4mb memory upgrade guidance

[ShawnT62]

Thank you again for words of encouragement, went back removed all solder (still a few plugged holes but nothing impactng this install). redid all solder, nice little silver mountains :)

Moved RAS/CAS wires over to the points labelled as such, also redid connection to mmu to ensure there was no short there,

Powered on, 3 Bombs, PROGRESS!!

Power off and on again, GEM DESKTOP!!!! Yippee

Powered off to insert SideCartridge so I could run Diagnostic, Powered on, loaded previously play floppy, still a good sign, power cycled and reset SideCartridge, Sidecartridge menu appeared for about 3 seconds, then garbled video :(

Pulled side cartridge and powered on,off numerous times, bunch of bombs, Fewer Garbled Bombs (I suspect, more like black bars where bombs would be)

Have presently powered done to let me reset, If it were a PC i would suspect the power supply, between added load of new memory, plus the albeit minor load of side cart may have stressed the poor 40 old yr old PS into cardiac arrest.

Again, thanks for the feedback and guidance, I truly appreciate it.

[exxos]

Moved RAS/CAS wires over to the points labelled as such,

What do you mean by this exactly ?

[ShawnT62]

Moved from R135,136, 137 over to the left on the points labelled RAS/CAS - should I move back?

[exxos]

Moved from R135,136, 137 over to the left on the points labelled RAS/CAS - should I move back?

Those points are likely not connected to anything... So those signals are likely floating now causing stability issues.

You can clearly see the tracks on the right pads going towards the MMU.

IMG_0483.jpg

You should continuity check from the RAM PCB end of wires to the MMU pins to verify good connection.

PLCC_MMU.JPG

As mentioned previously. Those 3 wires if disconnected can be tied to 5V via something like 1K (also tie that MMU wire (MAD9) to 5V via 1K. You will only get 1MB RAM, but if there's a problem with the higher RAM banks, then disabling the extra RAM may give some clues.

[ShawnT62]

Thanks - From memory expansion pins to MMU, I have connection between RAS, CAS Hi , Cas Lo.

Powering on, gives random results, white screen, white screen with row of BOMBS, random garbage.

Across memory expander the highest pin, has continuity with RAS line but I cannot find continuity with lower (second row, Left pins, how ever you would describe), should i find those points somewhere?

[ShawnT62]

So have gone backwards and cleaned up. Before going forward again wanted to check if I have damaged things in others opinion.

PXL_20250501_205301422.MP.jpg

PXL_20250501_205311362.MP.jpg

PXL_20250501_205315699.MP.jpg

Thanks

[exxos]

A lot of damage to vias. Totally missing. Even a seasoned guy, that's a few days work to fix. I'd suggest you get a new board and use your current one for spares.

[rubber_jonnie]

When I said clean up, I meant just tidy the solder on the bottom, not removal of the whole upgrade, as it looks like you've caused further damage.

Unfortunately you have damaged vias, typically this happens when heat is applied for too long and they lift. Often this is due to an iron that is not hot enough so is left on longer. As there is no photo of the bottom we can't see if there is via or trace damage there.

Also, there are a lot of scuff marks under where the old chips were and those, I presume caused by a screwdriver levering out the IC's, could have caused additional trace damage.

If you have to lever out an IC with a screwdriver, it isn't ready to come out and you'll likely pull vias and traces up. These boards are very fragile now and you have to be careful with them.

Just as a piece of advice, more light is always better for sharing this sort of photo, plus it needs to be in sharp focus to allow for the best assessment.

My suggestion right now, is to not do any more to it. It is fixable right now but it may not be if you continue.

I have repaired worse (STE Repair) but I'm not in a position to look at yours right now as I'm about to go on holiday, but I can have a look when I get back.

There are caveats though. No more work on it, you will need to arrange transport both ways, and it will be worked on to my own timetable as I am still working. For example the STE repair took about a month to do.

Let me know if you're interested.

[ShawnT62]

Probably best if I take you up on this offer. Much as I want to learn, this free 520ST is worth saving.

Let me know details for shipping (I will put a provision that if shipping is prohibitive, I will not proceed, coming from Canada and when i looked a professional install of U1MB it was ridiculous, $150 each way) which was 6 times what I paid for 800xl.

Thanks for your assistance and I promise, haven't touched since those pictures were taken. Also let me know if you would like pictures of underside.

you can reach me at stmith#sympatico.ca (change # to @)

[rubber_jonnie]

Probably best if I take you up on this offer. Much as I want to learn, this free 520ST is worth saving.

Let me know details for shipping (I will put a provision that if shipping is prohibitive, I will not proceed, coming from Canada and when i looked a professional install of U1MB it was ridiculous, $150 each way) which was 6 times what I paid for 800xl.

Thanks for your assistance and I promise, haven't touched since those pictures were taken. Also let me know if you would like pictures of underside.

you can reach me at stmith#sympatico.ca (change # to @)

Ok, let me know what you find out regarding the shipping to the UK, , it may be too expensive from Canada as you say.

Bear in mind that I don't want the whole thing, just the mainboard.

I tend not use email for communications, the PM system here is preferable.