Hi,
I have a couple of issue one ZX81’s and one has the early ULA without the back porch signal and early buggy ROM and the other the later ULA/ROM combo. I have fitted some homemade 555 composite mod boards to both and they work great, giving a stable clear picture.
When I fit the vLA81 to either all I get is either a black screen when the settings of MOD and BP are off or a white screen if the are set to on. If I switch to either one or other being off/on all I get is a black screen.
So I went and fitted a Transistor mod instead and removed the 555 mod and with an original ULA I got the “K” cursor back but with the vLA81 fitted it was the same behaviour as above.
I followed proper ESD procedures and it has not been out of the anti static bag since it arrived a couple of months ago.
Am I doing something wrong with this chip? or the settings or which video mod is used?
vLA81 white screen on working ZX81
- 1024MAK
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Re: vLA81 white screen on working ZX81
I suggest you reread the installation instructions. Check that ALL pins of the module are making good contact in the socket. Even better if you can test using a digital multimeter on the continuity range.
The composite mod. boards that use a 555 timer chip are intended only for use with ULAs that don’t generate the back porch part of the video signal. They are not needed when using a vLA81.
Then if you are still having problems, contact Charlie directly via his web site here. He also occasionally pops up here under the username of Bloodnok.
Mark
The composite mod. boards that use a 555 timer chip are intended only for use with ULAs that don’t generate the back porch part of the video signal. They are not needed when using a vLA81.
Then if you are still having problems, contact Charlie directly via his web site here. He also occasionally pops up here under the username of Bloodnok.
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: vLA81 white screen on working ZX81
I had read the manual several times before I posted on here and tried removing the 555 composite mod and fitted a simple 2n3904 transistor only as well. So even with the back porch composite mod removed I’m getting nada.
I fitted new 40 pin sockets before I tried the vLA81 and the fact that they both work with the original ULA’s I have makes me suspect the vLA81 has problems, realistically I’m not going to send it back to NZ, I’ll just stick it away for now and look for an issue 3 PCB to try it on.
I have two working ULA’s for now and I’m building a ZX81+38 so it is not urgent, I just thought I was doing something obviously wrong but it seems not.
I fitted new 40 pin sockets before I tried the vLA81 and the fact that they both work with the original ULA’s I have makes me suspect the vLA81 has problems, realistically I’m not going to send it back to NZ, I’ll just stick it away for now and look for an issue 3 PCB to try it on.
I have two working ULA’s for now and I’m building a ZX81+38 so it is not urgent, I just thought I was doing something obviously wrong but it seems not.
- 1024MAK
- Posts: 5118
- Joined: Mon Sep 26, 2011 10:56 am
- Location: Looking forward to summer in Somerset, UK...
Re: vLA81 white screen on working ZX81
There’s no significant difference in the circuitry between an issue one PCB and an issue three PCB.
Mark
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.