ZX81 - Missing vsync?

Discussions about Sinclair ZX80 and ZX81 Hardware
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Netaro
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Joined: Fri Sep 03, 2021 10:31 pm

ZX81 - Missing vsync?

Post by Netaro »

Hello!

So I've got a zx81, if I recognize correctly it's an issue 3 board:
IMG20210904201107.jpg
As I don't have any tv that accepts rf input, I've removed the rf modulator and inserted a composite mod inside, but tv didn't detect anything. Thinking it was a bad ULA, i've ordered a replacement, vla81, but result's the same, nothing.
After probing the output with my oscilloscope, i've noticed that there's no vsync signal visible (at least, that's what I think is missing). It appears that something's sent to the video output though, so what might be the problem there?
IMG20210904201107.jpg
IMG20210904201107.jpg
I've checked the z80 with a backbit chip tester, and it appears it's OK. Thinking that electrolytic caps might've gone bad and could cause the problem I've replaced them, but the problem still persists.
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bola_dor
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Re: ZX81 - Missing vsync?

Post by bola_dor »

What composite mod sis you implemented? I see the RF modulator is empty, does that ULA replacement output a proper composite signal? Does it need any configuration?
If you have an oscilloscope you can see what's the original ULA output at pin 16.
Use a transistor mod with a 1n4148 diodes at the ula output at least to try.. then you can use a more sophisticated mod..
In the last photo.. that's not a white screen.. and unless the cursor it should be al flat line at the upper range with horizontal sync pulsing down..
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BarryN
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Re: ZX81 - Missing vsync?

Post by BarryN »

First, many TVs will surprise you and STILL accept an analog RF signal on the cable TV input, because some cable companies STILL broadcast analog RF over cable. Second, looking at the oscilloscope trace, it looks like I see a horizontal sync pulse and a back porch signal, but I can't tell from the pictures what the voltage level is. It may be the signal does not have enough amplitude. There is also not enough time of the signal shown to see if there is a vertical sync pulse, but since the horizontal sync is there the vertical is probably present also, I suspect you just haven't boosted the signal to the required level, or maybe there is too much DC offset and you need to add a capacitor in series with the signal.
Netaro
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Re: ZX81 - Missing vsync?

Post by Netaro »

bola_dor wrote: Sun Sep 05, 2021 7:47 am What composite mod sis you implemented? I see the RF modulator is empty, does that ULA replacement output a proper composite signal? Does it need any configuration?
I've used this one: https://www.bytedelight.com/?page_id=3560. It didn't work because the signal lacked vsync.

According to vla81's page:
>Video level selectable between standard video (suitable for direct connection to a 75 ohm monitor input) and high sync level (suitable for the modulator input or some video mod boards).
So it should drive a proper composite signal.
bola_dor wrote: Sun Sep 05, 2021 7:47 am In the last photo.. that's not a white screen.. and unless the cursor it should be al flat line at the upper range with horizontal sync pulsing down..
Yes, it appears it tries to draw something, but as there is no vsync, tv won't 'catch' the signal and won't display anything.
BarryN wrote: Sun Sep 05, 2021 9:00 am There is also not enough time of the signal shown to see if there is a vertical sync pulse, but since the horizontal sync is there the vertical is probably present also
That's the problem: It isn't there. At all. That's what i wrote in my first post, that the vsync is missing. And I'm asking what might be the next troubleshooting step as to why it isn't there.
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1024MAK
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Re: ZX81 - Missing vsync?

Post by 1024MAK »

The vertical sync is a function of the ULA but under software control. That is, the software (the ROM program) controls the ULA, which in turn generates the vertical sync pulse.

In fact, the whole display system relies on the Z80, the ROM, the RAM and the ULA all working together.

The three electrolytic capacitors on an issue three board are fairly reliable and don’t normally cause problems. Two are to decouple the +9V (nominal) input supply. The remaining electrolytic capacitor is part of the Z80 reset circuit.

So, I suggest the next step, is to see what the Z80 is doing. What signals do you get on the following pins of the Z80 CPU?
Be very careful, if you short the probe between adjacent pins you may damage one or more of the chips.

Pin Function
6 Clock (3.25MHz square wave, but with slightly rounded edges)
16 /INT (should be pulsing)
17 /NMI (should be pulsing)
18 /HALT (should be pulsing)
19 /MREQ (should be pulsing rapidly)
21 /RD (should be pulsing rapidly)
22 /WR (should be pulsing)
24 /WAIT (should be pulsing)
25 /BUSRQ (should be high)
26 /RESET (should be high, except for a very short time after power is applied)
27 /M1 (should be pulsing)
28 /RFSH (should be pulsing)

Then test address lines A14 and A15. These give an idea of which memory the CPU is accessing and if it is trying to generate a TV display.
You can also test the remaining address lines A0 to A13 and the data lines D0 to D7. A lack of activity is suspicious.

Mark
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ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

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Netaro
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Re: ZX81 - Missing vsync?

Post by Netaro »

I've checked these lines, and they're all as active as you've described.

By accident I've noticed that when I heat one of the two ram chips, sometimes for a moment an image appears on the display, so I'm guessing that maybe one/both are broken somehow? I've ordered new ones, and check if new ram would solve the problem.
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1024MAK
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Re: ZX81 - Missing vsync?

Post by 1024MAK »

Failure of one of the RAM chips is certainly possible. The symptoms you have are unusual, I’ve not seen this problem before.

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 :!:
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Netaro
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Re: ZX81 - Missing vsync?

Post by Netaro »

New RAM just arrived, swapped it, and it appears that the problem has been fixed!

That was an unusual symptom, right :D
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