Help needed to debug QRP-02

[jamh]

heaveno Per Anders,

It has been a long time that I have corresponded with you. I ordered 4 of your QRP-02 boards when they came out. Maybe 10 years ago?

I populated them back then and put one pair in a beautiful chassis and they are still my main (and favorite) amps.

Here is my question. The other day I dusted out the other two boards to put then to use but there is something not right. How would you suggest I go about finding what's wrong? I'm confident the power supply is good. It's one of the smd's I think.

Failing that I should fix them, here's another question: could I somehow just use them as a regulated power supply? Where could I tap into the supply? And what should I sever?

Anyways, these are just some thoughts. Thanks!

[peranders]

What is not right? Can you describe the problem?

Check the ± 35 V and the regulated voltage for the opamps.

[jamh]

I'm gonna have to download the schematics again.  I think I've lost it.

[jamh]

Ok, did some measurements with a 10V torroid that I had in hand.

All measurements at the LM3886s:

Module 1:
1) +11
2) 0.015
3) -2.1
4) -11
5) 11
6) 0.19
7) 0
8) -2.0
9) -0.2
10) 0.09
11) 0

Module 2:
1) +10.75
2) 0.1
3) +10  <<<
4) -10.75
5) +10.75
6) 0.2
7) 0.1
8) -2
9) 0.2
10) 0.1
11) 0

I measured the output+, for module 1, it was -2V DC, 0.3 VAC.  For module 2 it was 1.5V DC, 1.4V AC.

Let me know if you want me to measure anywhere else.  Thanks.

[peranders]

You must have more than ± 12 VDC as supply voltage since you have both mute and under voltage protection in the LM3886. Besides that I have chosen a mute resistors for a 2 x 24 VAC transformer.

[jamh]

Yes, but don't you think it's a bit odd to have 10V at pin 3?  I'm just trying to figure out what's broken.  Also, the AC at output.

[peranders]

Is the feedback working? All solder joints OK? What supply voltage do you have. Are both amps broken? Have they worked before?

[jamh]

Well I don't remember if I tested them back then or not, but they've never been used before.  I could get a tranny, but I was hoping I could identify with lower voltages what's shorting or what's blown, something like measure the voltage at point x, and it should be vv, and at y should be v, etc..  If they're blown, I didn't want to spend money on an expensive tranny.

[peranders]

Can't you borrow a regulated power supply, 2 x 30 V or so? You'll need only 200-300 mA.

To start with you buffer should have 1-2 mV out.

[jamh]

Working on it. Will let you know.

[jamh]

So I got a 38V transformer and the results were not so good.  Two of the PCB traces got smokey and I shut it down quickly.  Prior to that I looked carefully at all the solder joints.  Not much wrong anywhere that I could see.

I'm not too hopeful we can debug this, nevertheless I took some resistance measurements from some of the PCB points (vs GND) and thought maybe something can be learned from them.  There were 21 little spots on the PCBs that I could measure.  Since they're not labeled, this is the best way I thought of reporting them:

http://i.imgur.com/X87JpE3.jpg

Maybe something will jump at you.  If not, that's ok, perhaps, like I said, I can use them as regulated power supplies.

[peranders]

It's hard to tell where the problem is just by doing resistance measurements. Which tracks got burnt? Could you find a regulated power supply?

[jamh]

The tracks that got burned have a pale red circle around them:

http://i.imgur.com/XTR0a4M.jpg

[peranders]

I think those is ± 12 V out from the regulators.

[jamh]

Which doesn't tell us much.  The problem is downstream from the regulators.  Is there anything I can measure to help us narrow it down?  I have a regulated PS at 12V but I'm not sure what you want me to do with it.  I also have a couple of torroids that can do a variety of AC values.