Thandar TS3022S current display problem - Page 1
Forum section: electronics / testgear

author: tomwalter
author_rank: Newbie
timestamp: January 09, 2019, 05:11:27 pm
content: I've just bought a Thandar TS3022S 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. I would be very grateful if someone could point me in the right direction. I'm recently retired and catching up on lost time with old radios! My electronics skills are pretty basic but I'm still keen to learn! This is my first posting here. Happy New Year to all. Regards, Tom G4KWL

author: bd139
author_rank: Super Contributor
timestamp: January 09, 2019, 05:49:03 pm
content: Good news. These are totally made from standard bits which are all available from RS etc unlike some TTi supplies.Service manual here:https://www.industrialcalibration.co.uk/downloads/TTi%20%27S%27%20series%20power%20supplies%20manual.pdfFirstly if you have a 2A one it'll kick out slightly more than 2A but wont read it. This is normal.Display issues on these are common. First step is to check all the supply voltages after the regulator on the control board. Then the IN and REF inputs to the ADC IC (pins 31, 30, 35, 36) to make sure they don't drift with the display. If they do, proceed to fix the regulator. If the IN drifts, it's probably IC7 (4066 analogue switch). Check supply voltages etc and then input voltages to that. If input side of the switches is different to output, replace the IC. Sometimes they go leaky.Also being Thandar, inspect the PCB for dry joints. very common from them.

author: tomwalter
author_rank: Newbie
timestamp: January 09, 2019, 10:14:07 pm
content: Very many thanks for the reply bd139. I'll follow up your advice and let you know how I get on.Regards, Tom

author: tomwalter
author_rank: Newbie
timestamp: January 11, 2019, 01:41:50 pm
content: Very many thanks bd139. 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. PS. it was IC7 (4066 analogue switch) that needed replacement.

author: bd139
author_rank: Super Contributor
timestamp: January 11, 2019, 01:43:36 pm
content: Glad to hearI don’t desolder the ICs, just cut all the legs off and then desolder them individually. Slightly less scary!

author: tomwalter
author_rank: Newbie
timestamp: January 12, 2019, 04:23:14 pm
content: Now why didn't I think of that. Excellent plan - will do next time!Cheers, Tom