Hi,

I have a v2.60 kit here (Werner's kit from the previous thread, I'm fixing it for him) - and R8 is blown (open circuit). It was marked as a 4M7 resistor, yet my v2.61 kit (and circuit diagram) have a 470k resistor in that position. I can't find a matching circuit diagram for the v2.60 kit to check what it's supposed to be, the only other one I've found on here has the resistor in this position marked as R10?

Anyone got a working V2.60 kit and can see what they've got in there for R8 (or have a schematic and can tell me?) - R8 on this pcb is the one under the GM tube, right next to the two high voltage capacitors C10 and C13.

I'm tempted to put a new 4M7 one in, but don't want it to blow again if it's supposed to be 470k.

Cheers,
Gary.

Aha, thank you! Well I think that explains it, R8 should be 470k, and this one had 4M7 in, poor thing was probably overheating and blew up. :-)
Cheers,
Gary.

It looks to me that this resistor is used to recharge C9 after each detection click at a predictable fast rate, so that high counts can be seen reliably. Using a value 10x higher would I think have little effect at low count rates (I could be wrong). But I don't see how a higher value would result in overheating; lower yes but not higher. I would check other components too for proper values, especially DZ4, R9 and C9, also R12 and R18, in case they are the cause of the over-current.

Hi Norbert.

Overheating was probably the wrong thing to say actually, it doesn't look at all discoloured like resistors usually do when they've overheated, but it did have a little piece of the coating missing at one end as if it had popped. I don't know if excessive voltage or current was the cause, or if it had just been damaged when it was put together and that eventually led to its failure.

The HV caps C10 and C13 (both 100nF/400V, not 220nF/630V as per the schematic) both had their legs stretched to fit the holes on the pcb and had broken the orange casings a little by doing this, I was worried they'd be dead too, but they seem ok.

DZ4, R9, R12 & R18 are all as per the schematic and seem fine. C9 is a 56pf/1kV cap - over twice the suggested value, do you think that would do it? I suppose it would need more current to charge up between pulses. R14 is 130k too, rather than the 100k specified.

Hmmm... I think I may swap C9, C10 and C13 for spec values, and R14 as well while I'm in there.