Strand Mini2 Dimmer pack faults/circuit sought

[ChrisC]

Hi -

I've just got a rather ancient Strand Mini-2 to fix, with two of the channels down. The symptoms are no output, with fuses intact. On prodding around with a DMM, the good channels have approx 16v DC across the bigger 220uF electrolytic capacitor (C1) on the PCB while the dead channels are 2v or less, so something is pulling the internal supply down. Before I start attacking it with a soldering iron, has anyone any stock fault fixes (eg replace the electrolytic capacitor!) ?

 

If not, a pdf of the PCB circuit diagram would be helpful - the pack wiring diagrams are already available on the Strand Archive site, but not the PCB internals. It's PCB 196/2 (I think), and seems to be common to Mini2's, Perma 12's and possibly STM dimmer modules as well.

 

Thanks in hopeful advance,

Chris

[ChrisC]

To answer my own question, I found the circuit for the drive PCBs in the MCM installation manual, which has just become available on the Strand Archive website.

It's the S-law trigger card, Ref 1101, which is identical to the Mini-2 trigger card apart from one transistor substitute.

The fault seems to be in the oscillator which drives the thyristors via a 4-winding pulse transformer....