ZX81 Repair (No video for starters!)

Discussions about Sinclair ZX80 and ZX81 Hardware
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fizzyade
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ZX81 Repair (No video for starters!)

Post by fizzyade » Wed Jul 11, 2018 5:11 pm

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

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1024MAK
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Re: ZX81 Repair (No video for starters!)

Post by 1024MAK » 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

fizzyade
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Re: ZX81 Repair (No video for starters!)

Post by fizzyade » Thu Jul 12, 2018 12:08 am

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.

fizzyade
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Re: ZX81 Repair (No video for starters!)

Post by fizzyade » Thu Jul 12, 2018 9:53 am

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
/RESET is correct, high.
/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?

fizzyade
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Re: ZX81 Repair (No video for starters!)

Post by fizzyade » 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.

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1024MAK
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Re: ZX81 Repair (No video for starters!)

Post by 1024MAK » Thu Jul 12, 2018 9:14 pm

fizzyade wrote:
Thu Jul 12, 2018 12:08 am
Doesn’t the ULA produce the modulation independant of the CPU? (I.e it halts the CPU and does a read of the ram).
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

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1024MAK
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Re: ZX81 Repair (No video for starters!)

Post by 1024MAK » Thu Jul 12, 2018 9:16 pm

fizzyade 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.
Bummer that you have a faulty ULA. But glad that one of Andy's ULA modules brings your Zeddy back to life :D

Mark

fizzyade
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Re: ZX81 Repair (No video for starters!)

Post by fizzyade » Thu Jul 12, 2018 10:33 pm

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!

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