[sdiy] OB8 assistance needed

[Cary Roberts]

Doug Terrebonne and I spent some time getting his OB8 up to speed on
Saturday, but we ran into an issue I was unable to solve.  Maybe someone
on the list with an OB8 and an oscope can assist?

The symptom is that the unit would latch up after being on for only a
few minutes.  I found a severe amount of ripple in the 5v line on the
front panel pot board so removed the LED driver (U2) to see if the LEDs
were the cause of the trouble.  That fixed things so then I went to
track down the culprit.  I was seeing the transistors that drive LED
rows 2 and 5 triggering twice per refresh cycle instead of once like the
other rows.  I traced this back to U58, a 74LS374 D flip flop, and
replaced it but that didn't rectify the problem.  In fact, looking at
just the U58s outputs with no connection to the front panel board pins 2
and 9 (rows 2 and 5) were still driven low twice per LED refresh cycle.
I thought maybe it was U40, a 74LS245, but if that were the case I would
think there would be issues with the keyscanning, DAC, and all the other
bits that are on that data buss.  I don't have a logic analyzer so it's
hard to tell if it's a possible issue with addressing.

So anyone with an OB8 and an oscope care to look at pins 2 and 9 on U58
and verify that they are only going low for one period as pins 5 6 12
and 15 are?

Oh yeah, and then tell us what we need to look at next.

-Cary