Mike's PCjr Page

Hello
I recently purchased pc Jr with tecmar Jr Capitan side card.
128k memory, clock and parallel port.
I would like to replace 64k bit memory chips with 256k bit,. Current chips are samsug km4164a-20.
Can somebody recommend a 256k-bit chip for that card?
Thx
Paul

I have Fujitsu MB81256-15 in my jrCaptain, it came with that.

Here is a link to a list of 41256 class RAM chips.

I did find this in a PCjr guide:


256K DRAM CHIP USING AND HANDLING

There are many types of 256K DRAM chips that will work
on the junior memory expansions. All should be CAS before RAS
refresh and at least 150 ns in speed. Anything faster than
120 ns is probably a waste of money speed wise on the junior.

The following is a list of compatible DRAMS by manufacturer;

NEC 41256 must be 1987 date or later
Texas Inst. TMS4256
Samsung KM41245 or KM41256A
Hitachi HM50256
Intel, Fujitsu and Mitsubishi

AVOID Toshiba, Motorola, OKI, Micron Tech. (MT),
Western Electric (WE), Seimens and NEC dated 1986
and earlier.

That advice seems like a mixed bag.

At 4.77Mhz the cycle time of the machine is 209ns. The RAM on the PCjr ran at 150ns. While 120ns would have been better than spec and probably a waste back then, now you are dealing with 30 year old chips. I'd take any extra margin that I could get - 120ns chips are just fine.

The list of specific chips to avoid is probably out of date too. If there were quality problems that was fair to point that out back then, but 30 years later, well, the weak chips have been pretty well weeded out. People give advice to use matched sets, but even that seems superstitious.

I have a stand-alone memory tester that I use for debugging fault RAM. It handles 4164s and 41256s (among others) just fine, and tests cycle time, heat, and voltage ranges as well as basic functionality. Kind of hard to find on eBay but if you have a bag of RAM in question let me know; I can test and sort reasonable quantities for you.

Well I found that list in "PCjr Power" copyright 1989 by Albert L. Allen

So yeah not a new list.



Heres another gem from that guide that I'm not sure is worth a try:

3. TECMAR JUNIOR CAPTAIN AT HIGHER SPEEDS

In answer to a lot of questions concerning speeding up the Tecmar memory sidecar.
The NICKEL EXPRESS BOARD has three speeds 6.66, 7.37 and 8 MHZ and comes with these speedup instructions.
To fix the Tecmar boards you will need to unsolder or remove two capacitors on the Tecmar board.
In the R/C network there is a metal plate and under it is two capacitors. You need to remove the large heatsink
with its power transistor from the card. One capacitor is marked C-l6 on the board and 101 on the component.
Remove it and put it in a safe location. Next remove the one marked C-l5 on the board and 331 component.
Replace it with a 120 picofarad pf capacitor. This should allow it to run at the 7.37 MHZ speed level.

As an advanced project, one may want to try a 68 picofarad capacitor in the C-15 location and put in
100 ns nanosecond memory chips. THEORETICALLY, this should allow 8 MHZ operation??


Although I got no idea what they mean by "NICKEL EXPRESS BOARD"

I did look around for chips from a list and decided to go with SAMSUNG KM41256AP-12 ( there are only $0.40 a piece so not a big pain if wont work )
here is a link https://www.utsource.net/itm/p/1741053.html?digipart=12
At the same time i will install a dip socket if need to return to old chips
big thanks for all answers

Your JrCaptain didnt come with sockets?

I got 2 JrCaptains and as far as I can tell if it was ordered with full 128kb the chips were soldered in but you could also order them in 0kb and then they came with sockets for future upgrades.

Of the 2 JrCaptains I have one has sockets and the other does not, the one that has sockets they look original but they might not be.


I posted this in another thread but going to post it here too:

Hello
Memory upgrade process - success
I was able to remove old 64k chips, saved them just in case, i installed sockets and inserted new 256k chips. power on, counter on the screen went to 640
Paul

The PCjr I just purchased came with the Jr Captain 512k, no power supply tho. What voltage power supply should I use with it?

Should have looked more before posting I found this http://www.emtcompany.com/logicode-dv-8 ... AnqP8P8HAQ hopefully it works