QRV-08 power up

[svarthvitt]

The reason I was asking, is that the amp was acting up when I last used it. Its been of little use in the summer anyways.

Now when I open it to measure, I see that the leds for the two regulators in the middle is disappearing 5 seconds after power up. One negative, and one positive. I´m sorry to bother you again, but do you have any suggestions on where to start?

I really want this to work correctly!


Thank you.

[peranders]

I must have more details in order to help you. Which supply voltages are OK?

[svarthvitt]

These are the supply voltages:

T27     13,86
T28    -0,94
V27    1,39
V28    -13,85

T28 and V27 are also +-13,86 at startup, but then the fuses goes after five seconds. It makes a hissing sound before the fuses cut the supply.

Just tell me what else you might need.

[peranders]

Some background: Both channels worked perfectly and now suddenly you have a problem with the both channels?

First I'll advice you to visually inspect the board.

If you remove R3, no current at all should flow. If it stills consumes current then check all transistors with diode test. Start with output transistors and go backwards.

[svarthvitt]

Hi,
I sort of gave up on troubleshooting this, and put it in a box somewhere. I found the amp again two weeks ago, and still cannot find any faults. I have replaced some op-amps, desoldered and tested some transistors against new ones, and still cannot find any defect parts.

I have visually inspected the board a lot of times now, and can not find any mistakes. It looks worse now than before the troubleshooting because of all the desoldering. The amp was working for a few weeks before it broke down. It's hard for me to say if it was working perfectly, since i did not bother to measure anything with my oscilloscope. It seemed ok.

I have a small humming sound from the board itself (transformers, I guess) that quits immediately after the fuses kick in. The weird part, and hopefully where the solution lies, is that the fault is in the positive side of one channel, and the negative side of the other channel. And its almost completely synchronized.

Is there any way i can test the parts while still on the pcb? The resistances around the board seems to be the same on a lot of random test points.

I really could use a good headphone amplifier to make my days at work more enjoyable. I hope you can help me out!

[peranders]

Have you removed the R3?

[svarthvitt]

Yes. Same result with r3 removed.

[svarthvitt]

heaveno again,
I tried making this amplifier work again. I replaced some parts and looked over everything again once more. I really cannot find the fault here.
I tried filming the amplifier with a thermal camera, and BR1 and BR4 are running really hot. None of the resistors or silicone in the middle seems to heat up, except E29 and R29.

Does this help at all? Any other input on what I could try? I really want this thing running.


Regards,
Svarthvitt

[peranders]

The R29 should be 470 kohm. IS it really this value? Is the opamp soldiered in the right direction?

[svarthvitt]

Yes, the resistors are 470k, and the opamps are in the right direction.

[peranders]

I'm trying to get the picture. The amp worked good and then after some time switched off it doesn't and the both channels seems to have the same fault?

I recommend that you cut the supply lines to both channels. Check that you have the correct voltage. 13.93 V is the typical value.

[svarthvitt]

The amp was working for a while before it went silent.

Should I disconnect the supply to the psu or the amp? I would guess the regulators behave badly with no load?

[peranders]

Are all electrolytic caps mounted correctly? Both channels are dead? If yes, then it's not a random error.

[svarthvitt]

Hi again,

Yes, the electrolytics are mounted correctly.

The supply lines seems to go to a lot of components in the amplifier, could you tell me the part numbers to remove and where to measure the voltage?


Thank you again!

[peranders]

I'll suggest that you try to get a working power supply first. Then you'll have to cut four traces, the power lines to each amp.