exxos's Atari, Amiga & retro forum

In my experience of installing MMU/Shifter based RAM expansions, there are several areas where things can go wrong. The black screen is a good indicator that the ST thinks it has no RAM installed, and suggests that the work done to disable the existing RAM banks is good.

Here's what I've seen in the past:

1. The MMU adapter isn't fully seated or has poor pin contact.
2. A pin on the MMU adaptor was bent during insertion.
3. The Shifter adapter isn't fully seated or has poor pin contact.
4. The Shifter adaptor is incorrectly inserted and one or more pins is not connected.

Items 3 & 4 are relatively simple to fix, and just require you to fully seat the adaptor and make sure the adapter is properly aligned.

Items 1 & 2 are more likely the problem, since the MMU can't see the memory.

For item 2 I'd advise you carefully remove the MMU adapter and check for bent pins, and if you find any you'll need to very carefully straighten them, paying very careful attention to alignment with the other pins.

For item 1, I'd suggest you go through my how to on how to tweak the socket that's here: https://www.exxosforum.co.uk/forum/viewt ... hilit=plcc, then carefully re-insert the adapter.

I recently upgraded a 520ST to use an MMU/Shifter based RAM module and had a similar problem with the black screen. Tweaking the socket and cleaning the chip contacts before re-inserting the MMU adapter resolved the problem for me.

Here is the first problem I found... But this may have blown the MMU...

Robert Tercsi found that and I fixed that. How can I test if the MMU is blown?

lrodriguezrullan wrote: 12 Mar 2019 17:49 Robert Tercsi found that and I fixed that. How can I test if the MMU is blown?

You will need to re-solder the resistors and remove the MMU board and cable and see if your machines boots as stock.

I did that and got yellow and blue horizontal bars. I was getting that before doing the upgrade and was told it was probably bad RAM. That's why I got the upgrade.

Do you have an MMU that I can buy?

Right, so the machine wasn't working in the first place then.. so this thread/fault isn't actually anything to do with the MMU RAM upgrade in that case. You need a stock machine working before anything else, so that is the actual problem you need to fix.

You should probably open up a new topic and start again with such information which hasn't been mentioned. We are trying to debug the MMU RAM upgrade, when it has nothing to do with the machine not working. So you need to be really clear on information as the fault is on the motherboard somewhere.

I only have the chips for sale which are in stock in my store. They are hard to find, it may be just easier to look for a tested working motherboard.

Without the upgrade, I get the yellow and blue bars. With it, I get a black screen. This thread was about the MMU RAM upgrade causing the black screen.

I will see if I can figure out the yellow and blue bars issue without the MMU RAM upgrade. Thanks.