Sinclair ZX80 / ZX81 / Z88 Forums

hi all,
sorry if this has been covered before, but i could do with some advice about removing a composite mod. i have a zx81 which has had the mod with the 555 done, and the output wired into the RCA rf output. all well and good but none of my old TVs have composite input. I want it to be rf again. at the minute in order to use it I have had to make a scart 'cable' by thrusting bent thick wire into scart holes 17 and 20, clipping bench supply ground to one end, and soldering the other to the output of the mod. this isnt terribly attractive. it was just to see my 81 work, as i lack a composite to scart cable. and anyway i really want to use my 81 with my 1976 b and w portable, not a 'modern' CRT with scart. it was an educational experience getting it to do anything at all as it came with no cables. at first my homemade power lead didn't work, not until i gave ground to both the sleeve, and the ring of a stereo 3.5mm jack i happened to have laying around.

should i remove the 555 mod, and try to re-solder the clipped wires in the um1233, or would i be better off taking a 1233 from a knackered pong TV game i have, and trying to replace the butchered one entirely? I am not even sure if all 1233 are identical? or maybe put the new 1233 in a little box behind my 81, take the output from ula pin 16 to it, then just plug that into my CRT?

what would more the far more knowledgeable people here do?
also the very topmost line of text on my TV is slightly warped. is this a 555 mod thing, or a product of my homemade scart cable, do you suppose?
thanks for your time.

my proud achievement- doris

I wouldn't know anything about bodging cables and connections. Your Honour, I would like thousands of similar offences to be taken into account....

And yes, as I have said on this forum and others, 3.5mm mono and stereo plugs/sockets don't work well when mixed...

DoktorDoris wrote: ↑Thu Jan 16, 2025 12:00 pm should i remove the 555 mod...

It depends.

I'm assuming you want a proper permanent 'fix'.

If there is enough length on the wires, to reinstate the existing modulator do the following:

Before removing the existing composite video board and the wiring for it I do recommend taking note of what is connected to what and taking photos just in case you need to know later on.

Disconnect the existing composite video board and it's wiring.

This applies only to UHF modulators as used in Europe. Viewed from the front of the board (edge-connector furthest from you) the middle hole in the modulator is for the video signal. The right hand wire is the +5V power. Originally the left hand hole was empty and unused.

Reconnect the wires to the appropriate PCB pads, extending them if needed.

Let me know which issue board you have if you need help identifying the correct PCB pads.

Inside the modulator, you need to find the vertically mounted resistor that is very near the signal tab of the phono socket. The top lead of this resistor has to be soldered to the signal tab of the phono socket.

That's it, just needs testing

The top line of text, or the cursor and/or edit line text, being distorted is caused by the ULA. I don't know why this happens. The symptoms may change as it heats up. It mostly affects the earlier ULA chips. The last ULA version is better. Apart from changing it, I don't know of any solution.

Mark

I'm sure either Mark or one of the other gurus will answer more fully your questions but just a few observations .

1) Please don't connect the ULA directly to the outside world without some form of buffer, again someone will come along and explain better than I can why.

2) The "bending" of the top line of the display. I have come to associate this with a Zeddy having the 2C158E ULA which was very earliest issue of the device fitted, I believe it has something to do with a timing issue in that device (some say that it's just the ULA on its way out so opinions differ.)

Could you check the number on the topside of the chip to confirm? If it says 2C184E or 2C210E then it's one of the later types, if all you can see is a date code then it's most likely to be an early 2C158E the code of which can be found on the underside of the chip.

It seems I've cross posted with Mark so do follow his advice.

Moggy wrote: ↑Thu Jan 16, 2025 1:18 pm 1) Please don't connect the ULA directly to the outside world without some form of buffer, again someone will come along and explain better than I can why.

Two reasons, first you (or your TV/display/monitor/converter) may damage the ULA and new chips are not available anymore. Secondly, the signal level is wrong if you are trying to do a cheap composite video modification.

Here’s a photo showing the resistor inside the modulator that I described in my earlier post.
(Modulator, the resistor connected to the phono signal pin)

Mark

Wow again, you've all been really helpful. I must admit I was a bit baffled by the fact that there are three wires, well three holes for wires in the modulator but you've answered all my questions yet again, I'm off to go and turn the iron on.

My ula is covered in little adhesive heatsinks. I presume it didn't come out of the factory like that? I do somewhere have some little adhesive heat transfer pads so I'll piggle the heatsinks off. Find out what's written on top of it and then replace them. Then report back to tell you what version ula I've got.

Doris

There is provision for three inputs in the case for other modulator types.

Mark

cheers mark. i must say i am awfully impressed by the way you reply to my questions with such alacrity, and in an unfailingly helpful way. i haven't used forum based websites for years, and as i recall the standard answer to a question used to be 'use the forum search facility you indolent bugger'. thanks for not saying that to me.

doris