ZX Spectrum 128K +2 Repair - Reset Switch and RAM faults

[@StuHo1mez]

Spot on Jim, i wish i'd kept my plus 2 now. I sold one during lockdown too and I really regret it.

[@lawrencemanning]

The first Sabateur was fantastic. Could never get into the second one. Oh and fabulous repair etc etc. ;)

[@borayurt66]

I think I missed something... The address bus was held LOW, and when you removed the ROM, the fault seemed to disappear, so far so good, you replaced the suspect ROM with another, and the fault returned. As the address bus was held LOW, the diagnostic ROM could not be paged in, so removed the replacement ROM to make the diagnostic ROM work, and it did, finding the faulty RAM chips. This is where you lost me... If the address bus was actually held LOW not by the ROM but by the faulty RAM chips, how did the removing the ROM seemed to clear the bus?

[@HappyLittleDiodes]

I have no idea! But you have it perfect, that's exactly what happened. Something about the failure mode of the RAM chips meant address lines were held low but only when the ROM was fitted

[@alrod6228]

hello, I'm trying to repair a 128 + 2 that has a similar problem, the magenta border and the black center. When placing the new 4164 chips on top of the existing ones nothing changes. Is the ROM broken? Is there any way to test the ROM? Where to get a new ROM? Thank you

[@HappyLittleDiodes]

What diagnostic tools do you have access to? Do you have an oscilloscope?

[@alrod6228]

@@HappyLittleDiodes no, only a multimeter...

[@HappyLittleDiodes]

OK then you're a little restricted, but you could start by checking your voltages, find a schematic and check +5, +12 and - 12 V at points that are convenient to measure, they should all be pretty close

[@alrod6228]

@@HappyLittleDiodes I just checked points of 5v and 12v. 4.84v (regulator) and 10.87v (tr4 leg). Any problem here?

[@HappyLittleDiodes]

10.87 seems a bit low. Is it a grey or a black +2?

[@WidowUK]

I pulled my old one out it was put away working but it has an issue now. It comes on as normal loads the game and it either crashes back to loader screen or it very slow or it just freezes in game. I’m thinking it might be a capacitor issue or a ram chip. It sometimes crashes while loading the first game loader screen photo on agent x the photo is scrambled

[@HappyLittleDiodes]

Could be an issue with your loading setup rather than the machine! If you're using audio

[@WidowUK]

@@HappyLittleDiodes I’m using its tape drive it seems ok some lower quality games are fine but the bigger games are having issues

[@HappyLittleDiodes]

@@WidowUK you could try loading from a wav file using your computer. Or the playzx app. It's either your loading setup or your upper ram

[@WidowUK]

@@HappyLittleDiodes like I said it was working fine last week but has suddenly become unstable. Normally if it was loading issue you would get an r tape error or it would just fail to load all together but as it loads the game and then has issues I’m looking at an hardware issue. It being very slow and freezing and crashing I think it might have a ram issue or maybe the power supply has a bad capacitor

[@HappyLittleDiodes]

Try the command PRINT PEEK 23732 + 256*PEEK 23733 tell me the result

[@BertGrink]

First upvote 😃

[@MrGigi-dz9cv]

What If one memory chip îs missing ? Could someone use a modern memory chip to replace the memory chips of these machines ?

[@HappyLittleDiodes]

Yes it's easy to find them. For example with the grey 128K in the video you can use 4164 memory chips (£1.50 per chip)

[@MrGigi-dz9cv]

@@HappyLittleDiodes Ok. I have some Sinclair like computers. Chips were made in Russia.

[@MrGigi-dz9cv]

@@HappyLittleDiodes Thanks anyway. It would be interesting to fit some modern chips, since i have a few older memory cards.

[@HappyLittleDiodes]

I'm sure you can find equivalents!

[@MrGigi-dz9cv]

@@HappyLittleDiodes I am not a repairman. Any suggestions greatly appreciated. It îs a 8kb chip KR565RU5.