issue loading program while video output is connected to TV

Discussions about Sinclair ZX80 and ZX81 Hardware
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ploughna
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Joined: Wed Oct 19, 2022 2:00 am

issue loading program while video output is connected to TV

Post by ploughna »

Hi all
First post on the forum! I recently resurrected my ZX-81 that I bought new with my hard-earned savings as a teenager back when it first came on the market. I've replaced the keyboard, installed 16K RAM and installed a composite video (with backporch) board from zxrenew.co.uk and am feeding video to a composite to HDMI adapter for display on a computer monitor. I'm using a Maxduino to load software.
Here's the issue: occasionally the monitor screen goes blank and comes back again after a second or two. I'm guessing it doesn't like something about the video signal (adjusting the pots on the board). The screen also goes blank when loading a program but I suspect this is normal.
Anyway, I decided to connect to the composite video input of an old CRT TV that I had to compare - no issues with this, picture is completely stable so I'm thinking it's simply more tolerant on whatever fluctuations might be happening with the signal. However, I cannot load programs successfully while connected to the TV! I see the bars on the screen, but the loading process fails every time (bars stay on the screen when the Maxduino has finished playing). If I go back to the HDMI adapter/monitor programs load fine. I can even load while connected to the monitor, then when loaded, switch over to the TV and it remains ok - it's just the loading that's a problem.
My theory is that having the video output connected to the TV input is somehow 'loading' the signal and degrading it somehow. Has anyone seen anything like this before and/or have any ideas?
Thanks!
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1024MAK
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Re: issue loading program while video output is connected to TV

Post by 1024MAK »

Hello and welcome ploughna :D

I’m not sure what’s going on. Do keep in mind that some video receivers/converters/modern TVs don’t like the ‘strange’ to them signal that the computer produces when loading. As it’s very far from being a standard video signal.

The ZX81 ULA does use the video output pin for both the video generation and the signal for saving (MIC). But the ULA has a separate tape input pin (for the cassette EAR). So it should not be affected by whatever video receivers/converters or TV is connected.

Is your TV a double-insulated type (no earth pin connection on the mains plug)?

Have you tried loading from a real cassette tape?

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

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ploughna
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Joined: Wed Oct 19, 2022 2:00 am

Re: issue loading program while video output is connected to TV

Post by ploughna »

thanks for the quick reply. yes, TV is double-insulated and no, I haven't tried a real cassette tape (I don't have one anymore!)
Honestly, I'm not overly concerned about the TV if I can make the HDMI converter work more reliably. I might just buy another brand for comparison as they're very cheap (the one I have is called AV2HDMI - I bought this one as I saw it referenced by a few people here on the forum)
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1024MAK
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Re: issue loading program while video output is connected to TV

Post by 1024MAK »

I only mention the TV being double insulated, as sometimes the system not having a mains earth connection can affect things. Yes, it shouldn’t, but…

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.
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