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Epson PX-8 repair

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laersn

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Mar 25, 2024
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Hello

I recently acquired an Epson PX-8 Geneva, which I believed to be in "almost" working condition but reached the end of my wits trying to repair it.

It fails to boot. Most of the time the screen will just be empty, very rarely it beeps, randomly turns on the keyboard LEDs, and gets to a password or initialization screen, but does never accept keyboard input.

What I've done so far:
- Replaced the missing main battery with a self made NiMh pack
- Removed the aux battery
- Burned the OS ROM to an EPROM (exactly the same behaviour)
- Checked all the voltages, power looks fine
- Checked all the clocks and compared with manual. Everything looks as expected
- The caps all look good, no bulging no leakage

Now, while probing with the oscilloscope, I've noticed that while the address lines look fine to me, the data lines don't look ok, right? See attached screenshot, which was measured at Q0 of the ROM, but it looks the same at the Z80 data lines, or at the RAM data out.

2024-03-26-144843_3762x1868_scrot.png
Could this potentially be a bus conflict? How could I now proceed to identify the faulty component?
I've also tried to burn a ROM with all NOPs, but have similar output on the datalines.

My apologies if this is a silly question, I'm still very new to this.
All help will be greatly appreciated :)

I've also tried the maptst.hex I've found in this forum, directly in the OS ROM slot, but didn't receive any output at the RS232.

Thanks so much!
Lars
 
Generally, when troubleshooting a boot issue, you want to look at three things, to start: Power, Clock, Reset. Check that the voltages are correct, check the clock is generating a valid clock, and that the reset is acting properly. Is it coming out of reset, is it activating reset? Based on what you mentioned, I don't see that you checked the reset, each cpu should have a reset pin. You may want to manually activate the reset button(s), as the ram will probably have some random values in it, if the batteries died.
 
Thank you very much for your reply.
I've checked the reset. Both resets seem to work correctly. All CPU control lines look ok to me too. Also, as mentioned before after a reset I see activity on the address bus that looks generally ok to me.
What really doesn't look too right to me is the data lines (yes?). I suspect one or more of the chips connected to the databus might be faulty (none get hot though).
Do you have any tips on how I could proceed to identify that component? I was thinking I could maybe pull the RAS/CAS of the RAM chips high one by one to see if one of them is causing this.. would this work? Any other tricks I could try?
 
Last edited:
laersn said:
Thank you very much for your reply.
I've checked the reset. Both resets seem to work correctly. All CPU control lines look ok to me too. Also, as mentioned before after a reset I see activity on the address bus that looks generally ok to me.
What really doesn't look too right to me is the data lines (yes?). I suspect one or more of the chips connected to the databus might be faulty (none get hot though).
Do you have any tips on how I could proceed to identify that component? I was thinking I could maybe pull the RAS/CAS of the RAM chips high one by one to see if one of them is causing this.. would this work? Any other tricks I could try?
Click to expand...
Could you share what "doesnt look right" looks like to you? What voltages are you seeing?
 
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