128k +2 Mysterious Videos issue.

A place to discuss the ZX80's and ZX81's younger brother - the Sinclair ZX Spectrum
User avatar
Tiger
Posts: 16
Joined: Tue Sep 01, 2020 10:37 am

Re: 128k +2 Mysterious Videos issue.

Post by Tiger »

In my case it was a faulty capacitator in the (switch) power supply. I repaired it today and now everything is ok. It seems that 105° caps are not a guaranty for a long lifetime... :o
User avatar
Erm
Posts: 186
Joined: Thu Jun 20, 2019 7:07 pm

Re: 128k +2 Mysterious Videos issue.

Post by Erm »

Tiger wrote: Thu Nov 19, 2020 9:54 pm Today I had nearly the same problem with "crawling dots scrolling over the screen". Tested an ZX Spectrum Issue 6a PCB. But an other Speccy, wich worked fine before also shows these dots. I used an adjustable bench top power supply and after I took an other power supply every thing works fine. Seems that the (two years fine working) bench top power supply suddenly produces interferences :roll: . So I think it is a good way to test your spectrum with an alternative power supply too. :idea:
You know what, I think you're absolutely right! I was using a modern 9v/2a PSU and have just changed to an original old Sinclair brick and certainly using the +2 with a RGB/scart connection is now much better. I still need to test it with the QL-VGA device as the interference with that was certainly a lot more pronounced. I'd already changed the internal voltage regulator from a switching one to an original 7805 but that didn't seem to do much.
I'm Chris, a Brit in Germany (Chr$ on the QL forum).
User avatar
Tiger
Posts: 16
Joined: Tue Sep 01, 2020 10:37 am

Re: 128k +2 Mysterious Videos issue.

Post by Tiger »

In my Speccy's I have mostly switching power regulators instead of the 7805, so this will do no change. The interference fault in my case was definitely the (faulty) external power supply. 8-)
User avatar
Erm
Posts: 186
Joined: Thu Jun 20, 2019 7:07 pm

Re: 128k +2 Mysterious Videos issue.

Post by Erm »

Well it certainly fixed the screen using an original PSU for now, but I seem to have done some damage to something else now as I get random patterns and junk on the (perfect) screen! Going to have to investigate.
I'm Chris, a Brit in Germany (Chr$ on the QL forum).
User avatar
Tiger
Posts: 16
Joined: Tue Sep 01, 2020 10:37 am

Re: 128k +2 Mysterious Videos issue.

Post by Tiger »

:( - random pattern - looks like (Upper-)RAM damage :shock:
User avatar
Erm
Posts: 186
Joined: Thu Jun 20, 2019 7:07 pm

Re: 128k +2 Mysterious Videos issue.

Post by Erm »

I swapped all the RAM (fortunately I'd socketed it all a while ago as this is my main tinkering Spectrum) and that changed nothing. I checked all the original RAM in another Spectrum and it seemed fine.

When I start it I get variations of this:
20201122_172121_resized.jpg
Sometimes there are a lot of random colour boxes, but the white horizontal bars are always there. The border is usually purple, sometimes other colours but rarely white.
20201122_172102_resized.jpg
Such a shame as this is my main Speccy and it was fine and very handy as it all worked. I wonder if the original PSU I reverted to is a problem, so I'll re-cap/check that. But in the mean time I have a broken +2. :?
I'm Chris, a Brit in Germany (Chr$ on the QL forum).
User avatar
1024MAK
Posts: 3246
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: 128k +2 Mysterious Videos issue.

Post by 1024MAK »

Can you burn EPROM chips?

Mark
User avatar
Erm
Posts: 186
Joined: Thu Jun 20, 2019 7:07 pm

Re: 128k +2 Mysterious Videos issue.

Post by Erm »

1024MAK wrote: Sun Nov 22, 2020 9:55 pm Can you burn EPROM chips?

Mark
Yes. You think it's the ROM? Or you mean to burn a diagnostic ROM? I do actually have the original saved ROM data somewhere.

I have the plug in DiagRom and that is obviously more focussed on 48k models but I did manage to get through to the 128k menu and it reported a couple of bad RAM chips, but I've ruled those out.
I'm Chris, a Brit in Germany (Chr$ on the QL forum).
User avatar
Erm
Posts: 186
Joined: Thu Jun 20, 2019 7:07 pm

Re: 128k +2 Mysterious Videos issue.

Post by Erm »

I just tried the Brendan Alford diagnostic ROM and the screen is illegible and then there are all 8 red bars in the border. Deary me.
I'm Chris, a Brit in Germany (Chr$ on the QL forum).
User avatar
1024MAK
Posts: 3246
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: 128k +2 Mysterious Videos issue.

Post by 1024MAK »

Yes, I was going to say, to run a diagnostic ROM. But you must have read my mind :shock:

If diagnostics are at least running, then the Z80 is mostly working. The ULA is also mostly working. It could be one of the chips that multiplexes the address lines to one of the groups of DRAM chips.

Mark
Post Reply