The Sinclair QL Forum

I have a QL with RAM issues (white screen) and some time ago I installed Minerva to locate the bad chip. The typical Mineva code triplets found IC1 faulty.
Recently I replaced it with a new RAM chip (purchased as tested and working).
But now Minerva displays an apparently infinite sequence of rendom code triplets. Also at each power on the codes are randomly different.
I noticed also another thing: if I remove the new RAM the screen area (under the codes) remains fully covered with random scribbles; but if I put the new chip, an horizontal strip full of the normal startup stipple background appears un the top of the screen.

Does someone have an idea of what is happening please?

This seems more like a problem with the 8301 ULA and surrounding chips (74LS257 and 74LS245), probably pins not making contact / dry joints. If you get the first line of the stipple, the multiplexing is not working (either 8301, or control signals from it to the two LS257)

Nasta wrote:This seems more like a problem with the 8301 ULA and surrounding chips (74LS257 and 74LS245), probably pins not making contact / dry joints. If you get the first line of the stipple, the multiplexing is not working (either 8301, or control signals from it to the two LS257)

Thank you very much for your suggestions!
I'm not very expert at hardware issues, but I suppose I could start to check the contacts and try to renew the solders, for the moment.

Nasta wrote:This seems more like a problem with the 8301 ULA and surrounding chips (74LS257 and 74LS245)

The 8301 is OK and I have the 74LS257 and 74LS245 replacements. May I adopt the testing tecnique of temporarily put the new chips on the old ones, or must I unsolder the old chips?

That depends on the nature of the failure in the RAM chip - if it fails in a manor where it drags the data line down or up, it will effectively lock out a piggy backed chip. If you have something like an oscilloscope, you should be able to see that by looking at each data line in turn. It's a pity the RAM chips are all soldered in the boards, as it would be so much easier if they were socketed.

Cristian wrote:

Nasta wrote:This seems more like a problem with the 8301 ULA and surrounding chips (74LS257 and 74LS245)

The 8301 is OK and I have the 74LS257 and 74LS245 replacements. May I adopt the testing tecnique of temporarily put the new chips on the old ones, or must I unsolder the old chips?

I have used use that technique for RAM chips in the past with success (it's not 100% definite, though - In case the situation doesn't improve with a piggy-packed good RAM, that doesn't necessarily mean all your RAM is OK - But if it does, you've found the bad chip).

I wouldn't use the piggy-pack technique for other-than-RAM chips, though. Put the multiplexers into sockets, that's better.

Tobias

I've already socketed the first Ram and replaced with a new one. I also tried the piggy-pack technique with another new ram chip, and apparently nothing changed.
So I'll try to install sockets for the multiplexers.
Thanks for your suggestions as usual!

A faulty 74LS257 multiplexer, or a faulty 74LS245 transceiver can cause all the main board RAM to appear to be faulty to the CPU.

If you have an oscilloscope, a logic analyser or a logic probe, check each input, output and control pin. You are looking to see and confirm that the logic levels on each pin toggle between logic high and logic low. That is, none of them are stuck low or high.

If you have a four channel oscilloscope, or a logic analyser, it is possible to connect them up so that you can check the function of each "channel" of each chip.

Mark

1024MAK wrote:If you have an oscilloscope

Thank you all very much for your suggestions. Unfortunately I don't have an oscilloscope nor skills to utilize it. But I have some soldering skills so my only choice is to adopt unsophisticated methods

In that case, I suggest you buy some suitable replacement 74LS257 and 74LS245 ICs and some turned pin DIL sockets. Then cut each pin of the existing chips as close to the body as possible. Then it is far easier to desolder the pins.

Fit the sockets and then try the new chips

Note that IIRC, one make of 74LS257 chips is not recommended in Sinclair machines, but I'm using a mobile device, so don't fancy going through the service manual.

Mark