Composite mod issue: left side of screen missing - solved!

Discussions about Sinclair ZX80 and ZX81 Hardware
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1024MAK
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Re: Composite mod issue - left side of screen missing

Post by 1024MAK »

patters wrote: Fri Mar 26, 2021 7:27 pm My modulator can has the wires crossing over each other a bit because I wanted them long enough to allow fitting and removing the can without having to re-solder everything - specifically in case of this kind of palaver!
You do have to be a bit careful when routing wires operating at video frequencies (5MHz). Please do try to keep all wires as short as possible. Especially the 0V/GND. And keep the input video feed as far as possible away from the video output. Otherwise sometimes, unintentional oscillation may result.

Mark
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Re: Composite mod issue - left side of screen missing

Post by patters »

Thanks, will do once I have something that isn't slicing part of the image away.

In your opinion, should I return the two composite mods I bought as defective? They were advertised as working on older ULAs. Or should I keep one since my other ZX81 has the newer ULA, meaning I ought to be able to run it without the 555?
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Re: Composite mod issue - left side of screen missing

Post by 1024MAK »

I’m working today, so won’t have much time to think about this until tonight. I think it is worthwhile doing a couple more tests before returning it/them. Once I’ve got what I would like you to do clear in my head, I post again.

If the ‘back porch’ is already being generated (because you have the third and final ULA version, the 2C210E), the NE555 does nothing useful, so it can be left out. Then you have a spare.

Mark
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Re: Composite mod issue - left side of screen missing

Post by patters »

Thanks. My hope is that it can be made to work by tweaking one of the resistor values for instance. I took some close up pictures of the second mod before I fitted it.
  • The electrolytic cap is 22uF
  • The ceramic caps are labelled 102 and 103
  • The trimmers are 10K
  • The resistor colour bands can be seen below (pretty out of focus unfortunately):
    resistors.jpg
I haven't mapped out the PCB, but hopefully the values above allow deduction of which components are which.
patters
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Re: Composite mod issue - left side of screen missing

Post by patters »

Incidentally, I just tested my now modded Issue 3 (which has the 2C210E ULA) by re-inserting the 555 into the mod and the result is the same slicing off of the left-hand border. So it's completely clear this circuit is a bad design and not fit for purpose. Remove the 555 and it's a perfect picture. The seller has sold lots of these in the wild and so it's strange I'm the first guy on here to report the issue. The auction listing I purchased from listed 13 units, and the link Moggy found shows a quantity of 23 sold. My test TV is a Sony Bravia that was expensive in its day, not some no-brand supermarket LCD. So if a composite signal doesn't work on this then it's no good.
patters
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Re: Composite mod issue - left side of screen missing

Post by patters »

The seller has ignored me for 48 hrs so I've messaged him once again, asking to return the defective item that I cannot use (for the Issue 1 board with the older ULA).

Also, I see the Zigg.net design has 3 caps, compared to the 2 in the mod I bought.
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1024MAK
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Re: Composite mod issue - left side of screen missing

Post by 1024MAK »

Okay, an update.

Earlier this year, before this topic was started, I bought one of these modules to see what they were like. Up until today, I had not got around to trying it.

I can confirm that as the OP says, the left hand side of the ZX81 display is missing.

The reason for this is due to the NE555 timer, timing for too long when it has been triggered.
Now the minimum time that a genuine NE555 is specified to time for is 10 µs. The pulse width of a line sync pulse from a ZX81 is about 5 µs. So under ideal conditions, a back porch of about 5 µs is about right.

But in this circuit, you are instead getting an extra long line sync pulse of about 15 µs, and hence why the left hand side of the picture is missing. The picture information is being overridden by this sync pulse that is too long.

So swapping the NE555 for another NE555 from another source may help (if it’s slightly quicker). Reducing the value of the timing capacitor (from 1 nF to 820 pF) may help.
Removing RV2, one of the 10 kΩ preset resistors and fitting a normal 10 kΩ resistor with a capacitor (82pF) across it (in parallel) should also help. The point of the capacitor, is that we need the transistor to turn off as fast as possible.

The level of the generated back porch is also wrong, it’s too black (the voltage drops to the sync level rather than to the black video level). So the 47 Ω resistor (R3) needs changing for a higher value (maybe 100 Ω).

I can’t say for certain which of these changes is definitely needed, as I need more time to do further testing.

Mark
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Re: Composite mod issue - left side of screen missing

Post by patters »

Is yours from this exact supplier, or one of the many similar variants? The reason I ask is that the seller refunded me for the one I can't use and so I was considering buying another mod from another source:
https://www.youmakerobots.com/home/73-zx2020.htm
https://rudedogretros.co.uk/product/sin ... rator-mod/
If they're all likely to exhibit this behaviour then I'll hang back and wait for your testing.

I noticed that GadgetUK164 built Joulespercoulomb's circuit and his produced a perfectly intact picture, even with an old ULA needing the back porch (as seen at 16m59s). The ULA can be seen labelled 8119 at 5m01s in the video.
Last edited by patters on Tue Mar 30, 2021 12:43 pm, edited 2 times in total.
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1024MAK
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Re: Composite mod issue - left side of screen missing

Post by 1024MAK »

The one I have, I bought from eBay from a seller in the U.K.
Here’s a photo of it before I started tinkering:
eBay bought, NE555 based back porch module
eBay bought, NE555 based back porch module
As I said above, this general design sails at the limit of what the bipolar NE555 can do. I intend to experiment some more later today.

I would suggest that it would be wise to wait a few days.

Mark
ZX81 Variations
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ZX81 Video Transistor Buffer Amp

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Re: Composite mod issue - left side of screen missing

Post by patters »

Yes that appears to be the identical PCB layout. Cool, I'll wait.
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