Debugging Apple IIc: no beep, no nothing

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Debugging Apple IIc: no beep, no nothing
Image icon UE8947.08 KB
Image icon Trace of Q335.76 KB

Hi Folks,

I have an Apple IIc that worked briefly before quitting. Now I get nothing: it starts up, I get a green power light and the drive spins, but nothing more. I'd love to get this running again. (I posted about this a few years back, but gave it up after a while, and just decided to pick this up again.)

So far, I have:

  • Looked at the external an internal power supplies, and they appear to be providing voltages that are within spec. (The -12V line is a little low, but within 10%.)
  • Reseated all the socketed ICs.
  • Checked that power seems to be making it to the major ICs.
  • Measured the various clocks -- it was something relatively easy to check with an oscilloscipe, and here's where I currently have questions.

I've attached a few pictures that may help. I measued the various frequencies on the chip labeled UE8 in my diagram (this chip) which appears to be a timing generator. What I see (see picture) is:

  • Pin 1 is spec'd to recieve the master clock signal at 14 Mhz; I measure 14.3 MHz. Good.
  • Pin 2 is spec'd to recieve half the master clock; I measure 7.14 MHz. Good.
  • Pins 14/15 are spec'd to output 1.027 MHz, roughly 180 degrees out of phase. The phasing is good, but I get .893 MHz. This seems suspect.

In addition, the trace of the Q3 line running into the MMU looks a little irregular (see picture). Suspciously, the timing for the short phase is in spec at 2.045 MHz, but the total for two phases (one long, one short) matches my .893 MHz from UE8. It's almost like it's lagging to make up for the low clock. (??)

I guess I have two questions:

  • Does it seem like a promising idea to replace the chip at UE8, or is this a red herring?
  • If it should be replaced, where the heck do I look for one? :)

Many thanks!