The ZX81 I purchased did indeed turn out to have a faulty ULA! Was so happy when this replacement bought the "K" to the screen!
Many thanks Andy!
Search found 8 matches
- Thu Jul 12, 2018 10:33 pm
- Forum: Hardware
- Topic: ZX81 Repair (No video for starters!)
- Replies: 7
- Views: 4901
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!
- Thu Jul 12, 2018 2:33 pm
- Forum: Hardware
- Topic: ZX81 Repair (No video for starters!)
- Replies: 7
- Views: 4901
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.
- Thu Jul 12, 2018 9:53 am
- Forum: Hardware
- Topic: ZX81 Repair (No video for starters!)
- Replies: 7
- Views: 4901
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, /...
- Thu Jul 12, 2018 12:09 am
- Forum: Welcome Area
- Topic: Hello
- Replies: 4
- Views: 3599
- Thu Jul 12, 2018 12:08 am
- Forum: Hardware
- Topic: ZX81 Repair (No video for starters!)
- Replies: 7
- Views: 4901
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.
- Wed Jul 11, 2018 5:39 pm
- Forum: Welcome Area
- Topic: Hello
- Replies: 4
- Views: 3599
Hello
Hi,
I've been frequenting various spectrum groups and BBC groups as these are the machines that I grew up with and have a huge amount of love for!
Just got a ZX81 which I am trying to repair, hence why I'm here!
Been a lurker for a while but thought I'd better sign up.
Ade
I've been frequenting various spectrum groups and BBC groups as these are the machines that I grew up with and have a huge amount of love for!
Just got a ZX81 which I am trying to repair, hence why I'm here!
Been a lurker for a while but thought I'd better sign up.
Ade
- Wed Jul 11, 2018 5:11 pm
- Forum: Hardware
- Topic: ZX81 Repair (No video for starters!)
- Replies: 7
- Views: 4901
ZX81 Repair (No video for starters!)
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...