16 Sep
2015
16 Sep
'15
12:19 p.m.
Yeah, as per my 'where are subsystem prints' page, they are in the 11/23
print set MP00740, pg. 81-87 (schematic on pg. 87):
OK, got it... A one page schematic, a board witht 4 ICs. Shouldn't be too hard...
(famous last words!)
OK, first thing, the things that look like op-amps are in fact comparators. They
have open collector outputs. I would read the LM339 datasheet if you've not
come across this chip before. It is a very common IC.
Because of the power connections to E1 and E4 (shown on the right hand side of
the schematic) the ouptus swtich between -12V and floating. This means that
resistors connected from said outputs to ground are actually pull-up
resistors. That threw me for a few seconds!
The 555s are being used unconventionally. But let's not panic for the moment.
Since you have a working board, the first thing I would compare is the reference
voltage from D2. Measure the voltage on pins 7 and 9 of the chip E1.
If that's the same on both boards, attempt to isolate the problem by checking
the voltage on in 3 of the 555 timers. Are those the same on the good and
faulty boards?
-tony