White screen of death

Discussions about Sinclair ZX80 and ZX81 Hardware
sirmorris
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Re: White screen of death

Post by sirmorris »

Yep.
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1024MAK
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Re: White screen of death

Post by 1024MAK »

There is no pull-up resistor shown on the schematic on the /ROMCS line. Just a series resistor between the ULA and the ROM. The ULA actively drives it high normally and pulls it low when access to the ROM is needed.
With the computer powered down, can you use your multimeter on the resistance range and test as follows:
ULA pin 13 to +5V and then to 0V/GND.
Do the same for ROM pin 20.
Then test between ULA pin 13 and ROM pin 20. This should pick up resistor R28 (680 ohms).

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

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sirmorris
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Re: White screen of death

Post by sirmorris »

I knew it was _something_ like that :roll: :oops: :lol: :lol:
nama
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Re: White screen of death

Post by nama »

Hi guys,
I didn't see your last replies as I was busy pulling the ULA.
Dropped in a socket and installed the ULA from my junker ZX81. It Works!!!
(Actually I first tried the dead ULA in the junker machine first, just to be safe and sure.)

It's a little saddening to see my old ULA dead and that with the new ULA the image quality has deteriorated greatly.
The old ULA was 2C210E, and the new one is 2C184E.
The old one gave a beautiful crisp image on a modern LCD screen using only RF. the new one is dark (have to dial up the brightness to full) and has terrible artifacting.

When is someone going to make a new design for a Super ULA???

So anyway, it's working now.
Much thanks for all your help and advice.

Cheers

Phil
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1024MAK
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Re: White screen of death

Post by 1024MAK »

If it just the /ROMCS output pin that is defective, you could build a circuit that replaces that function using either 74 series logic, or programmable logic (GAL, CPLD).

The 2C210E has a back porch generator, while the older 2C184E does not. A lot of colour TVs use the back porch part of the signal to set the black level. You could buy a ZX8-CCB to add the back porch and clean up the signal. This is designed to output composite video direct to a TV.

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
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1024MAK
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Re: White screen of death

Post by 1024MAK »

Here is an extract of a redrawn ZX80 schematic (from Grant's site):
IMG_5217.JPG
ZX80 /ROMCS
(331.85 KiB) Downloaded 298 times
IC6 and IC7 multiplexer chips switch their inputs from one set, or the other set depending on the state of the /REFSH signal. As one input for the /ROMCS output on the multiplexer is connected to 0V, it's possible to simplify the circuit a bit so you don't need a multiplexer chip.
Note that MREQ is an inverted version of /MREQ.

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
nama
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Re: White screen of death

Post by nama »

Thanks again for the reply.
Would something like this work?
Screen Shot 2017-09-02 at 8.15.55 AM.jpg
Screen Shot 2017-09-02 at 8.15.55 AM.jpg (41.99 KiB) Viewed 2829 times
One question.
If the issue is the CS line, would installing the ZXpand (with it's own copy of the ROM) fix this, or does it still rely on the CS line from the ULA for it to work?

Cheers
sirmorris
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Re: White screen of death

Post by sirmorris »

If the issue is the CS line, would installing the ZXpand (with it's own copy of the ROM) fix this, or does it still rely on the CS line from the ULA for it to work?
ZXpand takes total control when required, and does not rely on ROMCS for anything. It relinquishes control during video generation. The ULA needs to be able to drive the ROM chip in order to get the character data out of it at the right time.
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Andy Rea
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Re: White screen of death

Post by Andy Rea »

if it is just the romcs output from the ULA dead then a single 7432 or gate might work

maybe worth a go.... cuz the 210e output is way better.

regards Andy
what's that Smell.... smells like fresh flux and solder fumes...
nama
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Re: White screen of death

Post by nama »

Hi Andy,
Can you briefly explain how that would be hooked up?

Cheers

Philip
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