Sinclair ZX80 / ZX81 / Z88 Forums

Hello, let me run this past people here, perhaps someone has some suggestions.

I have a grey +2 with an issue. From completely cold I seem to get the normal boot screen. A few seconds later it starts to get corrupted and then is taken over by random colours with flashing squares and other nonsense. Resetting changes the random patterns somewhat (sometimes, but not always) and rarely after resetting I can see parts of the boot screen menu/copyright message but mostly it's just nonsense. Sometimes the patterns change, as if it were resetting itself. When I unplug, leave it a while then plug it in again, the same happens all over again.

I've tried the following: changed the Z80, checked the ROM (with Eprom reader... no obvious dodgyness there), I re-seated the long chip with the alu heatsink and I've removed all the 16 RAM chips, added sockets and installed 16 new RAM chips. None of that seems to have changed anything (but at least it hasn't made anything worse either!).

I suspect that maybe one of the multiplexer chips is misbehaving.

But before I continue speculating, do you have a multimeter, and have you tested the +5V supply and +9V supply rails?

The long chip with the heatsink is the ULA. And no, it is NOT the same type as used in the 48k.

How long (approximately) does it take before the “boot” screen starts becoming corrupted?

Mark

Hello Mark,

Before I answer you on those - I burned this diag ROM onto a 27C256 and gave it a go:

http://www.fruitcake.plus.com/Sinclair/ ... Tester.htm

It almost always results in a blue border with white stripes, differing numbers of stripes on each reset. The main paper colour is always black and it's a bit fuzzy (video issues that I'll sort if I can fix it!) but it looks like blue vertical lines on the black paper. I did so far only once get the diag "128 RAM tester" screen, so whatever it is, it's seems rather intermittent/random in nature.

I do have a multimeter but do not yet know (I'll look at schematics) where to find the +5 and +9v.

As for how long it takes before going wrong from cold, it varies between probably 5 seconds to 30 seconds if I'm really lucky.

Erm wrote: ↑Thu Oct 17, 2019 9:51 pmI do have a multimeter but do not yet know (I'll look at schematics) where to find the +5 and +9v.

Edge-connector (pin-out in the manual), or the header that connects the wires to the voltage regulator (mounted on a heatsink). The two outer pins, one is +9V to the regulator, the other is +5V output from the regulator. The negative / black probe goes to a 0V/GND point. The centre pin of this connector or the UHF output socket is another handy place
Wherever you use, just be careful you don’t slip and short anything out.

Erm wrote: ↑Thu Oct 17, 2019 9:51 pmAs for how long it takes before going wrong from cold, it varies between probably 5 seconds to 30 seconds if I'm really lucky.

That’s a bit quick for the normal warm up temperature, with a chip that has become heat sensitive

If the test ROM is showing an image as you describe, then it is not likely to be the multiplexer chips. And this fault could be tricky to find. Can you post up a photo of what the test ROM shows on screen.

Mark

Yes, of course, the voltages at the regulator! Will check those. And I'll do some pics of the diag screen shortly.

According to this:

https://spectrumforeveryone.com/wp-cont ... matics.jpg

R68 should be 100 ohms but was 1K ohms on some boards. It is indeed 1k ohms on mine, so I'll change that to 100 ohms, can't hurt. I've already replaced a dud TR9

Well, that was embarrassing.

On the 5v reg output (and exp port) I had 7.32v! Wish I'd checked that before swapping the RAM out. I assumed like a newbie that it must be something more serious because it sort of worked sometimes and couldn't possibly be the regulator.

I have some 150115 switched regulators with max 1.5A. The original 78S05 is max 2A. Do the grey +2's really need 2A? Think I might see if I can get away with the 150115.

Mark, thanks for your assistance yet again.

The circuitry on the main board does not need more than 1.5A, but I don’t know how current much the cassette deck needs.
I don’t think I have ever measured it.

Mark

Retroleum lists the reg as 2amp:

https://www.retroleum.co.uk/spectrum-128-modifications

Although it doesn’t mention whether it is the grey model specifically.

It says +2 and not +2A/B and I'm pretty sure that the +2 grey all had 2 amp originally, but I suppose I'm not really likely to be using the tape player all that often so it will probably be ok (at my own risk).

The +2A, +2B, +3 and +3B use an external PSU that provides a +5V rail (and +12V and -12V rails), so don’t have any regulators inside the computer.

Mark