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Epson PX-8 / Geneva repair

G

gw0udm

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Apr 15, 2022
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Hi all

I've recently got into the early Epson portable computers, and got hold of an HX-20 (which works perfectly) and most recently a PX-8 / Geneva. This is proving a bit more of a challenge though and I was wondering if anyone had any pointers.

I'm aware of the various issues with batteries - I've replaced the main battery with a new pack. The internal battery is dead but doesn't appear to have leaked although I will get it out soon before it causes any issues.

The issue is that on trying to power it up, nothing much happens. I've plugged in the AC adapter with the battery in place. The screen does show some signs of life, with the occasional line on it, and if I turn the contrast up to the max you can see a faint pattern on the screen but that's about it. It doesn't seem to make any difference whether the main power switch is on or off, and pressing the various reset buttons doesn't seem to do anything. There are no beeps, and none of the keys do anything nor do any of the lights come on.

Having a close look at the board, quite a few of the capacitors in the power supply section have got signs of corrosion on the solder joints, as have a few of the other components - although most of them look absolutely fine and overall the board looks clean.

So I'm thinking it probably needs recapping as a starting point... but wonder if anyone else has experienced this or has any advice?

Thanks

James
 
The contrast is very touchy, so you will want to slowly and carefully adjust it to find a workable contrast.

With a new battery pack and no A/C, it should be coming up.

There is a button inside the main door on the bottom of the unit that is a master reset type button. I've had to press it when the power switch was unresponsive. The power switch is not real in that it doesn't actually cut power to anything, but more of a sensed switch that the system notices and responds to and changes its state.

Good luck!
 
Thanks!

Unfortunately I do think there's something else wrong. I've got a new battery pack in there and given it a decent charge, but the behaviour is the same. All I can get out of it is a blank screen, although there is clearly some power getting through as you can see pixels on the screen but I don't think it's booting up properly.

As you say the power switch seems part of quite a complex arrangement (judging by the technical manual), and from looking at the board itself there is corrosion on quite a few of the capacitors in that area, plus one of the Zener diodes. It's quite odd though, in that most of the components are shiny and look like new whereas others have have a green deposit on them... maybe evidence of capacitor leakage? It would appear on the face of it that something's not getting powered up properly and the screen does look quite faint

So I'm thinking of replacing these capacitors as a starting point... anything else to consider?

James
 
Hi James, I wish I had better advice! Maybe try to find where the battery goes in and presumably it gets converted to near 5V somewhere. Is the main vcc rail at 5V when you turn it on? (I haven't seen the schematic so I am just guessing).
 
I would recommend to check and replace those capacitors. There are voltage converters and possibly the LCD clock which rely on capacitors.
If you have an oscilloscope you should test these clocks. With just a DVM it will be more difficult, but as per Alan's recommendation checking 5V should be the first step. Also ringing out possibly corroded traces could help.

I also had a faulty Zener in my PX-8 which broke the power supply for the plug-in EPROM.

Martin
 
Thanks very much all - I'll start with the capacitors and see where that leads. There are a few test points, and I've found 5V at the 5V test point which a good start. However there are a fair number of other tests in the technical manual but I don't (yet) have an oscilloscope.

I'd like to replace the crusty looking Zener although I'm not sure of the rating, will need to dig through the technical manual and see if I can find anything (unless you have any tips Martin?)

Thanks very much for your replies and I'll report back. Gives me an excuse to buy that desoldering station I've been promising myself too.

James
 
A desoldering station with a vacuum pump is a very useful tool. Ths simple solder suckers suck - not too well for me.

In my case the Zener diode was in the power supply for the plug in ROM, so this is probably not the same as in your system.
Best read through the technical manual - that was how I learned how "my" Zerner was responsible for the power suppl,y of the extra ROM.
Maybe replacing the the capacitors is enough.

Martin
 
I'm in the same repair process,
but I have 1.8V at the 5V rail. :(

The technical doc is a bit obscure regarding the main 5V rail ; I don't see how it's regulated (it just seems it's not, it goes thru Q6 and so be it) but it contains a very long chapter about the regulation/delaying of another 5V rail, which doesn't seem to be the main one.

Capacitors looks perfectly OK but I'll start with replacing them; I don't have a better idea. Q6 is the next candidate.

What would you do with the backup battery ? Is there any modern equivalent ?
 
> What would you do with the backup battery ? Is there any modern equivalent ?

The backup battery consists of four 1/3 AA NiCd batteries. I just replaced them in my PX-4 with NiMH equivalents (larger capacity though), spot welded together. Found the spot welder cheap in China, great tool for this!

Greetings & success,
Fred Jan
 
> DO you have a picture of your repair for inspiration ?

Yes, here it is:
newBatteries.jpg

Both main and backup batteries are made from normal cells, spot-welded together. The backup RB-301 in the PX-4 is slightly different from the soldered PX-8 battery, but consists of the same sized cells. Will fix the PX-8 later.

Greetings,
Fred Jan

P.S. Make sure the backup battery is fully charged before installing. The manual (PX-4 Personal Computer Technical Manual Aug 13, 1984) states it takes 30 hours to fully charge. That is for the original 90mAh battery, the new one is 250mAh...
 
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@fjkraan you mentionned doing your own spot welding
WOuld you please elaborate a bit on that ?

I see basically two type of cheapo spot welders, one with electrodes that look a bit like multimeter probes, and ont that look like two spikes next to each other in a single unit. WHich one did you use ?
How does this works exactly ? Also how could you spot weld batteries in front of each other ?

I think this will probably be of interest to many other people
 
You have to have a tool for it I think. I've successfully soldered to a battery directly before, but it is a pain. Do you have a local Interstate batteries store? They can spot well NiCD's together and usually do it for me at no charge if I am buying the batteries from them and just want tabs put on it (they will charge if you want a full battery pack made).
 
@CedsRepairs:
The tool I bought at Aliexpress is this one: "DIY Handheld 6 Gears 5000mAh Spot Welding Portable" from seller Docreate. This works OK and is probably better than directly soldering to batteries as the heat degrades the battery. Spot welding also produces heat, but for shorter periods. The device detects a short between the pins, beeps and applies a high current pulse (adjustable in six lengths) to the pins. Very simple to operate.

Greetings,
Fred Jan
 
Got my PX8 to start,

However it boots (after INIT) straight into A>
No menu, no nothing

It has two roms in the sockets (UTY utility and Portable Wordstar) but I can't read them ( drive err on b: select )

So basically it does'nt staty MENU, ahs an empty A:, and does not see B or C ...

I read the manual but none of this is supposed to happen so I'm a bit lost.
 
There is an option in CONFIG that determines whether it starts up with a menu or the drive prompt.

Did you try C: ? I think the A: is a ram drive, then B: and C: are the ROM drives if I am remembering correctly. Do you have other ROM's to try plugging in?
 
Looking at the schematics and erratic behaviour of all that is related to proms and menu : I believe I have an hardware problem on the 5V rail dedicated to the proms ; it would explain all I’m seeing .

The unit I got just needed the lcd screen to be properly replugged into the connector for it to start ; so I thought « hell that was an easy fix for once ! » but that would be too easy - right ? :)
 
Speaking of the screen connector ; it’s the first time I see a computer that comes straight out of the factory with a freaking paper clip (!) inside of it to tighten up the flat cables !
I’ve seen pictures of this on @Jeff_Birt channel but even here in europe ; yes ; paper clips as well !
 
@CedsRepairs:
I read the manual but none of this is supposed to happen so I'm a bit lost.
Click to expand...
If you tried all the reset methods (https://electrickery.nl/comp/px8/um/px8um2.html#2.2.5) and it didn't work, you might have a memory defect. The technical manual has some diagnostic flow diagrams and schematics, but it is usually not helpful for a specific problem.
The menu can be switched via Ctrl-HELP.
 
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