Hi All and thanks for your responses!

Mike, I thought the same that maybe the flash controls the output by sending a cutoff signal to the thyristor in the pack that cuts the HV. Would this make sense that the thyristor is placed close to the capacitor? Perhaps, since the secondary heads made for these have their own capacitor so they probably have their own thyristor as well. I thought perhaps the low voltage line both supplies (control) power to the head and carries a shut off control signal back to the pack??? Maybe the fault is in that line between the pack connector and the thyristor??? Or the thyristor itself. I know very little about these circuits so I'm just guessing.

I have checked the contacts and they look good. The whole flash is in excellent condition. I pulled the pack apart and could see nothing obvious inside. I didn't pull the main board off the alloy frame so I haven't seen under there for dry joints yet.

Matt, Interesting point about the N-20 that would shoot the above theory down, I actually thought the N-20 plugged into, and powered, the pack. I didn't know it powered the head directly from AC? I'll look into that point. And yep, I've tried both sockets, both behave the same.

I can't get the head or lead to fail on another good pack, and I can't get the pack to work (except about 3 random occurrences) with a second known good head and lead.

Diapositivo, I thought the thyristor was only responsible for cutting power to the head and it gets told to act by other circuitry, auto or manual? Again, not fully sure? Also the flikr page is about the 45 series i think? As an aside regarding them I have measured different (lower) voltages on the cap when connecting Nihm to the Alkaline contacts, they work but Nimh should go through the Nimh contacts. That is the 45's though, I'm running the 60's on li-ion batts.

Chan Tran, It bothers me too!!! The cap is definitely in the pack. BIG sucker too. Scary thing it is

Thanks again everyone for your helpful responses and I hope I've understood them correctly so far!

Any knowledge of how to get a schematic or service manual on the 60 CT-4?