TTI 354D PSU Repair (Logic board side not switch mode side) - now fixed!

[ircoopr]

Hello all,

I've recently acquired a TTI 354D PSU, a lovely piece of kit but unfortunately not fully working. Both channels seem to have a slightly different issue and whilst I can find a lot of people online fixing issues with the power board, I think my issue is actually on the logic board side of things.

The video linked here describes the core issues:
https://drive.google.com/file/d/1iPNesgVyYmaets6A3IWqGodfeVYDheKq/view?usp=sharing


Conveniently there is a full manual available for the unit here, though nothing is mentioned about the 0.FL displayed:
http://pa2hs.kg-uitdaging.nl/documenten/TTI%20EX354D,%20T%20&%20Tv%20Series%20Service%20Manual.pdf

Having a dig around I can see that all of the rails (from Power Board -> Logic Board) appear to be working OK, including the main output rail that for both sides is still controlable by switching the output on and changing the voltage set point. Additionally, by running channel 1 on channel 2's power board output (and vice-versa) the issues follow the logic board.

From a visual inspection the logic board appears OK, nothing obvious has failed.

Any advice is greatly appreciated! I've still got a lot of my own digging to do on this one but thought I'd throw it out there in case this is a common problem that my googl'ing as so far failed to find.
Thanks
Ian

[ircoopr]

To add to this story a little, I also did some testing under load to understand whether the issue was with the output itself or the display of the output.

• Both channels have voltage control and this can be changed by the setpoint potentiometers
• CH1 - Voltage and current control appear to work and appear on display as logical values, as does the current limit (tested using DC motor). When the output switch is ON this channel appears to work perfect, just with the switch OFF I am unable to set the preset voltage and current limit. Potentially this is something to do with Multiplexer IC4?
• CH2 - Voltage control works but neither voltage nor current appear on display in either the on or off states. The current control/limit doesn't appear to function.
• Again the issues follow the logic card channel, moving them to the 'other' power board output doesn't change the above.

[m k]

Ref voltages also checked?

My guess for 0.FL is overflow.

Adjustments seems to be classic pots so you can check and follow how OpAmp outputs are doing.
Seems also that sides are identical so if easy you can swap those PIC chips and see what happens.

[ircoopr]

All of the voltages from the power board seem to be OK (not sure what tolerence these should be measured within but appear reasonable).

I hadn't thought to swap the PIC's and that's a great idea.

Agreed that more investigation is going to be required, had slightly hoped that someone would swoop in exclaiming that this is a common known fault with the supply, alas it would seem not!

[TaylorD93]

Out of curiosity, I have a EX354D PSU, however it seems to be a pre V1 pcb as the layout doesn't quite match the general assy drawing in the service manual. Also some components didn't quite match the schematic. Mine is a single layer board with 555 starter.

What version is yours? Mine catastrophically blew up 2 months after I first repaired it and without all the documentation I'm feeling a bit lost as to where to start with the repair.

Sorry to hijack your thread, hope you have success in repairing yours. If there's anything you need checking on an alternative logic board, Mine seemed to be fine. So can compare it if needed.

[ircoopr]

Thanks for the offer of support! My board's appear to be version 3 I'm afraid, but I'm happy to measure bits if that'll help at all with your repairs.

[ircoopr]

Great news!

After doing some poking around I have replaced the analogue multiplexer (IC4), having noticed that the voltage setpoint input to this mux was being pulled down to ~-5V.  I now have 2 fully working channels. (The second channel which seemed worse before has been fixed by also replacing it's same multiplexer)

Great result, thanks for the help!

[m k]

Very good.

Maybe even better that you came back and closed the case.

[ircoopr]

Thanks! Figured it would be useful if someone else ends up in the same scenario.

Topic title changed now as well to reflect the fix