Hi all,
I'm looking for some help with a thandar ts3022s dual 30V 2A bench supply. It seems a fairly standard supply, both channels exhibit the same behaviour - power on, you can set voltage and current, but there is no current drive - when you add a load, the voltage collapses.
Investigation so far:
1. Close visual inspection under 3x mag to see any visual defects - none found
2. Clean all pots - all measuring smoothly
3. Measure all resistors, diodes on the control and PSU boards - all measure OK - but note this was done in circuit.
4. Did a diode test on the 2n3055s (darlington arrangement) - all showed ok
5. Replaced opamps anyway (TL072)
6. Pulled all the transistors - all tested Ok
7. Sense resistor is 0R1 - which is a challenge for any DMM to measure. I replaced it temporarily with a new (lower watt) one just to test, but no dice.
Basically I'm stumped. I was hoping there was a dead short on the output, but the diode tests good and no short at the terminals. I can only think that the 2n3055s aren't being driven (and they don't get warm) - and hence the voltage drops as the driver has very little current capability. I've attached the schematic here. The only odd thing I can see is that R26 (just under the 3055s in the schematic) gets quite hot - and is 1k rather than 2.2k. But its not burnt etc.
Any pointers you guys might have would be really appreciated. FYI, there is another thread here (
https://www.eevblog.com/forum/beginners/drifting-bench-power-supply-and-low-impedance-on-dmm-mv-range/) where the guy had drift, but thats not the same as my problem.
I've attached the schematic here
Fran