74S86 which is supposed to receive a clocking signal on pin 5 and output on pin 6.. doesn't see anything coming in on pin 5. Which in turn = nothing on pin 6 to go to the rest of the board.
That thread makes me sad... I've read it.. guy ended up selling it because he couldn't figure it out.. someone got a hell of a deal because at least video worked!74S86 is equivalent to 74LS86.
Pin 5 is connected to the oscillator Y1 (already a 14 Mhz clock if i am right)
Whenever my IC tester comes in I'll get all these tested though.Check the clock generation circuitry.
There are 4 x 74LS161 counters used as dividers for the 14MHz signal. Are these chips all working? If you don't have a chip tester, try them in an Apple II as it has a similar clock divider circuitry.
Unfortunately every ic looked like that. They were all sanded down and the legs are clean (now at least)It might just be the photo or the flash in the photo but the 6502B there still seems to have corroded/dirty legs? Depending on how determined you are, I'd say a new 6502 and new logic chips would be a decent place to start. 6502s seem to be available on eBay cheap, probably other places too.
Yeah I thought about a bad 74S86. Tried to swap 2 on the board to see if any difference, but a few slight mv is all I really got. Could just be two bad IC though!Not too common for crystals to fail but given the condition your board is in, it wouldn't be all that surprising. Could also be a bad 74S86, but also check all the other components in the oscillator circuit around the crystal (R90, R91, Q10, Q11, etc). Somewhat sensitive circuit, if any of them drift/go open/have broken connections it will not oscillate.
R stands for resistor. Q for transistor. Not sure if that's what you were asking, though.If I can also ask a question... In these diagrams, what exactly is R90/R91/Q10/Q11? Those are directly IC on the board.. I see them in the diagram just not sure what it really means.
You should expect to see something, but when probing the legs of the crystal itself, the load presented by the oscilloscope probe *may* cause it to stop oscillating or at least attenuate the signal. The fact that you get nothing on pin 5 of the 74S86 proves the oscillator circuit definitely isn't running though (or that the 74S86's socket is bad, so you might also probe the end of R51 that's connected to the 74S86 to see if you get a signal there).Not sure it that's truly where it starts or what I'd see from it. I expect I would see a waveform and not just flat 5v though
Is there any part number written on the side of the transistors opposite the colored stripes? Schematic is a little too blurry to tell what part they're supposed to be.
Seems it is R51 that connects to the 74S86. I'll try and probe each side to see if I can get anything from it later tonightso you might also probe the end of R51 that's connected to the 74S86 to see if you get a signal there).