[time-nuts] DG535 triggering problem

[Kevin Rosenberg]

On Dec 16, 2011, at 11:25 AM, Brooke Clarke wrote:
> Just a guess.  If line triggering does that there may be a problem with the power transformer.
> Use an Ohm meter to locate the transformer terminals for the AC line and then check from one of those to all the other terminals.
> I suspect there's a short in the transformer.
> Look at U502 to see of it's burned up.

Thanks for the thoughts. U502 is interesting. It's a small
PCB with an 8-pin SOIC and an SMD cap on one side soldered
into with a two rows of 0.1" pins into the main PCB. Since
it's only a 8-pin IC on top, and U502 is supposed to be
a quad comparator, likely there's another 8-pin IC on
the bottom on the adapter board. I'd need to desolder the
14-pins to check that. I suspect that SOIC to DIP board is
there because the original U502, a MC12040, became unavailable.

I'll need to do some more disassembly to check resistance
between the primary and the secondary taps. I was wondering
about the transformer or other power rails as well. That's why 
I checked the 6 regulated outputs. 

While the sonic noise with 'line' triggering is an important clue, 
I was thinking it was probably more in the logic circuitry since 
the other 4 trigger modes won't trigger. Unfortunately, I don't
have a schematic.

Anyway, thanks again for the information on trimming C512.

Kevin