Hi, I have one of the old brown / cream Thurlby PL320 (30v / 2amp) bench power supply units with a current meter issue.
The unit is working, i.e. with the output on, the output voltage is good and it'll take a load ok.
The voltage meter is accurate / stable, both with the output on and off.
The current meter, however, displays random, varying numbers, and sometimes OFL, changing continually. Essentially it's erratic, and all over the shop. Switching on the output changes the values, but still erratic / random. Values have no resemblance to set (output off) or active current (output on with/without load). It just doesn't sit still or give a reading that makes any sense. Could be 000, 2615, 3954, etc. etc., changing several times a second.
With the aid of the service manual, I've checked / tested many components but yet to find anything overly amiss:
- Capacitors seem ok (changed a couple that were wide of tolerance).
- Resistors seem ok - checked pretty much all in the current control side.
- Diodes all seem ok (think I've checked them all)
- Transistors seem ok (only checked the 2 or 3 around the current side of things - rest seem to drive the display, hence assumed ok.
- R8 (0.5R W/W current sense) seems ok and with a 5v / 80mA load got a v-drop of approx. 0.04v.
- VR1 (10k Log Pot for Current control) was iffy - reading 17k and very jumpy, so have swapped that for a 10k linear pot for now.
- Also swapped the ADD3701 chips over and issue remained on current side, so both of those appear good.
Any ideas on what to check next much appreciated.
Thanks.