[DC LO] YEs. The outptu (pin 3) of the 555 is rounted there, it then feeds a transtor buffer on the lgoic board which drivs the reset input of the clock generator IC. The trigger input of the 555. That's pin 2 of that chip. I don'tknow wh yDEC drew that 555 the way they did, but it is not easy to figure out what is going on. It's not obvious at first glance which line is the +12V power -- or at least it took me a few seconds to figure it out. Not directly. I would exptect no movement on the display at all if the CPU wasn;t running. I wonder... If a smoothing capacitor had dreied up, there migth be excessiveripple on one of the power lines. This might cause the output of the comparator stage -- the trigger input of that 555 -- to be toggling at twice line frequency. At which point the 555 would be contiulaly retrigggered, but maybe occasionally it drops resrt for long enough for the CPU to do something. AHve you put a 'scope on the power liens to check for ripple? And again, Iwould look at that trigger input of the 555. -tony

As has been said before, one purpose of a schematic is to help you understand how somethign works (and if you are repariing somethign, in general you ahve to understnad how it works so you can see what is not working). There are conventioanl ways to dra a lot of sub-circuits, and it helps -- a lot -- if they are used. The way that 555 is drawn, wit hte timing compohnets above it, whi;e electrically correct, is not clear. Exactly. Ah, or perhaps Argh! :-). What test equipment do you have? Sicne you can't detect these signals driectly, you need soem instrument to do that,. I am tryign to think if there's any way you could find the fault with just a multimeter... A 'scope is vertainly soemthing worth obtaining. Of course, if you get a non-wroking one, you can have another project of fixing the 'scope. Which often needs a working 'scope :-) -tony

Do you have the 555 data sheet? It _must_ be on the web somewhere, but I've never looked for it as I have at least 2 versions on paper. The 555 data sheet shows a component level schematic of the chip, and it's not that hard to follow. It also shows the standard applicaiton circu The instruments I used to use most when repairing calssic computers were the multimeter and a simple logic probe. The latter, sued with thought, can detect a laot of problems (it would be useful here to see if that trigger input is toggling, for example). Hower, I do find a good (if old) 'scope and a logic analyser to be very useful. The latter is important if you are working on the older, mostly-TTL-built machines as you are. Now, I use a multimeter and the HP LogicDart. The latter takes the palce of the logic probe, and is also a 3 cahnnel logic analyser, which is enough for a lot of things. Still use the 'scope and a larger LA when I need them, of course. -tony

I'd thoguht of that, but as you saiod there are some 'unknowns' -- what size of capcitor to sue, whether you need a shut resisotr across the meter and if so, what vlaue, etc. You may not know what hte meter readign realy means, particulalry if you don't ahve much experiecne with this sort of circuitry. I would start with a1uF non-polarised (not electrolytic) capacitor and a 1M resistor in parallel with the multimeter. Try it on a known-smooth DC rail (like a battery) and a low-voltage mains-frequency supply 9e.g. from a small transformer) to ge some idea of the sort of readings you get and how it behaves. -tony

On Fri, 17 Jan 2014, Tony Duell wrote: I /should/ have a 1uf non-polarised disc cap and a 1M ?watt resistor laying around. -- Cory Smelosky http://gewt.net Personal stuff http://gimme-sympathy.org Projects