Mike's PCjr Page

Hello,
New to this very helpful forum, and new to the PC jr. I have been doing a lot of reading in here and elsewhere about PC jr memory issues. Seems the lack of a parity chip really seems to cause a lot of problems.
Well, that is what I seem to have.
I have received 8 PC Jr's that a friend wants me to clean up and get working. Each unit has it's own problems. Most I can figure out.
But a couple seem to have base ram issues. I have stripped them down to only the motherboard and a known working floppy drive and card.
They can boot DoS 3.2 or 2.1, and do some functions. But coping files, or writing system files after a format and I get corrupted data on the disk. If I load a program that almost files available memory the units crash. Usually after I press a key after they load.
I have run the JRMD ram test,and I get no errors, but there are parts of ram it can not test.
I have felt for hot ram chips, and none feel warmer then others. I have felt all the chips and none seem exceptionally warm.
I would rather not de-solder all the ram chips and put in sockets. That always can introduce more problems.
Any suggestions about how to proceed with finding and fix this?
Thanks

What are you using to power the boards? I only ask because I have seen some unusual issues with the floppy drive + jrATX + picoATX power supplies that I am slowly trying to get resolution on.

Do you have a Gotek you can use in place of the floppy drive?

Have you tried the built in tests (ctrl+alt+ins)? It does a memory test while idling. I'm not sure if it is any better or worse than JRMD.

What programs are you attempting to load that are crashing and if it is a bare system are you using the 64k memory expansion or just the bare 64k system? (i.e. is it a 64k or 128k machine right now).

Thanks for the reply. Much Appreciated!
I have several long and short power-boards with brick PSU's. I also have a ATX adapter with and without the external power supply.
I have purchased several Gotek's and can never get then to work.
Yes I have used the built in test. No errors reported
I am using just a 64K machine and DOS 2.1 or 3.2.
I am not running any programs. Just using DOS copy. It fills available ram during the read, then writes it back out. It appears that the systems in question write out garbled data. But I'm not sure if the "write" subroutine or the stored data is corrupted. Same issue with formatting at the end when /s system files are written to disk.
Obviously I have tested these DOS disks 'copy' & format commands on other systems without problems.

I've honestly never had the base 64K in a PCjr cause problems. The lack of parity checking is annoying, but if you think about it those were 64K DRAMs soldered onto the board in a pretty isolated system.

The original IBM PC *needed* parity because it was using those god-awful 4116 chips that used three different voltage levels. By the time you get to the PCjr you are using 64K DRAMs that have just one voltage level and are far more reliable. Parity checking is great for telling you if you have an error, but if you don't trust the 8 chips in a bank to be reliable, then adding a 9th chip in the bank actually makes it less reliable. The difference is that at least you'll know when you are flipping bits.

Back to your specific problem ...

DOS requires a 128K system to run. If you are running on a 64K system, then the errors are probably expected. 16K is usually reserved for the video buffer. The BIOS needs 2 to 4KB at the lower end of memory for the interrupt vectors and BIOS data areas. So you are trying to load DOS and run in about 44KB, at best. Copying a file might require more buffer space than you have available; it should break the copy into multiple chunks or refuse to execute the command, but who knows what bugs you are hitting because you are way off the beaten path.

Here is what I would do:

• In a 64K system, just run the built-in diagnostics and let them sit there. It will just keep counting memory. Do that for a few hours to see if anything breaks. Running the other tests (video) would be interesting as well, but the key part is to let the machine just keep touching and counting the 64K of RAM.
• If you have the manufacturing test plugs, use those - the manufacturing test plugs might do a more memory rigorous memory test, or at least tell you exactly which chip it thinks might be bad if it does detect a problem.
• If that gets too boring, put a known good 64K expansion card in the system and try DOS 2.1 again. Then run a memory tester.
• Don't bother copying files around because you are introducing too many variables into the equation - the controller, cable, floppy drive and diskette make things much more complicated.

Understood. I will follow your advice. I'll post results later
ps
I am missing several badges. I want to make simple sticker replacements. What font(s) are used for the "PC jr" letters?
(I found a good image for the "IBM")
Thanks

KLund1 wrote: ↑Mon Apr 11, 2022 7:57 pm Understood. I will follow your advice. I'll post results later
ps
I am missing several badges. I want to make simple sticker replacements. What font(s) are used for the "PC jr" letters?
(I found a good image for the "IBM")
Thanks

Hmm, no idea, but PM me if you need a high res scan. I have a loose badge laying around I can scan in.