White screen of death
- 1024MAK
- Posts: 5104
- Joined: Mon Sep 26, 2011 10:56 am
- Location: Looking forward to summer in Somerset, UK...
Re: White screen of death
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
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
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.
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.
Re: White screen of death
I knew it was _something_ like that
Re: White screen of death
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
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
- 1024MAK
- Posts: 5104
- Joined: Mon Sep 26, 2011 10:56 am
- Location: Looking forward to summer in Somerset, UK...
Re: White screen of death
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
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.
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.
- 1024MAK
- Posts: 5104
- Joined: Mon Sep 26, 2011 10:56 am
- Location: Looking forward to summer in Somerset, UK...
Re: White screen of death
Here is an extract of a redrawn ZX80 schematic (from Grant's site):
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
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.
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.
Re: White screen of death
Thanks again for the reply.
Would something like this work?
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
Would something like this work?
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
Re: White screen of death
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.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?
Re: White screen of death
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
maybe worth a go.... cuz the 210e output is way better.
regards Andy
what's that Smell.... smells like fresh flux and solder fumes...
Re: White screen of death
Hi Andy,
Can you briefly explain how that would be hooked up?
Cheers
Philip
Can you briefly explain how that would be hooked up?
Cheers
Philip