modem7
10k Member
Yes, suspicious. Oscillations of some sort are expected there. But because this is the "keyboard processor", are we to expect a keyboard related error on-screen ?
Caps, crystal, or 80C39 ?
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Zenith 171 - New project - no boot
- Thread starter Scruit
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Caps, crystal, or 80C39 ?
Confirmed that there is no oscillation on the XTAL pins of the 80c39. They read 2v and 1v, flat. On powerup they go direct to those readings with no delay, and stay there.
The keyboard does not work, even when I got to the 'world map' screen while I had the oscilloscope hooked up, no keyboard response.
I figured I'd follow the oscillator direction last night, but it wound up being just one of those nights in the workshop. Test the capacitors? My multimeter won't measure in the pF range. Test the 10mhz crystal? I don't have a function generator that can do 10Mhz. Test the 80c39? Don't have a way to test it, or spares.
Well, I've been looking for an excuse to get an LCR meter, and a AD9850-based signal generator can be had for $19. It would be cheaper to just replace components until it works, but I'm trying to teach myself to learn, understand, test and identify the bad components instead.
The keyboard does not work, even when I got to the 'world map' screen while I had the oscilloscope hooked up, no keyboard response.
I figured I'd follow the oscillator direction last night, but it wound up being just one of those nights in the workshop. Test the capacitors? My multimeter won't measure in the pF range. Test the 10mhz crystal? I don't have a function generator that can do 10Mhz. Test the 80c39? Don't have a way to test it, or spares.
Well, I've been looking for an excuse to get an LCR meter, and a AD9850-based signal generator can be had for $19. It would be cheaper to just replace components until it works, but I'm trying to teach myself to learn, understand, test and identify the bad components instead.
Twospruces
Veteran Member
I would find any scrap crystal that is similar clock and just see if using that would allow a clock to be generated. That would rule out the crystal and maybe point to the controller.
I don't have any spare crystals lying around, but ordered a pack of cheap crystals in different frequencies, they will be here in a few days.
The two capacitors tested OK on the LCR meter, 20.5 pF, which is close enough given the rating of 20 pF +/- 5%.
I am putting together a cheap signal generator using an AD9850 and an Arduino Nano. I should have this done this afternoon and that will allow me to test the original crystal.
The two capacitors tested OK on the LCR meter, 20.5 pF, which is close enough given the rating of 20 pF +/- 5%.
I am putting together a cheap signal generator using an AD9850 and an Arduino Nano. I should have this done this afternoon and that will allow me to test the original crystal.
The crystal tests ok. Tested by building an AD9850-based signal generator and pushing 9mz and 10mhz through the 10mhz crystal. Output jumps when I hit the correct frequency. Actually found the frequency to be 10.001 Mhz
Attached imaged show a red trace which is the output from the signal generator, and a blue trace is the output from the crystal.
Next steps: 80c39s on order. Will do some sanity checks on the board/traces before I put the replacement 80c39 in.
Attached imaged show a red trace which is the output from the signal generator, and a blue trace is the output from the crystal.
Next steps: 80c39s on order. Will do some sanity checks on the board/traces before I put the replacement 80c39 in.
