TTI PL303QMD power supply – left channel not working - Page 1
Forum section: electronics / repair

author: chris_kz
author_rank: Newbie
timestamp: February 05, 2025, 04:52:02 pm
content: Hi,I have a TTI PL303QMD power supply with a defective left channel that I’m trying to fix.The right side is working as expected. The left side however, outputs around 50v when the voltage is turned all the way up and about 2v when the voltage is all the way down. At the same time the volt meter for the left side shows from 0.2v to 10.5v when the output is enabled. It actually works as expected when the output is disabled. Also, when the output is disabled the voltage on the binding posts reads around 1.8v.My first guess is that there is something wrong with the power board for the left side. Although the volt meter not matching the actual output voltage is a bit concerning.I would appreciate any help in troubleshooting this, especially if anyone has the schematic for the power supply as I’m not finding anything out there.I’m also going to email TTI support and see what they say.Thanks!

author: benj38
author_rank: Frequent Contributor
timestamp: February 05, 2025, 09:22:03 pm
content: I never worked with this model of PSU before, so what I am saying may make no sense, but it may be worth verifying that you don't have the PSU in "Isolated Tracking" mode.

author: chris_kz
author_rank: Newbie
timestamp: February 11, 2025, 03:51:17 pm
content: Finally found some time to work on the power supply this weekend.The power boards for the two channels look identical so I tried swapping them. The left channel still had the same exact issue but now the right one started acting all weird. It was not regulating any longer and was just outputting some arbitrary voltage. I did not mess with it for too long because I was worried something will start smoking. Luckily swapping the boards back reverted thing to their original state and now the right channel works again.This makes me thing there might be an issue in the control board for the left channel in addition to any issues with the power board. Will keep prodding around but unfortunately without the schematics this might be a lost cause. Still waiting toe hear back from TTI support.I would appreciate any ideas on what to try next.

author: voltsandjolts
author_rank: Unknown
timestamp: February 11, 2025, 04:10:52 pm
content: Maybe there is some hints here...https://www.eevblog.com/forum/repair/venerable-aim-tti-pl303qmd-p-needs-help-output-trip-error/

author: Zucca
author_rank: Unknown
timestamp: February 11, 2025, 04:30:29 pm
content: you need the schematics, shoot me a PM

author: chris_kz
author_rank: Newbie
timestamp: February 11, 2025, 05:07:34 pm
content: Thanks for the suggestion! Unfortunately that was one of the first things I checked.

author: Zucca
author_rank: Unknown
timestamp: February 11, 2025, 06:00:33 pm
content: Quote from: chris_kz on February 11, 2025, 03:51:17 pmThey are not, look at the jumper wire(s).

author: chris_kz
author_rank: Newbie
timestamp: February 26, 2025, 01:41:19 pm
content: Thanks, I'll take a look tonight when I get to my workbench.

author: tom66
author_rank: Super Contributor
timestamp: February 26, 2025, 01:48:55 pm
content: In my experience, TTI will provide schematics for their equipment if you ask, but it can take them a bit of time to respond. Give them a week or so. Good company and only up the road from me in Huntingdon, UK.