Redgate gets a Zeddy

[redgatemodels]

Wow, I can't believe that I've got an issue that hasn't been seen before

I'll see about taking a picture of the screen and post it up, it's not a perfect inverted display. I've a membrane on the way so once fitted I can do further tests etc.

Hope the ULA is not toast though.

[Moggy]

@Mark.

When you say done in the factory do you mean this particular one or that these components were fitted topside as the rule in the factory?

The reason I ask is that looking at the motherboard there looks to have been a lot of heat applied around the sockets as if they have been crudely de-soldered with a view to replacement. I checked against a picture of another board plus one of my own and the sockets on Redgatemodels board look newer and different to the ones fitted as standard.

If you check the sockets footprint in the pictures, the ones on Reds board are slightly wider which leads me to think they are replacements and being wider, the filter resistor and cap has had to be soldered to the underside of the board as there is no longer room topside ( very very crudely it has to be said ) and I don't think Timex would have let something like this pass muster.

Click on the images to magnify and see if you agree.

[redgatemodels]

Here’s an couple of pics, whole screen plus a closeup of the cursor. On closer examination it’s not inverted, but has a black background rather than white. K looks black too to my eyes. Camera has made the black look blue.

This is uhf, not composite

[1024MAK]

Ahh, that may be symptoms of a colour TV getting the black level wrong because it’s confused by the lack of the back porch part of the video waveform.

The ULA that you have does not generate the back porch. As Sinclair designed the ZX80 and the ZX81 to work with inexpensive black and white / monochrome TVs, this did not matter. But it does matter for most colour TVs.

You may be able to get a grey picture by playing with the contrast and brightness controls.

Otherwise you need a back porch generator module.

Mark

[redgatemodels]

A comp mod with back Porch is on the list, this will sort it out?

[1024MAK]

Should do. I recommend the ZX8-CCB.

Mark

[redgatemodels]

How about this? https://www.ebay.co.uk/itm/225407247107 ... media=COPY

I'd rather get something from the UK

[Lardo Boffin]

The postage is a bit higher for the ZX8-CCB but you could also get the rather good ZX-KDLX keyboard from there at the same time and save on costs!

I have fitted a lot (lost count) of ZX8-CCBs. They are very good!

[redgatemodels]

As an aside, why "back Porch" what are the origins of the phrase? I know its a timing signal etc, just curious as to the name.

[Moggy]

How about this? https://www.ebay.co.uk/itm/225407247107 ... media=COPY

I'd rather get something from the UK

The ZX8-CCB has two adjustable pot's so as to tailor the output for different TV's, something the board in your link doesn't have so it has to be hoped that it would work on your particular TV.

The ZX8-CCB also has a couple of solder pads to which you can solder a pair of thin wires, leading to the outside of the zeddy, which can then either have a jumper or small button switch fitted which when used gives inverse video and border. Something I use on all my ZX8-CCB fitted 81's.