One of the perks visiting office these days was an opportunity to grab a split-rail PSU which was destined to the dumpster.
The symptoms:
- On power up you can hear some relays click
- You also can hear the buzz of toroidal transformer
- Screen and buttons did not work
- Screen and buttons could occasionally turn on (e.g. after smacking or shaking the unit) and then randomly turn off a second or two later
- When screen was on - device was functional, I could enable PSU outputs which stayed active even after screen went back off
- I could enable outputs with the screen OFF by holding output button and smacking/shaking the unit, but could not see a voltage/current setting
Obviously something was wrong with control panel and more likely its power supply circuitry (likely loose contact somewhere).
Taking the cover off was relatively quick, but the control panel itself is a dual PCB sandwich connected with pin-headers and held up by terminals:
So I had to remove all terminals to take the panel apparat completely:
This allowed to trace power supply circuit to the panel components:
The control panel is supplied by AC input from transformer (4-grey wires connecting to the top of the control panel).
After rectifier there are two LDOs in parallel:
- 3V3 to supply a PIC microcontroller - the brain of this PSU
- 5V with 2x 1N4004 in series on its output
This second circuitry produces approx. 3.5V to the front board, which is used to light up buttons and 7-segment LEDs.
The problem was that one of these 1N4004 diodes was glitchy. Its internal contacts were loose.
So depending on surrounding temperature or vibrations 1N4004 was either working or it was a broken circuit.
Just tapping on it or bending the pin turned the PSU panel ON.
https://youtu.be/yT6edyKEJuoEventually I ended up replacing 1N4004 as a quickest fix, but if it will ever break again I will replace diodes with a small buck converter module
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