Thandar TS3022S PSU current display problem

[tomwalter]

I've just bought a Thandar TS3022S PSU off Ebay and it all works fine, except one of the two current displays is misbehaving. Its actual power output is normal, ie it produces the correct displayed voltage and the current limiting works correctly, but the display has a mind of its own. On mains power on, with no outputs on, both supplies display and set as expected. If I have the faulty display set at 1.99A, when I turn on power out, the current display starts to drop to around -1.3A over about 2 minutes, irrespective of whether it has a load or not or if I change its setting whilst it does this. When I monitored actual current with my Fluke 175 DMM, it was obviously behaving as it should, limiting correctly, compared with the other supply when I tried the same with this. I swapped over the display unit boards and confirmed the fault is on this board as it transferred the fault with the display unit board. Has anyone any ideas? Regards, Tom

[SiriusHardware]

If this is one of the ones which uses a 'Four element CMOS switch' IC like a 4016 or 4066 as the meter mode selector, I have fixed several cases of 'Current meter going haywire' on those by replacing that IC. We have a lot of those Thandar bench supplies at work, of various generations.

[MrBungle]

IC7. Been there

[tomwalter]

Many thanks for the advice - I'll let you know how I get on.
Regards, Tom

[tomwalter]

Very many thanks MrBungle and SiriusHardware! Clear, precise and accurate diagnosis. Repair completed with total success!! My first successful ic repair. I still find desoldering ics on double sided boards a bit scary but alls well that ends well! Regards and thanks again! Tom

[tomwalter]

Just to be clear for any future visitors, it was IC7 (4066 analogue switch) that needed replacement. Tom

[See_Mos]

My advice from many years repairing industrial electronics is never try to desolder IC's on double sided boards. The IC's are far cheaper than the boards and if you loose a plated through hole fault finding can be a pig, especially if you don't have the circuit diagram.

The way I do it is to take a fine pair of flush side cutters and cut the pins close to the IC body. Lift out the old IC then using a fine soldering iron and a small magnetised screwdriver heat and lift out the individual pins one at a time then finally desolder the holes from the solder side. If you can remove the board completely heat the pins from the solder side and tap the board gently against a raised object such as the reel of solder. The pins drop out and then you can desolder the holes without damage.

[SiriusHardware]

I agree, my standard approach for anything with a lot of through hole pins soldered into a double sided PCB.

Provided

(a) you are sure the original part is faulty, and

(b) you definitely have another one.

Tom, glad our collective memories did not prove false.

If there was room I might have considered putting an IC socket in to make the IC easier to change in future since it is not unusual for it to fail. I don't remember if there is room to squeeze a socket in there or not.

[MrBungle]

I chop them off too. Should be enough room in those for a socket if I remember. I didn’t add one as I didn’t have any handy and didn’t want to buy any at the time.

Nice to know my IC failure crystal ball still works

[tomwalter]

Thanks guys - brilliant stuff! I did put in a socket for future proofing. I still can't quite get over this 'forum' environment. Pure magic! If only it had existed 50 years ago, my struggles with mini engines, TVs, radios etc would have been childs play! Thanks again, Tom