On Mon, Jan 23, 2017 at 9:23 PM, Adrian Graham <wit...@binarydinosaurs.co.uk> wrote: > On 23/01/2017 19:07, "Tony Duell" <ard.p850...@gmail.com> wrote: > >>> I didn't think of a 555 in that scenario but that makes sense. I've marked >>> it as such for now, cheers! >> >> Is there another capacitor from the 2/6 junction to chassis? Any other >> components >> around? It might be one of the special SMPSU ICs, but it doesn't >> instantly ring a >> bell with me. > > Yes, just found it after some leg cleaning. This board is a big fan of > running small traces under other components. Looks like it's a 68pF ceramic > type.
Sounds reasonable... I normally trace connections using my DMM, but of course you have to know what will fool it (i.e. low resistance components). I find I have to desolder relays, switches, inductors, transformers, low value resistors, etc before starting to trace the circuit. > >>> Tonight it's 'address pins and why I get inconsistent results on a logic >>> analyser while looking at everything that touches A7' :) >> >> Short from A7 to another pin? Failed address latch? > > I've been looking for shorts but I'd have thought that anything that's A7 > should go high at the same time as long as it's part of the same trace, so > the LS373 to MC3242A to pin 3 on all four ROMs should pulse together and > they don't. If there's a short they should all get it. Oh... The trace isn't open-circuit, is it? Check it with the multimeter. Given the corrosion damage to the tape drive I could well believe PCB problems of a similar nature. -tony
