Zenford Zeigler Retrofit

Correct Tommy, thanks for the tip. Will definately have a look at them. Maybe a simple question, but do you know the function of the diodes between the transistors?

I would suggest putting the multimeter on Diode-check mode an then check every combination of the three pins on the transistors in both directions (6 measurements) and be very suspicious of any that show different numbers to the good drives.

What Andy said and those diodes are there to protect those transistors from the back EMF and voltage generated from motors when free wheeling or loss of power and from induction created when switching on/off etc. Do check those diodes too, but you have to separate them before you do so.

Thanks guys. Will be working on the machine this afternoon. Appreciate the help. Will let you know what i find.

Well no great luck with the transistors / diodes on the bottom of the drive. I pretty much tested all the drives and they seemed good. No shorted components, or any high / low diode drops. The drives have 2 sections that can separate. The heatsink side has the transistors, diodes and some resistors, and that plugs into the pcb (control circuit). I ended up switching over one of the good running drives transistor section with a faulty full speed on enable transistor section, and the fault stayed with the pcb control, not the transistor section.

Will have to look into it a bit more, checking components etc.

Anything else you recommend to check out in particular?

Did you already replace all old capacitors? They are normally a ticking time bomb in power electronics.
Mark