Green Screen problem and poss. a new ROM.

[Chr$]

I took delivery of an Italian QL today. Excellent condition, I was very pleased with it (it means I have a spare Italian QL now as I only bought this one because it came with the ​original Italian manual).

I powered it up and it worked fine, with the exception of the CTL key (and possibly others) so clearly it needs a membrane. I opened it up to see exactly what was inside and found an Issue 6 board with an Italian repair sticker and an unusual ROM combination - FP as the 0000 ROM and MGI as the 8000 ROM. I always thought that the 8000 right hand MGx language ROMs were always paired with MG in the left socket. So I removed them to dump the data.

I reassembled after cutting off a cm of one of the k/b connectors as it was visually split near the socket.

And it's now broken! When powered up or reset it goes through the tweed screen and then all I get is a plain green screen.

I've re-socketed all the socketed chips, de-oxited the sockets, checked the pins and alignment was correct and still a green screen. I even tried a Minerva ROM to see if that would offer more info, but it does exactly the same and also just shows a green screen.

Has it developed a RAM issue or do you think it's ROM related and perhaps a dodgy ROM socket that didn't like having its chips pulled. I thought it was odd that Minerva does the same.

[Chr$]

I think I've fixed it, or rather, found the issue. I went through pickabacking a known working 4164 on top of the RAM chips. Most made no difference, some resulted in a white screen instead of green, but one (IC3) resulted in a normal F1/F2 boot screen!

So out comes the m/b. Must have just been a coincidence I suppose.

[Ruptor]

I think I've fixed it, or rather, found the issue. I went through pickabacking a known working 4164 on top of the RAM chips. Most made no difference, some resulted in a white screen instead of green, but one (IC3) resulted in a normal F1/F2 boot screen!

One bad ram wouldn't stop the processor from doing something to the screen would it otherwise the minerva couldn't say which chip was faulty. Take a close look at the board around that Ram chip and perhaps gently press the board as is to see if things change.

[Chr$]

I checked for shorts too in the address and data lines. It's very stable with the extra chip RAMmed (see what I did there) onto IC3. Could a really bad RAM chip cause a green screen? The service manual says a few things about the green screen, including:

2.2.14 To summarise, if on power up a white or green screen is displayed, it indicates that the RAMs are in general working, and that the machine is very close to being fully operational. It suggests a data or address line short-circuit or a faulty RAM chip.

Regarding the FP ROM - it's very similar to MG, I think about 80 bytes are different. Looks like FP is an earlier version of MG because it has a 1983 copyright date and the other MG related chips are all 1985. Here is a combined FP+MGI attached, MGI starts as normal at 8000h.

[Ruptor]

It's very stable with the extra chip RAMmed (see what I did there) onto IC3.

Do you mean IC5? It won't hurt to change the chip. If the ram was pulling or shorting lines I can't see how the piggyback would work. Perhaps the original chip is not in circuit maybe broken inside.

[tofro]

The piggypack RAM diagnostics method works in surprisingly many cases. Obviously, you don't want to keep it like that forever and the faulty one must go.

Minerva will be doing its best to even boot with faulty RAM and might even work with less than 128kBytes intact - But there are cases (when the "working" RAM is just not enough to get the system up) where it must wave a flag.

[Ruptor]

Minerva will be doing its best to even boot with faulty RAM and might even work with less than 128kBytes ntact

I was assuming Minerva was only using CPU registers and ROM until it had checked the RAM which is the normal way to write boot code but clearly not.

[mk79]

Minerva will be doing its best to even boot with faulty RAM and might even work with less than 128kBytes ntact

I was assuming Minerva was only using CPU registers and ROM until it had checked the RAM which is the normal way to write boot code but clearly not.

Of course that is the case, it does not use the RAM until after the test.

[tofro]

What I am trying to say is that Minerva may return from the test (passed) with a lesser amount than 128kBytes and go on to boot.

Tobias

[Pr0f]

What I am trying to say is that Minerva may return from the test (passed) with a lesser amount than 128kBytes and go on to boot.

Tobias

Agreed - it does say this in the Minerva manual - and warns you may end up with not very much RAM if this happens!

My guess is that if the faulty RAM is in the first bank that handles the 2 screens - then that's when you are likely to get the solid screen colour if the faulty memory is very low in the memory range. Sometimes pressing reset multiple times can sometimes get it to show F1/F2 screen - this might suggest a refresh issue - could be faulty RAM or ZX8301 chip. Also - make sure to remove any cards that also extend the RAM - such as Expansion RAM, Trump, or Gold (+super) card.