Help fix amp - rail voltage gone crazy

[manina]

I had an issue with my amp - a Pioneer GM-x922.

Long story short, now it turns on however there is a difference in the rail voltage

The positive is 31V and the negative 45. According to the instructions, it should be in the 30V range so the negative is way out of range, especially taking into consideration that the rail capacitors are 35V rated.

I checked the output voltage of the TL494 Pin 9 and 10 and there is a difference here. Pin 9 is 1.1V while Pin 10 is 0.2V

My understanding is that these two should be equal however I can not figure out why they are not.

Any suggestion?

Thanks

[Perry Babin]

If you place the black meter probe on one of the non-bridging speaker terminals and the red probe on the center legs of the output transistors, do you still get different positive and negative rail voltages?

Are you sure you have the pin numbering on the 494 correct (see attached)?

[manina]

Thanks Perry - I was hoping you was going to answer.

I actually removed the finals as this amp mount some old Sanken that are difficult to find here - I'd like to avoid to fry them.

I double checked the numbering and I did it right. Pin 9 and 10 are the one driving the IRFIZ44N according to the amp service manual.

This amp has the rail capacitors rated 35V so anything in the 50V range is not well tolerated by them and if they short, due to the overvoltage exposure, the Sanken may toast - this was my assumption, at least.

Tomorrow I'll re-solder the Sanken and read the voltage as suggested.

In the meantime, I have measured the voltage at all pins of the 494. Pins 2 and 3 looks high to me - 3.52V

[Perry Babin]

If 9 and 10 have significantly different voltage, the IC may be defective, something is loading one of the outputs or some input to it is causing it to malfunction.

You can measure the rail voltage at the center legs of the rectifiers (D961/962) without reinstalling the outputs.

Without seeing the voltage on all of the pins, it's difficult to determine what 2 and 3 should be. Can you post the DC voltage on all of the pins of the 494? Place the black meter probe on chassis ground. Place the red meter probe on the point where you need to measure the voltage.

IC951
Pin 1:
Pin 2:
Pin 3:
Pin 4:
Pin 5:
Pin 6:
Pin 7:
Pin 8:
Pin 9:
Pin 10:
Pin 11:
Pin 12:
Pin 13:
Pin 14:
Pin 15:
Pin 16:

[manina]

Originally Posted by Perry Babin

If 9 and 10 have significantly different voltage, the IC may be defective, something is loading one of the outputs or some input to it is causing it to malfunction.

You can measure the rail voltage at the center legs of the rectifiers (D961/962) without reinstalling the outputs.

Without seeing the voltage on all of the pins, it's difficult to determine what 2 and 3 should be. Can you post the DC voltage on all of the pins of the 494? Place the black meter probe on chassis ground. Place the red meter probe on the point where you need to measure the voltage.

IC951
Pin 1: 0
Pin 2: 3.5 (after 1 minute it goes up to 4)
Pin 3: 3.5 (after 1 minute it goes up to 4)
Pin 4: 0.28
Pin 5: 1.66
Pin 6: 3.7
Pin 7: 0
Pin 8: 13.2 (the amp is supplied with 13.2 volts)
Pin 9: 1.28 (after 1 minute it goes down to 0.57)
Pin 10: 0.44 (after 1 minute it goes up to 0.57)
Pin 11: 13.2
Pin 12: 13.2
Pin 13: 5
Pin 14: 5
Pin 15: 2.4
Pin 16: 2.4

I originally measured the voltage at the diodes and re-done it today: +34 and -47.5. Measured after 1 minute the amp is ON, they are +33 and -48 while some of the IC951 voltages changes significantly (to me)

I also measured leg of the IRFIZ. Q963/5/7 have 0.7V while Q 964/6/8 have 0.65.

Giving the high negative rail voltage, I removed the rail capacitors. Hope this is not causing any problem to the readings

[Perry Babin]

For now, I think the IC may be OK. The voltage could be different because the soft start causes the voltage to vary when starting. If you don't read them at the same time, they may be different.

Which legs had ~0.7v (gate?)?

You need to have rail caps in while testing. Lower value caps rated for 50v would be OK.

Does the voltage adjustment potentiometer for the rail voltage (VR951) have any effect on the rail voltage? If so and you can get the negative rail to -35, reinstall the original rail caps.

You should pull the rectifiers and check them. Confirm that both diodes in each rectifier are OK. With your meter on diode check, you should read ~0.4 in one direction and open in the other direction (from the center leg to either outside leg).

The rail caps (one positive and one negative rail cap) have two terminals connected together (one terminal will be positive and one negative). Do you read 0 ohms from that point to the chassis ground terminal?

And also from the chassis ground terminal to the non-bridging speaker terminals?

[manina]

Soldered the 50V caps - the negative rails goes up to 55V while the positive is still OK.

I moved the VR951 and I can see the positive voltage going up and down (25V to 36V) while the negative has minimal variations (52-55).

The 0.7V were on the leg1.

Diodes are OK and the readings are ok - 0 ohm

Looks like the VR951 has no effect on the negative rail. I'll have a look to the components between it and the IC951

[Perry Babin]

Someone emailed me and said they've seen several Pioneer amps with open traces with similar problems. Have you looked at the board carefully to see if there were any open traces?

You may have to use a stiff (non-conductive) brush to loosen any solder mask that may have lifted but hasn't fallen away.

[manina]

Thanks for the help.

I guess I should first check the negative rail section between IC951 and the toroid for the open traces.

[manina]

Just found out that one of the IRFIZ was not properly working.

I ordered a new set of them and will re-check the voltage once they are in place.