Sinclair ZX80 / ZX81 / Z88 Forums

redgatemodels wrote: ↑Wed Nov 29, 2023 11:51 pm How about this? https://www.ebay.co.uk/itm/225407247107 ... media=COPY

I'd rather get something from the UK

While modules based in the circuit design by zx.zigg.net are okay, their performance very much depends on the NE555 meeting the datasheet specification. Some “NE555” chips from the far east, don’t. I’ve investigated this on a different module and concluded that the 7555 chip is better in this application.

But I have not tried the module from this seller.

redgatemodels wrote: ↑Thu Nov 30, 2023 2:32 pm 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.

It’s part of the history of the analogue TV signal. A video signal has to contain the following elements:

• The signal containing the picture information,
• A horizontal pulse to synchronise the horizontal position of the electron beam so that lines of the picture align correctly on the screen,
• A vertical pulse to synchronise the vertical position of the image.

That’s all you need for monochrome video.

But with colour images, you need more control signal information.

Either side of the sync pulses, there is a fixed level signal at the voltage for the ‘black’ video level. Note that the bottom of the sync pulses is at a lower (more negative) level. The level before the sync pulse is called the front porch. The one after is the back porch. It’s believed the name comes from the shape of this part of the waveform when viewed on an oscilloscope.

With colour TVs, the circuitry samples the voltage of the signal after the sync pulse, where the back porch is. This helps it automatically determine the black video level of the video signal. This means the TV can cope far better with variations in the received signal level and produce a better picture.

However, the first two versions of the ULA chips for the ZX81 go straight from the sync pulse to the border area “colour”, which is white.

This confuses most colour TVs, hence they then display a very dark picture (dark grey to black).

Mark

Good news, bad news

Received the cheapie comp mod yesterday. Fitted it and on the crappy portable tv get this
I also got this duplication on rf before comp mod. With the big tv get a nice clear display, yay!

The replacement keyboard membrane (bought from RWAP) sadly did not last long, all worked ok until I screwed the case together then no response from keyboard. Top and bottom of it is that the long connector literally bit into the tails and scraped off the conductive material. I’ve ordered a new pair of connectors and another membrane

Question for you all - should there be an insulator underneath the heatsink where it screws to the pcb?

I think so, as this was the root of my keyboard problems. Even after replacing the keyboard connectors and fitting another membrane I was getting no working keys when the case was screwed together. There’s a track from the keyboard that runs right under the heatsink. I guess when the case was screwed up the earthed heatsink contacted this track. A bit of insulating tape later and a fully working Zeddy

16K internal memory upgrade next ……

No insulator required, however it seems you have the heatsink incorrectly orientated hence the shorting out problem.

See the pictures below.

The top one is yours and the heat sink touching the track is obvious, the second picture is how it should be.

Ah ha!

thanks for that, next time it's opened I'll reposition. Not sure if I mentioned this before but this example was taken apart and screwed into a frame or on the wall in it's time. I guess the previous owner put it on wrong ...

redgatemodels wrote: ↑Sat Dec 09, 2023 3:37 pm Question for you all - should there be an insulator underneath the heatsink where it screws to the pcb?

Not normally on a ZX81 (some other boards do need insulation).

Is it an original heatsink?

It’s also possible that the heatsink was skewed or not cut to the correct size, that would do it. As would having it fitted the wrong way round…

Mark

Just the wrong way round, the picture says it all. It's the same shape as the half dozen I have in my scrap pile just incorrectly fitted.
I would also say they are stamped out of aluminium rather than cut,the tool marks on the edges being the give away.

Yeah, apologies, I started writing my reply before reading the most recent posts

I agree, the original heatsinks are stamped as you say.

Mark

Apology appreciated Mark, but in your case unwarranted considering the amount of computers you've saved here plus the excellent advice you've given over the years.

internal 16K has arrived, now to find time to get it fitted