White screen on a ZX81

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Quantum
Posts: 7
Joined: Sun Dec 12, 2021 10:26 pm

White screen on a ZX81

Post by Quantum »

Hi to all,
I am waiting on a vLA81 ULA from NZ to fix my TS1000, while waiting I purchased a ZX81 for parts hoping to swap out the ULA.

When I got around to open up the ZX81 I was dissappointed to see all the IC were soldered in, it even has the 2C210 ULA dated 8240. Just what the TS1000 needs.

I decided to keep it all original, but did do a composite mod so that I could use it as is. I did the mod, and all I get is a very nice white screen with no
K cursor

I have tried a ram pack, checked voltage readings all seem OK, I even checked for a clock signal on pin 6 of the Z80 and got a reading of 3.238 MHz.
What other tests can I do? Is there a step by step testing method? Any help is muchly appreciated.

Cheers.
RJ
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1024MAK
Posts: 3989
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: White screen on a ZX81

Post by 1024MAK »

I don’t know of a step by step guide, other than spending some time browsing and reading through the hardware sub forums here…

The ULA appears to be working if you have a white screen and the correct clock signal at pin 6 of the Z80.

Because EVERYTHING (apart from the keyboard) has to be working in order to get to having a “K”, it could be the Z80, the ROM or the RAM.

Did you clean the edge-connector contacts on the board before trying the RAM pack? And is the RAM pack a known good working unit?

If the answers to both are yes, then that tends to indicate the internal 1K bytes RAM chip(s) are not the cause. Leaving the ROM and the Z80 as prime suspects.

So, I suggest the next step, is to see what the Z80 is doing. The following assumes you are using a logic probe or oscilloscope. What signals do you get on the following pins of the Z80 CPU?
Be very careful, if you short the probe between adjacent pins you may damage one or more of the chips.

Pin Function
16 /INT (should be pulsing)
17 /NMI (should be pulsing)
18 /HALT (should be pulsing)
19 /MREQ (should be pulsing rapidly)
21 /RD (should be pulsing rapidly)
22 /WR (should be pulsing)
24 /WAIT (should be pulsing)
25 /BUSRQ (should be high)
26 /RESET (should be high, except for a very short time after power is applied)
27 /M1 (should be pulsing)
28 /RFSH (should be pulsing)

Then test address lines A14 and A15. These give an idea of which memory the CPU is accessing and if it is trying to generate a TV display.
You can also test the remaining address lines A0 to A13 and the data lines D0 to D7. A lack of activity is suspicious.

Mark
Quantum
Posts: 7
Joined: Sun Dec 12, 2021 10:26 pm

Re: White screen on a ZX81

Post by Quantum »

Hi Mark,
Thanks for the reply and information. I did clean the pcb contacts for the rampack, it is a Memotech 64K unit but not sure if it's good.
I do not have a scope or a logic probe, although I have ordered a logic probe and will test the pins as per your last post.
I will let you know the results.
Cheers.

Richard.
Quantum
Posts: 7
Joined: Sun Dec 12, 2021 10:26 pm

Re: White screen on a ZX81

Post by Quantum »

Hi Mark,
I got a logic probe and tested the pins, below are my findings.

Pin Function
16 / INT is pulsing, showing H & L
17 / NMI is pulsing, but rapidly, showing H only
18 / HALT is pulsing, but is rapidly, showing H & L
19 / MREQ is pulsing rapidly, showing H & L
21 / RD is pulsing but not rapidly, showing H & L
22 / WR is pulsing, showing H only
24 / WAIT is pulsing, but rapidly, showing H only
25 / BUSRQ is HIGH
26 / RESET is HIGH, LOW for a short time after power is applied
27 / M1 is pulsing, showing H & L
28 / RFSH is pulsing, showisng H & L

A0 is pulsing, showing H & L
TO
A12 is pulsing, showing H & L
A13 is pulsing rapidly, showing L
A14 is pulsing, showing H & L
A15 is pulsing rapidly, showing H & L

D0 to D7 all have activity.

Let me know what you think.
Cheers.

Richard.
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1024MAK
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Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: White screen on a ZX81

Post by 1024MAK »

The only one of these that concerns me is A13.

Can you please test this again.

Assuming address line A13 is working, then that indicates that the Z80 is likely okay.

Can you test pin 20 on the ROM.

And can you either take a photo of your board (make sure it’s possible to see the details on the ROM) and post the picture here. Or describe the details on the ROM.

Mark
Quantum
Posts: 7
Joined: Sun Dec 12, 2021 10:26 pm

Re: White screen on a ZX81

Post by Quantum »

Hi Mark,

A13 is pulsing very rapidly, showing L only

ROM pin 20 is pulsing showing H & L The ROM is a Motorola ZCM38818P date 82 40
HPIM1809.JPG
HPIM1810.JPG
My old camera is only 5MP but works great for Ebay, I hope the pictures come out OK for you.

Cheers.

Richard.
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