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!
What is not right? Can you describe the problem?
Check the ± 35 V and the regulated voltage for the opamps.
I'm gonna have to download the schematics again. I think I've lost it.
Ok, did some measurements with a 10V torroid that I had in hand.
All measurements at the LM3886s:
3) +10 <<<
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.
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.
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.
Is the feedback working? All solder joints OK? What supply voltage do you have. Are both amps broken? Have they worked before?
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.
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.
Working on it. Will let you know.
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:
Maybe something will jump at you. If not, that's ok, perhaps, like I said, I can use them as regulated power supplies.
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?
The tracks that got burned have a pale red circle around them:
I think those is ± 12 V out from the regulators.
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.
If you look at the schematics you see that the 12 V is for the opamps only. The opamp is OK? Do you have the polyester caps on the solder side?
Yes, the caps are on the solder side. Perhaps the opamp is not ok. How do I tell without removing it?
Check first that you have mounted in correctly, then you can measure the resistance between pin 4 and 8. If it's blown I'll you should measre near zero ohm.
I think we're on to something. While res between 4 and 8 is good in both (18.k), I started measuring all the pins and there is something not right between 7 and 6 in the one with the blown traces. I measured 140ohm between outb and -inb. In the good one this is 2.3M. Time to take out the AD8620?
C6 is also blown. It shows 140ohm as well.
C6, is it measured only this or in circuit? The opamp seems to be broken.
In circuit, but in circuit, the value of C6 in the other channel was in the Ms. I'll change them anyways while I'm at it. Got some nice sounding wimas.
Still waiting on parts..