Epson PX-8 repair: help needed!

[dutchacorn]

A week ago I picked up an Epson PX-8 through a local auction site. I had discovered the HX-20 before and I'm eager to learn more about these early Epson portables.

The issue with this one is: it does not work.


The basic voltages are there and I also found the clock signals which all seem to be present. Also, after a reset, switching on the machine using the main on/off switch actually brings up the 5V on the board. Switching off does not change the the state after that. The system ROM verifies ok.

The LCD screen stays blank and the CAPS/NUM/INS leds don't blink or switch on (should they?). The LCD does respond to the view angle slide; it gets darker and lighter.

The CAS and RAS signals on the PD4265 DRAM are there.

The only useful documentation I found was a fair scan (PDF) of the Technical Manual.

I'm looking for advice on next steps and also more resources for this machine. I found the F.J. Kraan site with a number of links, some working, some dead. Mainly looking for a good PDF of the operating system reference manual and any other resources.

 

[GeoffB17]

I don't have a PX-8, but I do have an HX-20, which is just about working.

Two things you might need to try to check.

Firstly, the LCD unit might have failed. Totally. This has happened with the HX unit (not mine, luckily). This can arise if the unit has spend a lot of nime not being used, and not being changed.

Associated with this fairly terminal problem, there may also be problems with certain elec caps connected with the LCD, if this is the problem then the repl of the caps can get things right.

You say the contrast (view angle) control does something. Please detail. This should change from totally black to totally white, if that's the case then the LCD may be OK.

Geoff

 

[dutchacorn]

Thanks for the suggestions, Geoff. If the issue would be the LCD I would expect the machine to do a startup beep? Or at least respond to the on/off switch. It seems not to get trough the start-up process and hang somewhere before a beep or screen output is generated.

Here's what I mean with the view angle control:

 

[alank2]

There usually is a startup beep and it looks like the contrast switch is working from your pictures. There seems like I remember a reset of some type on the underside when you open the plastic door. A few times I had my PX-8 in a state where it did not respond to the power switch, but pressing that reset helped.

 

[dutchacorn]

Yes, that’s the initial reset switch. If I switch on (5V kicks in), then slide the switch to off (no effect), then press the initial reset, then computer actually switches off. So the 7508 sub cpu seems operational.