[SOLVED] TTi PL303QMD Left side has wild voltages - Page 1
Forum section: electronics / repair

author: Cyclotron
author_rank: Unknown
timestamp: April 12, 2025, 07:43:44 pm
content: I acquired this TTi PL303QMD as needing repairs. As far as I can tell, the right side operates properly.The left side of the unit has ~1.4-1.5 volts across the positive and negative terminals even when the output is turned off.When I turn the output to "on" for the left, the voltage supply out put does not agree with the display at all. In fact the voltage adjustment knob can only be turned in the lower ~10-15% of its range and it will adjust the output voltage read at the postive and negative terminals from the ~1.4-1.5 to over 50v which is pretty high given the PSU is rated at 30v.I've just started to explore the unit. I've checked the voltages in some places and those appear to be fine. I've swapped the power boards left to right and the issue remains on the left. I've swapped the control boards left to right and issue remains with the left.The only think I havn't swapped is the transformers. The display board doesn't appear to have anything on it but the control for the adjustment knob. My plan is to test the knob output across the front panel.I have this haunt about that voltage while output is "off". I guessed out the gate that I had bad FET or BJT or similar. But given the board swaps that contain nearly all of those, it seems that might not be the case. Anyway, I just started down this explorative path and wanted to post here as I've seen references to other units of the same model exhibiting the same problem. I hope to learn of and repair this issue in hopes it can be used by others.Any insights or suggestions welcome!

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: April 13, 2025, 09:54:24 am
content: Hello Cyclotron,Quote from: Cyclotron on April 12, 2025, 07:43:44 pmThat's really weird.If the boards seem to be healthy, is there a problem with a cut wire, a loose connexion, the 'local - remote sense' switch ?Could not find any schematic for the moment.Contact Aim-TTI for help : describe the issue and the checks you already made and ask for schematics. Sometimes they respond (but not always).Does the output ON-OFF front button activate a relay ?Take a few hi-res pictures of its guts.Jot down the references of the main ICs (LM324 ?)

author: lowimpedance
author_rank: Super Contributor
timestamp: April 14, 2025, 03:58:36 am
content: link to thread by @zucca about his adventures with one of these supplies. Perhaps some useful info there.https://www.eevblog.com/forum/repair/venerable-aim-tti-pl303qmd-p-needs-help-output-trip-error/

author: Cyclotron
author_rank: Unknown
timestamp: May 01, 2025, 02:14:42 pm
content: I'm back to working on the "original" problem after making a bonehead mistake and shorting something on the right (master) side of the unit. I fixed that, and now I have a master side that works perfectly fine.The left side partially works; I can adjust voltages and current while the output is off.When the output is turned on, the displayed voltage drops to 10V or lower, though the true voltage output is much higher. The amperage side is fully controllable, and the display agrees.So, to state the problem more precisely, the left side has a full range of voltage (actually goes higher than it should) and a full range of current on the outputs.The current display is accurate, but the voltage display is not.Now, I need to figure out from the schematics how it determines the voltage for the display. I also need to find out if there's a way to calibrate the unit because that might be the problem.

author: Cyclotron
author_rank: Unknown
timestamp: May 04, 2025, 10:38:06 pm
content: I finally got back to this again. The part numbers and initial inspection made me believe the power and control boards are identical. But, there's at least one difference I found today.The power board for the master side (right side from the front of the PSU) has a wire link soldered across "LK-1".The LK-1 bypasses the PJ4-B and PJ4-A pins. If you are looking at the PJ4 connector for the master (right) side, the connector with the single brown wire can be on either of those pins, as they are identical electrically.This brown wire on PJ4 on the master side carries the gate control single through the "independent, tracking, parallel" switch to slave the PSU board on the slave side when in parallel mode.PJ4 on the slave is wired through that same switch to act as a link during independent and to connect brown to brown in parallel modes.Again, the master is working fine, and I've even calibrated it, and it is dead on.The two control boards appear to be identical and have a serial connection across the front board that establishes their relationship. The center switch mentioned above also lays out circuits unique to each side. I have swapped the two control boards left to right, and the master remains 100% working.However, the slave's gate voltage to control the output is wild. The dial controls it from 0 to 46 volts when it should be 0-30. The display of the voltage on that side, when enabled, is wrong, too, which is a symptom of the problem, I'm sure. For some reason, this side can't detect the true output voltage. Either that or it believes the current load is higher than it is, and wants to drive the output MOSFET up.The slave with the output turned off appears normal. The voltage and current controls function, and there appear to be no issues. Only when the output is turned on does the voltage reading immediately drop to 12.4 or below on the display, even though the output voltage is way higher. Once you get down low enough on the dial, the voltage display will start to drop, but it will never equal the output.I've run through the calibration on the slave, and you can't set the max voltage output or display.There is a chip that I'm not familiar with on this power board, and I've been studying it. The TCA785P phase control chip.Does anyone have any thoughts on the voltage monitoring and control circuit for this PSU?Thanks

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: May 04, 2025, 11:01:24 pm
content: Hello Cyclotron,TheTCA785is an old phase control chip that dates from the end of the 70'.I can hardly believe that such an oldie is still used in a recent lab power supply.Does that mean that the primary of the mains transformer is switched at 50 Hz like in a light dimmer ?Could you spot an SCR or a triac anywhere ?

author: Cyclotron
author_rank: Unknown
timestamp: May 05, 2025, 01:58:19 am
content: Quote from: timeandfrequency on May 04, 2025, 11:01:24 pmIndeed, the TCA785 is no longer availalbe per what I've looked into. And I'm perplexed as to what the unit uses it for. Q1 and Q2 are connected via diodes to the gate of a mosfet in the high 39v ciruit. Then Q1 and Q2 are connected to op amps that appeart to be terminating at a header that is unused in my model. I'm guessing the header is used in the model that has LAN/USB/GPIB?

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: May 05, 2025, 05:29:27 am
content: TCA785 is a PPM/PWM controller and looking at the waveforms in the datasheet, its operation is rather straightforward.Still waiting for the answers to the questions asked at Reply #1 (three weeks ago).I can't even figure out if it's a linear or a switching mode PS.

author: Cyclotron
author_rank: Unknown
timestamp: May 09, 2025, 02:23:18 am
content: @timeandfrequency has been a great resource to chat with on this project.After learning about LK-1 I replaced the hard wired connection with a jumper header so I could move the power boards around. I've swapped the power boards and the master continues to work and I've swapped the control boards and the master continues to work. The slave continues to exhibit its strange behavior.So, I examined the front board that the two control boards attach to for issues. So far I can't find anything. I've replaced the diode between the polarity outputs as they were not reading exactly the same but probably were fine. I've examined the board for cracks, resoldered the joints. At this point I am lost. I am going to replace the optocouplers and BJTs they use for the board to board serial between the control boards. I don't have evidence that there's a problem there but I don't know what else to look at either.I will go over the board again for possible components that could cause the strange behavior. I hate the parts shotgun but I don't have a lot of options at this point.

author: Cyclotron
author_rank: Unknown
timestamp: May 09, 2025, 01:53:50 pm
content: Here's a pic of the front display board.

author: Cyclotron
author_rank: Unknown
timestamp: May 09, 2025, 01:57:35 pm
content: Here's a pic of the control boards. As best I can tell, they are identical.Swapping them left to right does not move the issue and the master side always works fine.

author: Cyclotron
author_rank: Unknown
timestamp: May 09, 2025, 02:15:28 pm
content: Another interesting behavior. In parallel mode, the unit performs as expected. The combined left and right can output up to 30 v and 6 amps.

author: Cyclotron
author_rank: Unknown
timestamp: May 09, 2025, 02:54:56 pm
content: And we have a winner. R81 is in the sense circuitry on the front board (as seen in the red box in the attached image). I had not noticed the two resistors on that board before the circuits passed to the control boards..I'm taking a strong lesson from this project. Don't believe it just because the initial look at the boards indicates they are identical. That rabbit hole took a lot of time in my diagnostics. Swaptronics can bite you.Yesterday, I configured the power boards with a jumper for Lk-1 so I could configure them for master or slave. Swapping the power and control boards gave me a solid idea that the issue had to be on this front board.So, I started tracing the circuits visually under magnification, and there they were. I tested, and R81 was open. Replaced that, and it appears to be working.I will reassemble everything, run it through calibration, and post back how that turned out.

author: Cyclotron
author_rank: Unknown
timestamp: May 09, 2025, 07:23:43 pm
content: I'm just wrapping it up. The supply calibrated both sides just fine as expected. Ready to put it to work.A very, very big thank you to you all. I appreciate the collaboration.

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: May 09, 2025, 07:47:43 pm
content: Great achivement !A nice repair : just an SMD resistor to replace and this PS is back in businessYou may update the title of the post and add the magic word :

author: Cyclotron
author_rank: Unknown
timestamp: May 09, 2025, 07:54:09 pm
content: Quote from: timeandfrequency on April 13, 2025, 09:54:24 am@timeandfrequency, you've been a fantastic help and lab partner while completing this.I should also shout out to Aim-TTI, as they did respond to me and provided me with the documentation I asked for without hesitation. This certainly influences my future purchases.@Gertjan thank you very much for sharing information, tips, and encouragement.Last, because its humorous. I reached out to the Reddit poster to see how they got along with their unit. It turns out they didn't fix it but sold it to me... We had a good chuckle about that.