Help fix amp - rail voltage gone crazy

[Perry Babin]

The rail voltage isn't likely to change with the new FETs. You can operate the amp with one missing power supply FET to allow you to continue troubleshooting.

Did you look for the open traces?

[manina]

I spent few time looking at all traces and did not found any open one.

I fitted two FETs back (one per rail side) and the voltage is still crazy.

I checked another 922 amp and found that the IC951 pin 9&10 voltage is higher - it's in the 4V range vs the 0.5V of the defective amp.

Now, randomly, the amp does not start when I connect the remote. To be precise, it turns on but the leds flash and the rail voltage stays low (10-15V) - when stop flashing, the rails are back to the "normal" +30/-52V.

[Perry Babin]

The voltage on 9 and 10 isn't important right now as long as the IC is producing pulses.

Do you have an oscilloscope?

[manina]

Unfortunately I'm missing the scope.

[Eva]

Is that PSU regulated? Does it have output inductors right after the secondary side diodes without electrolytic capacitors first? Does it have two separate inductors or one with two windings on the same core? A couple of pictures would help.

Does the negative rail drop when some load is applied? (Use a power resistor to draw 0.1A to 1A or so...)

Taking a look at transformer waveforms with an oscilloscope would give a lot of information about the problem. Higher than expected output voltage in unregulated SMPS can result from unexpected resonances (shorted turns in the transformer, small ceramic caps shorted, etc...)

Note that some not-so-good push-pull SMPS designs will produce higher than expected output voltage just when the output is not loaded. Operating the SMPS at the wrong frequency can do the same.

Once I got an amplifier with wrong rail voltages that cooked supply electrolytics and snubber resistors. Oscilloscope revealed that it was operating at 300Khz rather than 30Khz. Timing capacitor was open. The SMPS was no longer operated in resonant mode (zero voltage turn on for the primary side MOSFET) resulting in heavy overshoot.

[manina]

Here it is the amp as it stands right now.

It's regulated an hads two inductors (left side of the pic) just after the diodes.

As I removed the finals, I can not apply any load.

Unfortunately, I have no access to a scope so it's hard to verify what you mentioned, Eva.

As only the negative rail is wrong, has to something in the negative regulator path.

I have the pdf of the service manual, if this may help to help me.

[Eva]

I mean applying a load from the negative rail to ground (or from the positive rail to the negative one), drawing current directly from the supply rails. The voltage will probably drop with some load current.

On that kind of power supply, due to the separate inductors, the negative rail is usually not under control and voltage is very likely to rise if there is not a minimum load current. The bias current of the output stage was probably helping a lot with that (most car amplifiers are class B but I think this one is true AB as older classic ones from Pioneer).

So with output devices removed the negative rail is very likely to be higher than expected, even on a perfectly working amplifier.

If you have changed output devices (particularly if the new ones are of a different type), you should check bias current too.

btw: The image does not expand when I click on it, I think it already appears full size (quite small).

[manina]

OK -Understood. I'll apply the load as you said.

I may have reduced the pic when I uploaded - let me try again.

The output device are still the "old" ones. I just removed them to check if they were still OK and never re-mounted as I was afraid to fry them.

Pioneer claims this is a class-A amp so it has to be an AB with an high bias.

[manina]

I tried a 3.9ohm/10W resistor and the fuse blew so I used a 470ohm/10W resistor and the negative voltage drops but only of very few volts - from 52 to 49

[Eva]

3.9 ohms was a too low load (over 10A and 500W with 47V), and 470 is a bit on the high side (100mA with 47V) but should have been enough to produce a substantial voltage drop.

The inductor of the negative rail may have shorted turns. Try to swap the output inductors of the PSU.

If you have access to a frequency meter, try to check the operating freq of the SMPS.

Also, could you mail me the service manual? Use my messenger address (it appears by clicking on the icon under "Location:" in my posts).