USA ZX80 fitting zx8-ccb low video output voltage? Solved

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RetroByte
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Joined: Mon Mar 12, 2018 1:33 pm

USA ZX80 fitting zx8-ccb low video output voltage? Solved

Post by RetroByte »

Hello all,

I have a USA ZX80 here in the UK, I've removed the diode to make it 50Hz and it works fine on a B&W camping CRT and I get the normal very dark picture on more modern LCD and plasm TVs.

The problem is, I can't get a ZX8-CCB working on it, I have four of them and they all work on another ZX80.
I've compered the video output on my scope (I'm not good at using oscilloscopes) and the ZX80 that works with the ZX8-CCBs puts out about 3V from the connection to the RF box, but the one I can't get working only puts out about 1V, so I assume that's my issue?

If so, how can I fix that?

BTW: I've tried adjusting the pots and haven't had any luck, but maybe one of them needs a major adjustment to make up for the low voltage?

Thanks for looking, I hope someone can help :)
Last edited by RetroByte on Sat Aug 12, 2023 4:54 pm, edited 2 times in total.
RetroByte
Posts: 51
Joined: Mon Mar 12, 2018 1:33 pm

Re: USA ZX80 fitting zx8-ccb low video output voltage?

Post by RetroByte »

I'll read this later, maybe it will help.

https://www.sinclairzxworld.com/viewtop ... put#p50632
RetroByte
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Re: USA ZX80 fitting zx8-ccb low video output voltage?

Post by RetroByte »

I've found that R30 is 1K on the working UK machine, but 2.2K on the USA one, I'm going to try another 2.2K in parallel to see if it works.
RetroByte
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Re: USA ZX80 fitting zx8-ccb low video output voltage?

Post by RetroByte »

This USA board is a lot different, it has TR1 installed.
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1024MAK
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Re: USA ZX80 fitting zx8-ccb low video output voltage?

Post by 1024MAK »

You may have to change the resistors and other components to convert the USA board to UK specification.

As far as I can tell, the different modulators used in the different versions had different input requirements.

I don’t know of a schematic that shows clearly the USA circuit design, so before starting work, take plenty of photos from multiple angles and make a note of all the differences you find.

Mark
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Paul
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Re: USA ZX80 fitting zx8-ccb low video output voltage?

Post by Paul »

1024MAK wrote: Fri Aug 11, 2023 1:02 pm I don’t know of a schematic that shows clearly the USA circuit design, so before starting work, take plenty of photos from multiple angles and make a note of all the differences you find.
Mark
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RetroByte
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Re: USA ZX80 fitting zx8-ccb low video output voltage?

Post by RetroByte »

Thanks for the reply, I had already started changing some of the components before seeing your reply, but luckily I did take a few pictures before.

Have a look at one of the photos of the Mic jack, it has a short, which I thought would fix my problem, but it didn't.

I think I have completely made this machine the same as the UK version, by removing TR1, R33, R31, adding R32 and replacing R30 for the different value.

By connecting the composite output or the ZX8-CCB to a TV, I now get the grey screen with the 'K' that immediately goes up the screen and disappears and the screen goes black.

The video voltage is still low compared to the working machine. I have found that from R32 (the sync resistor) to 0v there is a low reading of 0.5ohm, which is not on the other machine. Looking at the circuit diagram, I can only imagine that it's getting that reading, either through IC21 or IC19.

Please let me know your thoughts and let me know if you think I’m on the right track.
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RetroByte
Posts: 51
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Re: USA ZX80 fitting zx8-ccb low video output voltage?

Post by RetroByte »

BTW, the third photo is just showing R36 that I have lifted one end.
Last edited by RetroByte on Sat Aug 12, 2023 4:40 pm, edited 1 time in total.
RetroByte
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Re: USA ZX80 fitting zx8-ccb low video output voltage?

Post by RetroByte »

I'M A COMPLETE STUPID IDIOT! :oops:

When I first got this machine years ago, the RF box had been removed, after getting the machine working on my CRT, I soldered the RF box back on so I could fit the ZX8-CCB, but I didn't notice the insulation part missing, so it was shorting. I'm lucky it didn't do other damage.

I now have it working :) and it only wasted two days :oops:

I hope the photos are of some help to others, let me know if you want more info on this machine :)
RetroByte
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Re: USA ZX80 fitting zx8-ccb low video output voltage?

Post by RetroByte »

A few more photos.
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