The Zurich 30 Amp Regulated Power Supply was marketed under several other names, including MFJ. I have had mine for some 12 or 15 years now I recon. It is a ‘real’ PSU with a chunky transformer.
When it started to play up I was on air, I saw 10 Amp or more pulses of current indicated on the Ammeter. I was operating on PSK31 with a friend. When I mentioned it he said ‘does that not worry you it could damage your rig’.
No, no worries here, this was the practical demonstration of one more reason for the system of 12 Volt supply I use in the shack. The PSU float charges two 40 Amp Hour Lead Acid Batteries individually via Schottky Diode isolators. A fault like this did not apply 15 to 17 Volts to my expensive radios, it might just do the batteries a little good by giving them a few de-sulphating pulses.
‘Surely’ he said ‘your power supply is protected against Over Voltage’.
Yes it is, although not Over Current, but this fault was obviously not related to over voltage, the voltage involved must be a ‘legal’ one. Any voltage between 0 and 15 is ‘legal’ it being a variable voltage supply with a front panel adjustment pot.
This should have been a good clue to the cause of the problem, I didn’t find it instantly, adjusting the voltage set pot and wriggling it made no difference to the pulses of extra current so I ruled it out at first. Finally I substituted a large preset pot and the problem was cured. I suspect that the connection of the earthy end of the potentiometer track was intermittent possibly under the riveted tag.
I ordered three 10K Ohm pots from different sources and one turned out to be very similar physically to the original.
The original has an indent at the center 13.8 Volt point, the replacement does not. No problem I don’t use that point anyway, remember the Shottky Diodes, their forward volt drop means I set the output above 13.80. The original also has a splined shaft the new one is plain so I provided a different pointer knob.
Whilst inside I soldered the wire wrap connections on the control PCB.
Also the wire wrap connections to the load share resistors, these connections do get warm, I am much happier to see some solder on these after several years of use and possible corrosion of contact.
The Regulator Transistor connections on these boards had also had many hot /cold cycles so the joints were all re soldered.
Finally back in use on the rack, Battery Terminal Voltages set to 13.8 Volts.
I now have circuits of several (not all) models of the Zurich family of PSU’s they are stored on the cloud, go HERE.
Jim G3KAF contacted me, as have several others over the years, about a problem he had with this well loved PSU. Jim had a voltage regulation problem. The Voltage dropped when current increased.
Jim like me would have liked a circuit diagram for the PSU, he managed to identify the value of a capacitor from my photograph. The fault remained though, only a few days later he found the problem.
On the small board with all the components the diode which rectifies the low voltage ac supply (next to the 470uF 12v capacitor) was highly suspect because the 470uF was getting warm!. When I took the diode out and checked it, I found it was a dead short circuit. Having replaced it the PSU is now working superbly well. I had changed all the 8 smoothing capacitors and the bridge rectifier and now I can go from zero amps to 20 amps and the voltage only drops 10mV (0.01 of a volt). I suspect it has not performed this well for years.
Thanks Jim for the information, if anyone has a circuit diagram or other information / faults found, please contact me.