TTi ts3022s bench supply current problem

[fran]

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

[Performa01]

You say it's the same problem on both channels, so there aren't that many options left.

Make sure the Isolated/Tracking switch is set to Isolated. Still the same problem on both channels?

If yes, then load one output (you say the voltage collapses) - does the 2nd output collapse as well (even though it's not loaded)?

If yes, then observe the voltages over C2 and C3 (maybe also C1) and see how much they drop when the output is loaded.

Best of luck!

[fran]

Right, so I actually got this sorted out..... But through brute force and ignorance rather than knowledge and elegance. I reflowed all solder joints on the board, replaced all transistors bar the 3055s and it worked. I truthfully can't say which it was, or if it was both. I can say I could see no evidence of cold solder joints under low magnification. As a result I suspect it may well have been interplay between several components.

The main thing now is she's back up and running hopefully for years to come.

Another thing crossed off the fixit list.

Thanks for the comments...

Fran