Another Mastech HY3005 overvoltage problem [Fixed]

[bborisov567]

Here is the problem - when there is a sudden load the supply puts out 50v for a second and then it goes back to the set voltage. If i hook a pulsing load, for example a pwm generating board the output stays constanly at 50v. I have replace all electrolytic caps. Measured the output transistors with multimeter - they appear to be ok. Check the supply rails for the op-amp. +12v, -6v are good, -12v measures -15. Also i have changed the trimmer pots on the board. Where to look for the problem? Could it be trimmers not adjusted properly or bad output transistors?

[bborisov567]

I fixed it! After a bit more digging i found that voltage base emiter on V13 barely changes when the voltage spiking occurs while voltage base emitter on V14 goes to almost zero. Also there is a small SMD cap between base and collector on V13 (pay attentention to that cap). Replaced V13 but being lazy i didnt solder back the cap immediately. Voila! The supply is working flawlessly. Happy i fixed it soldered back the smd cap. Tested it and the problem came up again. Then reallised the whole problem is coming from the cap. Probably when there is some kind of oscilation the cap is letting current from collector to collector to base while charging. I dont know if somebody added it at some point because it is not in the schematic.