Sinclair ZX80 / ZX81 / Z88 Forums

Hi all,

Got a Spectrum +2a (black) that works just fine. It had an issue with a dead RAM chip which I replaced.

My issue is that any expansion card that I plug into the back and power up results in a garbled screen once more. I thought it may have been an issue with a faulty Z80 with a bad /M1 line, but having replaced that with a known working CPU, the issue persists.

Do you guys think that I'm looking at a partially faulty ULA?

If I dump Retroleum's diagrom onto an EPROM and stick it in the ROM socket onboard, it boots and all tests run fine with no errors.

Any thoughts would be helpful!

EDIT: I have also cleaned the edge connector

KbJ

Which expansion cards / interfaces have you tried?

Mark

I had a +3 where if I plugged in my DIVMMC future and didn’t get it in exactly the right place it either wouldn’t boot or would give a garbled screen. The edge connector was cleaned.
I always assumed it was worn and therefore would only make contact and work in certain spots.

1024MAK wrote: ↑Wed Mar 20, 2024 7:31 pm Which expansion cards / interfaces have you tried?

Mark

Hi Mark,

I've tried the ByteDelight diagnostics card, 2 different V3 SmartCards from Retroleum and a Dandanator Mini - all which work fine on other Spectrum machines (including another 2a that I have).

The edge connector has been cleaned, and I've even checked continuity between the fingers of the SmartCard all the way down to the board and measured each track to where they connect to other Via's or points away from the edge connector.

Thanks,

Mark.

Okay.

All those require the machine to page out the internal ROM chips. Do you have any other expansions/interfaces to try, such as a Kempston (or compatible) joystick interface?

Mark

Hey Mark,

Yes, I just tried a kempston joystick interface (which works on a 48K machine) and it's not working on the +2a.

I guess I'm possibly looking at a faulty ULA then?

Thanks,

Mark.

Not sure it's the ULA. Depending on which version (there were a surprising number not including the various clones), a basic Kempston joystick interface only typically uses the eight data bus lines (D0 to D7), address line A5 (and sometimes A6 and A7), /RD, /IORQ and sometimes /M1. And obviously, +5V and 0V.

Apart from the power lines, these are all Z80 inputs or outputs. The ULA or more correctly, the gate array shouldn't be involved. And anyway, there are no spares that we know of for the +2A /+2B / +3.

Do you have a 'scope, logic analyser or logic probe?

Mark

Yes, I have a scope and a logic probe

Thanks

Mark.

With your Kempston joystick interface fitted, test the signals I mentioned in my earlier post to see if any are flatlining or otherwise wrong.

So, test these pins on the Z80:

Pin Signal
6 Clock
7 D4
8 D3
9 D5
10 D6
12 D2
13 D7
14 D0
15 D1
17 /NMI (should be always high)
18 /HALT (should normally be high)
19 /MREQ
20 /IORQ
21 /RD
22 /WR
25 /BUSRQ (should be always high)
26 /RESET (should be high unless the reset button is pressed)
27 /M1
35 A5
36 A6
37 A7

Z80 pin-out here.

Mark