this is the best i could find on the net (a few transistors have different markings). i don't have the original papers...
Thanks for that. It's an improvement... I think!
if the offset would vary a few mv, it wouldn't be a problem but from +30mv to -50mv in 2 min (and still going down till oscillation) or less is odd. the other channel is fine and it stabilizes it's self at +20mv.
OK.
looking at the pcb there isn't any sign of trouble (leaking caps, signs of burned components,....).
There can be subtle clues. Also this is an unusual amplifier. I think it could be quite helpful to see some photos of the amplifier. Both sides of the PCB could be relevant.
as for the diodes of vt6 there are measured and fine, but could try replacement if i knew the equivalent as i can't find any info on the net....
When semiconductors get old, the faults they exhibit can be quite subtle. It's quite possible for a diode to measure OK, but still cause DC shifts as it warms up. They can also become sensitive to being moved or tapped, as well as temperature of course. It MAY be possible to locate these components by measuring voltages and currents, but this may be impractical and it's often easiest to locate them using freeze spray and/or a heat gun, or just replace any suspect components.
What is the part number for the diodes? I can't read it on that schematic.
as for the cooling spray i can't say that i have access to any cans (i have to order one).
I think you should do that.
after replacing the mirror transistors and the offset pot it seems that the hole thing offsets (maybe i didn't measure right the first time) right after c1(5uF). so could it be the supply ? i have replaced the filtering caps as they were bad, so could the rectifying bridge be faulty or something simple like that?
I doubt it would be the power supply. But this raises another issue. What is the history of this amplifier? When was it manufactured? How did it fail? What else have you done to it? Tell me everything you know about it
There may be a clue in there somewhere.
Having a look at the schematic, I notice that VT1 (type KT315A) is used as a zener diode to regulate the current in the differential amplifier (VT2 and VT4). This probably isn't a good idea. You could measure the collector-emitter voltage of VT1 and see whether it varies noticeably as the unit warms up.
I searched for KT315 (several transistors of this type in this amplifier) and discovered that it's a Russian device. Actually, it's a series of devices with significantly different specifications - VCEmax ranges from 15 to 40 volts! This might indicate poor control of the manufacturing process.
The data is at
http://www.5v.ru/ds/trnz/kt315.htm and there are photos at
http://www.freewebs.com/projektai/fm/tranzistoriai.htm and
http://www.155la3.ru/kt315_2.htm.
They seem to be packaged in some kind of potting resin, rather than being hermetically sealed. If that's what you have in that amplifier, I would be suspicious of all of them.
BC547 is a suitable replacement for these KT315 transistors, although the pin order is different and you would have to choose a gain suffix according to the specification of the KT315 variant you're replacing. The 2SC945 would be better, but neither Digikey nor Mouser carries it :-(
If the main smoothing capacitors had failed, I would replace (or at least measure) any other electrolytics. There's one in the feedback path back to the input stage, C5 I think.
Beyond that, I would suspect VT2 and VT4.