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Epson PX-8 / Geneva repair

I seem to have the exact same problem as in your notes @Martin Hepperle this is extremely helpful

I was gonna debug the 5V powerboost as it looked like the most likely source of the problem, but I'm glad to see I'm not alone
 
To add something useful of my own, I recapped the entire PX8 and tested each removed capacitor (as I always do)

Only C16 was reported as bad by an ESR meter, it also had visible cristals starting to grow just under it.

All the others tested good.
 
Sadly my problem seems different than the one of @Martin Hepperle

Although some of the symptoms are the same ; I can’t access my Rom sockets ; and when I do I receive garbled directory listings
Sometimes I’m able to see “ws.com” but if I launch it ; it freezes
Also trying to access D: sometimes makes a 200mA spike and gives up with “charge battery” message
I tried replacing the roms with 27c256 copies but it’s the same
I’ve got 5V on the vcc of the roms
Unit is fullly recapped

Bit out of options sadly :(
 
Put a scope/logic analyzer on some of those pins and see if there is noise or something odd going on with the signals.
 
I've tried replacing the CPU( easily done as it's the same one in the ram expansion), nope
Tried reflowing the pins of the GAH40S chip - as this is basically the interface to the two ROMs, nope
Triple checked the ZD5 zener and 5V rail, tried to power the rom out of external +5 ( putting pin 1 & 28 in the air), nope.
The unit is recapped, super clean, works in the CTRL-HELP menu, I can mount a cassette for example

But I'm fairly limited as I can't read ROMs my PX8 boots to "A>", never displays the MENU, and I've got no CP/M commands (no, pip no nothing)

If someone with a PX8 could confirm what a working PX8 does when it has no ROM in the sockets, that would already help me a bit.
Does it boot to A> without menu ? Could anyone confirm (ping @Crashedfiesta too :) )
 
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interesting; with the two roms removed, my HC-80(Japanese version of the PX-8) boots into just the A> prompt. I've never tried that before so was a bit surprised. I guess it makes sense that CP/M is in the internal rom.
I forgot to mention this is with the RAM disk attached. Doing a DIR shows "NO FILE"
 
Thanks a lot @mk2s

Which would confirm that somehow my PX8 refuses to load ROMs but other than that works just fine. But it's of little use without ROMs.

I'm afraid the only diagnostic I can come up is that the GAH40S surface mounted chip (in charge of dealing with those roms) is toasted. What's frustrating is that it looks so much like Martin's problem but in my case the 5V rail to the ROMs is nice and steady. The only bizarre thing is that it takes 10ms to raise, which might be normal - so I tried powering up the ROMs with an external +5V directly on pins 1 & 28, and the result was exactly the same.

Actually it does read the ROMs sometimes, I can get a directory listing, then only half of the files, then garbled names like " : : J : :" then sometimes BDOS error, then again the directory listing. Loading or TYPEing anything from the roms freezes the computer.

It could also be a RAM problem I suppose, but that's not a better news ; the RAMs this uses look like unobtainium (at least on their NEC reference, uPD4265)
I find it unlikely as I see no sign of crash using the CTRL-HELP menu or mounting cassettes (altough that's not much of RAM usage)
Technically it could also be a problem with either the molex socket or the pull up/down resistors array but that is becoming very unlikely.

There is really nothing in between the ROM sockets and the Programmable gate aray "GAH40S" so the causes of problems are rather simple ; it's either the 5V rail or the GAL itself :(
 
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It could also be a RAM problem I suppose, but that's not a better news ; the RAMs this uses look like unobtainium (at least on their NEC reference, uPD4265)
Click to expand...
I always assumed the RAM was just standard 64kx1 dynamic RAM, very common in computers from that area. The Technical Manual doesn't suggest otherwise.

Fred Jan
 
This is for now the result of some disassembly and running the ROM on the PX-8. Will test the PX-4 later.

D-RAM Test ROM, to be installed in the O.S. socket, 2A.
There are two jumpers close to the socket J4 and J5, but
they are not related to the O.S. ROM.

Output:
PX-8,HC-80/88 D-RAM Test Program Ver. 1.0 Bit 76543210 1 : ........ 2 : ........ 3 : ........ ....
( I stopped it at 101.)

This Test ROM relies on the 8251 USART to be working and reports
its progress on the RS232C port, 4800 Bd, 8N1. I used a #724 type
cable. No output is generated for the LCD screen. Each test run takes
about 43 seconds. It might be intended as an indefinite burn-in test.
At the first run the keyboard LEDs are on, for later runs they are off.

The program code first test the upper half of RAM running from ROM,
then copies (part of) itself to RAM to test the lower half of RAM.
The program does not rely on RAM for its status, all is kept in the
registers.

Reporting includes defective bits in the RAM I.C. on the PCB.
PX-8: 5D 7E 4E 6D 5E 6E 4D 7D
PX-4: 4C 4D 5C 5D 6C 6D 7D 7C
(can't show this as no bits failed)

The checksum for the ROM that my programmer reports is 5B8AF1. The
handwritten label on my 'original' is "MapPin Ram Test V.10".

Addendum:
The PX-4 version is very much the same. The header is:

PX-4,HC-40/41 D-RAM Test Program

A run on the PX-4 takes about 30 seconds.
 
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Thanks for the useful (at least for me) info @fjkraan
I've ordered a NoS 6803 processor (dip40) in the hope of maybe solving my prom problems. And that's also what the tech manual suggests if you can't access your proms. The GAL (which is the tech manual next suggestion) I can't replace anyway... So let's cross fingers because this is my last hope.

Getting back in the PX8 TEST ROMS ;
Unless I'm mistaken, the TEST ROM you're talking about above (the one that is silent but has serial output) replaces the main ROM.

However, the technical manual refers to a "test rom" that clearly goes into the optionnal ROM sockets, and has user input and output/menues on the screen.
I don't see this ROM in your ROM collection on your website (maybe I'm not looking properly)
Does anyone have a dump of that particular "tets rom that goes into option socket and is referred in the tech manual" ?
 
that is indeed the same program, in the form of a .COM inside an ARC. The menues and options are clearly the same.
I guess what the tech manual talks about is this same thing but in the form of a ROM
It would be interesting to recreate that ROM as I think nobody has it shared on the internet.
 
Here a first second version. The MAPTST program starts up from disk, but I didn't try all options. The ROM is created with PXPROM2 on a PX-8. The ROM is untested. It also contains MCTUTY, a micro-cassette-control-program. And a fixed version of MTEST.BAS

Fred Jan
 

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Hey @fjkraan thanks again Fred Jan,

I've been able to successfully run a RAM test with your RAM test that goes into the system rom socket

Capture d’écran 2023-05-10 à 16.09.11.png

Even if that is still running I can smell that my RAM is OK

So I'm really out of options here ; according to the tech manual if you're option ROM don't work, first replace the 6303 and then replace the (unrepleceable imho) surface mounted GAL. I've also confirmed all ROMs get their 5V.

I know this a long shot but... would anyone reading this have a "spare parts" PX8, that can be completely broken I don't care I just need a replacement 6303 from it. Or just a motherboard, again could be broken as long as it has the sub processor.
If I read the doc right, the 6303 has a "masked ROM", in short it's a custom ship - I've tried replacing it with an HD6303RP and the system doesn't boot .

It is super frustrating for me to have a 100% working PX8 (in a decent physical shape too), that can't read it's freaking option ROM sockets, because without that there isn't much you ca do with it.
Oscilloscope probing is inconclusive. I see data coming out. At most, I find it a bit noisy with noise spikes, but that may just be the way it is, after all it's 1982 tech.

Message in a bottle :)
 
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Hi CedsRepairs,

Nice to see you got the test program working, and now at least you know its not the RAM. In the past I reduced some non-working PX-8's to salvageable parts, including the 6301 chips. But I would have to test them before shipping, because their questionable origin. And no, I didn't keep the PCB's with the Gate Array's (not GAL as that is another technique).

Fred Jan
 
RetroHospital said:
I have 1.8V at the 5V rail
Click to expand...
Where did you test that? VRF is voltage reference for the 7508 cpu and it's just a sampling, if it's below 1.8 then it's "critical low voltage" and you get a warning. Otherwise, it should be between 1.8 and 2.x on that VRF test point. That's the input for the ADC and you can adjust that with the V1 variable resistor. VB+ and 5V are the ones that show the actual power levels.
 
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@RetroHospital Have you tried vfloppy to serial port? I believe that utility running on a linux box will provide you with drives d: and e: over a serial cable and it also comes with some utility disk images which could be helpful. If you are still sometimes seeing ws.com I wouldn't give up on 6303 and GAH40S yet; as they must be functional in order for dir to show ws.com. I just today tried vfloppy from https://github.com/electrickery/vfloppy and it worked as advertised.(except for the forth image) Hoping things work out for you.
 
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