Repair of Philips PM5712 pulse generator

[unitelex]

Acquired this nice compact Philips PM5712 and it arrive today.
The on-off switch was not latching in the ON position but a slight spray of switch cleaning lubricant at the base of the switch where the latch spring is visible immediately solved that problem. No need to remove the red heat-shrunk covering over and insulating the switch body.

I tested the functions using my Tek scope and found that the 100uS setting on the DURATION selector switch resulted in no output. All other switch settings gave the expected output. I initially suspected the switch contacts but these showed good continuity by my Fluke DVM. No dry solder joints were evident. Inspecting the schematic the only possible remaining cause was C143, a 680nF 35V red boxed capacitor. It looks like a polycarbonate, but being so small for its value and the "+" symbol suggests it could be a tantalum...

I removed this capacitor and also one of the same value from the DELAY switch circuit.

My Peak LCR45 indicated the correct capacitance for both capacitors but critically the suspect capacitor has a much higher impedance, 17 Ohms vs 8 Ohms for its friend. A NOS 680nF 63V boxed white polycarbonate from my stock was somewhat larger but measured 2 Ohms, so both capacitors were replaced by the NOS Polycarbonate caps.

This solved the problem.

There are at least 4 more of the same red boxed "tant" capacitors in this unit. I will most likely replace those as they may be marginal or due to fail in the not too distant future.

Happy with this pulse generator, fits nicely in the last remaining shelf space I have free in the workshop.

Regards
Chris

[Phil G4SPZ]

Nicely done, Chris. I always like reading about fault-finding to component level.

I’ve had little experience with tantalums (or should that be ‘tantala’?) but have read numerous reports about their unreliability.

My own pet hate capacitors are those little clear polystyrene jobs with thin lead-out wires. They can be responsible for a host of intermittent faults and they tend to go low-capacitance or open-circuit. Intermittently, of course!