No question is stupid at this point. Yes I checked also for overlapping soldering, broken copper paths, cold/broken joints etc.
I agree, sure there's more than one thing not working. Caps can cause erratic behavior, but maybe more can do logic IC, controllers etc. It not my territory.
I'm quite sure the problem is one (or more) of the ICs. Tomorrow I'll do a last series of checks and tests. After that, if nothing new will emerge, I'll have to give up. Too time consuming and, even if if not much, I already spent enough money on that.
My goal is to improve my skill in vintage audio electronics repair, so it doesn't matter if I spend 30 euros on a piece of gear valued 20, as long as I learn something and improve my knowledge. But at this point, there's not much else I can do with this one.
I agree, sure there's more than one thing not working. Caps can cause erratic behavior, but maybe more can do logic IC, controllers etc. It not my territory.
I'm quite sure the problem is one (or more) of the ICs. Tomorrow I'll do a last series of checks and tests. After that, if nothing new will emerge, I'll have to give up. Too time consuming and, even if if not much, I already spent enough money on that.
My goal is to improve my skill in vintage audio electronics repair, so it doesn't matter if I spend 30 euros on a piece of gear valued 20, as long as I learn something and improve my knowledge. But at this point, there's not much else I can do with this one.
Hi m-tagg,
Something should be getting warm. They do make current trace probes, not a common item.
Anyway you have a diode getting warm and fuses going open. You can disconnect various circuits from the negative power rail to see when the current drops. What I would do first is let it run in standby and wait until something begins warming up. Feel around with your fingers. A thermal camera would be nice right about now too. Not easy, but you can do it.
I would have grabbed an oscilloscope by now to look at the power lines. I know you can't use one at the moment, but here is where one can be useful.
We had a few devices where the logic would fail, an entire PCB full of TTL chips. I simply connected a 5 VDC power supply (disconnecting the unit supply) and turned the voltage up until I had some current, maybe about 1/2 an ampere. I kept it below 5 VDC. Let it cook a short while and the defective chip would get pretty darned warm. That saved a lot of troubleshooting time, and worked every time. I wouldn't recommend doing the every time, but when the situation is right it just works.
Something should be getting warm. They do make current trace probes, not a common item.
Anyway you have a diode getting warm and fuses going open. You can disconnect various circuits from the negative power rail to see when the current drops. What I would do first is let it run in standby and wait until something begins warming up. Feel around with your fingers. A thermal camera would be nice right about now too. Not easy, but you can do it.
I would have grabbed an oscilloscope by now to look at the power lines. I know you can't use one at the moment, but here is where one can be useful.
We had a few devices where the logic would fail, an entire PCB full of TTL chips. I simply connected a 5 VDC power supply (disconnecting the unit supply) and turned the voltage up until I had some current, maybe about 1/2 an ampere. I kept it below 5 VDC. Let it cook a short while and the defective chip would get pretty darned warm. That saved a lot of troubleshooting time, and worked every time. I wouldn't recommend doing the every time, but when the situation is right it just works.
Thank you anatech, these are my last (and likely final) checks/measurements.
PS201 (N20) is rated 0,8A, I have 2.8 mA and 2.0 mA respectively in standby and power on modes. So it shouldn’t blow, but it does. Maybe there are some peaks when a particular operation is performed (pushing buttons or something, or it's just a matter of how it's kept on, or extra currents when switched on/off)).
D341 becomes hot quickly, but if I put the ammeter in series with it, I read almost nothing, and it’s Q209 becoming hot, no matter if I am in standby or power on mode. Measuring in place of the diode on its PCB pads I have 15 (stb) or 17 mA (on) and Q209 hot. Really confusing.
At this point I’m almost sure the problem is IC305, which is clearly not replaceable. Another suspect could be the op amp IC203, easy to replace. The very last thing I could try is to replace the op amp, but much more suspected is the other Sony IC.
PS201 (N20) is rated 0,8A, I have 2.8 mA and 2.0 mA respectively in standby and power on modes. So it shouldn’t blow, but it does. Maybe there are some peaks when a particular operation is performed (pushing buttons or something, or it's just a matter of how it's kept on, or extra currents when switched on/off)).
D341 becomes hot quickly, but if I put the ammeter in series with it, I read almost nothing, and it’s Q209 becoming hot, no matter if I am in standby or power on mode. Measuring in place of the diode on its PCB pads I have 15 (stb) or 17 mA (on) and Q209 hot. Really confusing.
At this point I’m almost sure the problem is IC305, which is clearly not replaceable. Another suspect could be the op amp IC203, easy to replace. The very last thing I could try is to replace the op amp, but much more suspected is the other Sony IC.
Check that ammeter as this is not logical. I think it is broken as the values are absolute nonsense.
An AC current, or very high crest factor pulses may not show up at all depending on the ammeter used.
One thing for sure is that at some point your current draw exceeds the 800 mA rating of the fuse. Why not replace the fuse with low value resistors? You can then measure the AC voltage and observe it getting warm without constantly replacing the fuse.
One thing for sure is that at some point your current draw exceeds the 800 mA rating of the fuse. Why not replace the fuse with low value resistors? You can then measure the AC voltage and observe it getting warm without constantly replacing the fuse.