Sinclair ZX80 / ZX81 / Z88 Forums

Hi all,

Just wondering if anyone has managed to successfully fit a CCB module to a ZX81 with the early 2C158E ULA.

I have such a machine, that gives the following output when simply composite modded:


'Scoping this output gives the following waveform, looks like the output swing from the ULA is quite weak (2v/div)


So after fitting the CCB module I get the following waveform after careful twiddling, it's extremely hard to get any kind of output that includes a back porch:


And this is the best picture I can manage:


Any words of wisdom for me, or am I just better off restoring the original modulator and keeping it original? I'd rather not swap the ULA as that would ruin the originality (but I have tried the CCB in conjunction with a 2C210E on this board, and it works fine).

Cheers,
B

They are pre-adjusted for the 2C184 ULA and have to be calibrated newly for the very old ULA (which I don't have).

As you have a scope to see it should be more easy to adjust.
I am not sure if my instructions are reliable after rereading them a few years later.
I can adjust my CCB in the same way when wrong adjusting R1 (sync level).
If sync level gets the video pixel level there will be a backporch created at the position of the K cursor which results in a long black bar as seen.

So try to turn the sync level to maximum input level while turning clockwise till end (carefully - can be overturned easily).
Then try to get the cursor with the other trimmer R2 while moving R2 slowly.
As the sync level has a wider range you may also try 4 or 5 different positions of sync level when dividing the 300 degree turn circle in equal parts while trying to get the cursor with the other trimmer.

Cool, I'll try this again tomorrow. There's very little adjustment leeway though - turning R1 a hair either way loses all signal on the scope, and very fine adjustment is needed to get the back porch to fire.

B

Don't spend too much time on the back porch - this is done automatically if hsync is present or found.
I would concentrate to get the hsync steady first, you would see the back porch then as well.
There maybe an issue with the voltage levels from the very old ULA 2C158 as well.
Could you please measure out with your scope with is going in to the ZX8-CCB via the yellow cable, so the voltage level of sync (lowest), white (highest) and if you can see the black level (pixel black). Maybe you have to type some more chars in the first line to see good. It could also be done with a small program when the RF interface is still working:
You may use an inverted space as well to see more better.



Here is an example what would be good to know as voltage levels:
If the trigger of the sync is moved too wide up to the black level (seen as "K cursor") you would get the black bar as in your pictures.

Ok, here's a capture with R1 turned all the way to the right, and R2 adjusted so the cursor can be seen. Input from the ULA is the yellow trace, output from the CCB is the green, 1v/div.



Input white level measures as 4.21v, sync and black level both measure at 3.85v. Output is 1.14v white, 0.43v black and sync (no back porch). TV output is equivalent to a straight composite mod at this point. Next I'll adjust R1 to try and achieve a stable back porch.

Um, the output from the ULA on the video output/tape pin is suppose to be a three level output:
Peak white level = highest voltage
Black level=medium voltage
Bottom of line sync pulse = lowest voltage.

In your 'scope traces, I only see two levels...

If possible, use a 0.5V/div or better still a 0.2V/div setting on your 'scope please (may need to use AC coupling if you run out of adjustment range).

Does this Zeddy work okay via UHF on a Zeddy friendly telly (which of course, you don't have, otherwise you would not have bought a ZX8-CC8 )

Mark

Correct, there are only two levels on the input trace, even after adjusting down to the 100mV range there's negligible difference in voltage level between the sync pulse bottom and the black level (coupled through a 10uF cap):


Screen display at the time (appears brighter than it actually is)


Also you're right, I don't have a Zeddy friendly TV (even the mid 80's Toshiba residing in the attic hated 2C184E equipped machines), however I've seen pictures of this working on a B&W TV prior to purchase, and the seller is a fellow Sinclair nut and knows his stuff.

I think I may have to leave this one as is and live with the dark output. I know of one other person who's in the process of modifying his 158E equipped Zeddy with a CCB module, will be interesting to see how he gets on (I'm sure he'll be along in due course )

It most likely will fine work on a B&W TV, as it has no colour circuitry...

I would go and test the ZX81 that I have with that version of ULA, except the only one to hand with that version of ULA (2C158) is one where the ULA produces no clock output for the CPU... (it's an early date code one, so it's in the list of "do I repair it or leave it as it is?").

I forget if I have a working ZX81 with another 2C158. Most of mine are either 2C184E or 2C210E. I normally use a Sharp Aquos TV that works fine with both these

I would look at my ZX81 info details file, but that went to silicon heaven with the death of the old laptop some years ago

Mark

Not to worry Mark. I think tonight's 'scoping and your reminding me of the three level ULA output makes clear that either this is a characteristic of the 158 ULA, or mine is faulty in some way.

Either way, as the black level is at the same voltage as the sync pulse, the CCB module can't tell the difference between the two and can't reconstruct the back porch without also getting in the way of black pixel output.

Cheers

B

I have a few ZX81s, three are V3 motherboards and work perfectly with the CCB board. My fourth will tune into a 22" flat screen Phillips TV giving a fuzzy but recognisable cursor. When I added the ZX8 CCB I get the same image as Balfords Fairly sharp cursor but about 4 or 5 characters wide. No amount of tuning will get it right.

Im using the latest ULA (210E?) on a very early V1 board. Could the problem be caused by the motherboard rather than the ULA or ZX8CCB?