> > 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
