NMS8250 missing VRAM

Expert (73)

16-08-2021, 15:29

Hi everyone,

I was trying to upgrade my Philips NMS 8250 to 512kb using a document i found on Bas Kornalijnslijper's site. I made a mistake soldering to the wrong pins and got a black screen. I reverted my work, but still had that black screen.

After replacing IC149, a HD74LS670P, the system came back to life, but on the bootscreen it displayed the message it had only 096kb VRAM (i have a ROM with memory counter installed). When i want to load the program TestRam.com, the system hangs, so i can't check if any of the vram chips is failing. I checked all pins on continuity using the schema in the service manual and everything seems ok. Also checked the data lines with a logic probe and that looked fine too.

Since the memory upgrade also uses IC131 (HD74LS125AP) and IC159 (HD74LS139P), my guess would be that one of those IC's is failing, but also checked them with the logic probe and it looked fine to me.

So, i thought it was time to check with the experts Does anyone have any idea what i could check, or try?

Kind regards,
Dennis de Haan

By ddehaan

Expert (73)

16-08-2021, 18:41

Tried the "piggyback" method on the VRAM chips, but no luck there.

By marcoo

Expert (112)

16-08-2021, 19:18

Didn't you pushed by accident the switch in between the drives?

By ddehaan

Expert (73)

16-08-2021, 19:25

If you mean the SW101 switch that puts the 8250 in service mode, than no Can't find any other switch at that place.

By ddehaan

Expert (73)

17-08-2021, 13:49

Placed a socket on IC131 (HD74LS125) and replaced it. Tested the removed chip in my TL866 and it stated that it was working correctly. Also checked all the traces from that chip and they were all ok. Only other chip i worked on was IC159, so maybe something's wrong there...

By Jipe

Paragon (1625)

17-08-2021, 15:42

don't you confuse RAM and VRAM ?
because the memory counter should show 512k RAM and 128k vram at init
on which chips have you soldered the additional memories ?

By ddehaan

Expert (73)

17-08-2021, 16:06

Thanks for the extra info, but i was getting 128kb RAM and 096kb VRAM at the bootscreen. I hadn't soldered the RAM chips yet, but only the 74 IC's. When that is done, the computer should start again with the new mapper, but with only 128kb.

But... I found the problem! In an attempt to fix my mistakes i checked all the traces using the Service Manual and it looked to me that pin 8 and 9 on IC141 should be connected so i did a bodge wire. After putting a new IC, i did not remove that bodge and got the 096kb vRAM. When i looked at the schema again this afternoon, i noticed that those pins should not connect, but the trace of pin 9 runs to a via under pin 8.

After removing the bodge wire, i was presented with the full 128kb vRAM again

Turns out, the IC was indeed bad and i was a dumbass for reading the schema wrong Test Ram reported no issues, so i now can retry the 512kb mod again in the morning

To everybody that responded to this problem: Thank you all for helping!

By Wild_Penguin

Hero (644)

27-08-2021, 00:52

Hi ddehaan,

IIRC there is somewhere on this forum there is a thread about my endeavors on doing the upgrade. Just a few things to note:

1. There is a fault (a trivial one) on some of the instructions for this kind of RAM upgrade. It has something to do with the 74LS138 vs 74LS139 - out of which either one could be used in the mapper circuit. So be sure to double-check the instructions, by thinking how the mapper circuit should work, and datasheets handy!

2. It was noted that using a more modern RAM chip (SRAM) could be more sensible these days, as they are cheap, more easily available, and take up less space - and can be easily installed in a less hacky way than making a skyscraper out of the 4464's. But there are no ready-made instructions available (when the old instructions were made, different kinds of chips were in use / more common, than today...).

3. I've noticed that a stack of 4 chips can be relatively heavy. If you plan on soldering sockets in place of the original four 4464s, make sure you use sockets of high quality! I've noticed that if my 8255 has been "bumped" since last use, the stacks can get a bit loose in the sockets (which, I'm afraid, are not of good quality!). So, every now and then I need to wiggle them a bit so that the RAM starts working again . It is on my TODO list to do something about the sockets, some time in the future, but as my TODO list is not shortening at all, I've never come around to do it (this might actually include ditching them in favor of some more modern RAM chip and rebuilding the mapper).

Good luck!