solder joints and totally fried mostly but lot of stuff was 1-5 bucks!

 

[2bdangerous]

I have 1 with lower volume on left, can a nood with soldering skills fix it?I know this is old thread, so is the volume