Hi,
I recently obtained my first ZX81! That's the good news, the bad news is that it doesn't work.
I've probed around the board at various stages, I'm seeing a 3.25Mhz oscillation going to the Z80 which appears to be generating reads and addresses.
The oscillator input on the ULA appears to be at 3v with the oscillation superimposed on that, maybe a few hundred millivolts, but I assume this is OK as it's outputting the 3.25Mhz signal.
I'm seeing a static DC signal on the video out on the ULA, no modulation. I've tried replacing the ULA with one of Andys recreated ULA's but I still get no video output. Humn.
Wondering if it might be the modulator pulling the line to a voltage, but thought I'd ask around here to see if anybody has seen this problem before or has any suggestions of where to look.
I don't have another working unit, so I can't compare signals to anything so I'm working in the dark a little here!
Thanks
Ade
ZX81 Repair (No video for starters!)
- 1024MAK
- Posts: 5142
- Joined: Mon Sep 26, 2011 10:56 am
- Location: Looking forward to summer in Somerset, UK...
Re: ZX81 Repair (No video for starters!)
The video generation is done by the Z80 CPU and the ULA working together. If the CPU is not running, for whatever reason, the ULA will not and cannot produce a valid video signal.
So we start with testing the Z80 pins with either a logic probe, a logic analyser or a oscilloscope.
What are /RESET, /MREQ, A14 and A15 lines doing?
A thread with pin-outs is linked to in my signature
Also what voltage do you get on the ULA video pin (pin 16)?
Mark
So we start with testing the Z80 pins with either a logic probe, a logic analyser or a oscilloscope.
What are /RESET, /MREQ, A14 and A15 lines doing?
A thread with pin-outs is linked to in my signature
Also what voltage do you get on the ULA video pin (pin 16)?
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: ZX81 Repair (No video for starters!)
I’ll have a check in the morning, address lines were going as was RD, i’ll Prove them specifically tomorrow.
Doesn’t the ULA produce the modulation independant of the CPU? (I.e it halts the CPU and does a read of the ram).
Thanks for the response. New to the ZX81.
Doesn’t the ULA produce the modulation independant of the CPU? (I.e it halts the CPU and does a read of the ram).
Thanks for the response. New to the ZX81.
Re: ZX81 Repair (No video for starters!)
/RESET is correct, high.1024MAK wrote: ↑Wed Jul 11, 2018 11:43 pm The video generation is done by the Z80 CPU and the ULA working together. If the CPU is not running, for whatever reason, the ULA will not and cannot produce a valid video signal.
So we start with testing the Z80 pins with either a logic probe, a logic analyser or a oscilloscope.
What are /RESET, /MREQ, A14 and A15 lines doing?
A thread with pin-outs is linked to in my signature
Also what voltage do you get on the ULA video pin (pin 16)?
Mark
/MREQ, A14, A15 are pulsing as would be expected.
ULA pin 16 4V
I've bent pin 16 to isolate it and still no modulation.
Must be the ULA?
Re: ZX81 Repair (No video for starters!)
It's the ULA.
I noticed that IOREQ wasn't pulsing, swapped out to Andys ULA and hey presto pulsing.
Looked at the video signal in more detail and there was indeed modulation (very noisy signal), held a composite cable against the modulator case and signal and whoosh a "K" on the TV.
I noticed that IOREQ wasn't pulsing, swapped out to Andys ULA and hey presto pulsing.
Looked at the video signal in more detail and there was indeed modulation (very noisy signal), held a composite cable against the modulator case and signal and whoosh a "K" on the TV.
- 1024MAK
- Posts: 5142
- Joined: Mon Sep 26, 2011 10:56 am
- Location: Looking forward to summer in Somerset, UK...
Re: ZX81 Repair (No video for starters!)
No, it's more complicated than that. HALT is used for synchronisation. Read the information available here
For a more in depth explanation of the video system, read this
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: 5142
- Joined: Mon Sep 26, 2011 10:56 am
- Location: Looking forward to summer in Somerset, UK...
Re: ZX81 Repair (No video for starters!)
Bummer that you have a faulty ULA. But glad that one of Andy's ULA modules brings your Zeddy back to lifefizzyade wrote: ↑Thu Jul 12, 2018 2:33 pm It's the ULA.
I noticed that IOREQ wasn't pulsing, swapped out to Andys ULA and hey presto pulsing.
Looked at the video signal in more detail and there was indeed modulation (very noisy signal), held a composite cable against the modulator case and signal and whoosh a "K" on the 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.
Re: ZX81 Repair (No video for starters!)
Yeah, i’ll Probably reunite the machine with an original from Ian at some point.
Happy to see it come back to life, even happier when the keyboard worked without issue as well!
Happy to see it come back to life, even happier when the keyboard worked without issue as well!