Zx81

[SiriusHardware]

Just found it, reading it now.

https://vdrivezx.files.wordpress.com...ual_v1.3-1.pdf

[SiriusHardware]

I would say the correct settings for your case would be all switches OFF, which equates to

SW1 off = direct 75R output to composite video input
SW2 off = 'Back porch' enabled, like on 2C210 ULA
SW3/4 both off, as you are not using any special memory modes.

Leave all the ULA switches in the OFF position and make a straight through connection from modulator video IN to modulator OUT, bypassing the modulator. Do not use or include a transistor buffer, as the new ULA already includes one.

[John Earland]

Here is the link.
https://retrorevivalshop.co.uk/index...roducts_id=560

As I said I did get a picture but not now returned to original setting. I will modify it as I did with the Spectrum and try that. Thanks again.

[John Earland]

Here’s the manual:
https://vdrivezx.files.wordpress.com...ual_v1.3-1.pdf

[julie_m]

Well, that's certainly not bad for a first attempt! Well done If the damaged pad was not connected to anything on its side of the board, don't worry too much about it. Just make sure that there is some solder on the side where it is connected to something. If any of the holes has lost its through-plating, be sure to solder the socket pin on both sides of the board so as to complete the circuit. Start by soldering just two opposite corners of the new socket, and double-check it is sitting level and the right way around (the "pin 1/pin 40" end will look different from the "pin 20/pin 21" end somehow, depending on the manufacturer) before continuing. Been there, done that, grown out of the T-shirt ..... If any more pads break off when soldering in the new socket, cut the track cleanly with a sharp knife blade to limit the damage, solder a single strand taken from some hookup wire to the end of the track and wrap it around the pin. Use long-nosed pliers to shape it. If you grip the wire between the track and the pin, the pliers should conduct some of the heat away so as not to melt the first joint undone. You probably won't need to do this, but forewarned is forearmed.

I've been thinking about getting a "proper" desoldering station, rather than ing about with de-solder braid, plunger pumps that need priming each time or even just deft flicks of the wrist. I mean, just because you can do a good job without an expensive, special tool is no good reason not to get one, is it?

EDIT: Oh, too late, I see! Never mind, it's a great job.

[SiriusHardware]

I managed for decades without an assisted (electric) desoldering iron and then I went to a job where every bench had one as standard, and that ruined me. I wouldn't consider not using one now. I have an oldish Weller at home, and an Oki / Metcal at work.

I'm not sure what it is that John has, the simplest type of 'electric' desoldering iron is very similar to the manual teflon tipped plunger type, except that the nozzle is metal, and heated.

[John Earland]

Originally Posted by SiriusHardware

I managed for decades without an assisted (electric) desoldering iron and then I went to a job where every bench had one as standard, and that ruined me. I wouldn't consider not using one now. I have an oldish Weller at home, and an Oki / Metcal at work.

I'm not sure what it is that John has, the simplest type of 'electric' desoldering iron is very similar to the manual teflon tipped plunger type, except that the nozzle is metal, and heated.

I have a manual desoldering device with a plunger which has to be reset everytime. I'm doing the mod now.

[John Earland]

Well, going for the simple mod worked. However there are still some problems. The cursory looks like it crackles with tiny pinpoints of light (photo 1-though don’t know if it shows properly) then intermittently I get the second picture! Mmmm. The picture improves when I set the ULA to the configuration shown in the third picture-though I still get the colour image. However that was earlier. I’ve left the ZX81 for a couple of hours and it is stone cold now and I’ve just switched it on and it seems to be behaving itself! I shall put it through it’s paces in the morning and have a good look at the manual!

[John Earland]

Just switched it on this morning - this is what I’ve got from cold. Apart from this, the output is really crisp once it settles down! There is definitely colour trying to get through though. The switches on the ULA are set to (I think) ON OFF OFF ON.

I‘ve had it running for about 10 mins and has seemed to settle though it’s not terribly stable! Colour continues to deep through into the characters. As you can see from the second picture, when it does stabilise the image is crisp! Maybe I need to adjust the switches. Back to the manual.

[John Earland]

Just a quick update: I've changed the switch settings to ON ON OFF OFF - doesn't seem to make a difference - though it should! Switch 2 is for the back porch which I needed with the chip I had which was 2C210E. However, I have turned the colour off on the LCD and that has done the trick it seems! Not really a solution but it appears to work. It could also be that as the machine warms up that affects it too!

[SiriusHardware]

The ZX81 doesn't have the fairly complex circuitry needed to generate a colour signal for the set to 'see', but in the absence of a colour carrier from the source, a circuit called a 'colour killer' should really come into play on the TV.

Maybe this particular TV is a little bit taken aback when confronted with a monochrome source. (Being an LCD set, it will have been made many, many years after mono broadcast transmissions and other monochrome-only sources were common).

You could try letting the fault clear and then try cooling various things including the regulator, electrolytic capacitors, etc, to see if the colour noise comes back when you do that. Don't blast the electrolytics too hard, they contain liquid inside which needs to remain liquid in order for them to work...

[John Earland]

Originally Posted by SiriusHardware

The ZX81 doesn't have the fairly complex circuitry needed to generate a colour signal for the set to 'see', but in the absence of a colour carrier from the source, a circuit called a 'colour killer' should really come into play on the TV.

Maybe this particular TV is a little bit taken aback when confronted with a monochrome source. (Being an LCD set, it will have been made many, many years after mono broadcast transmissions and other monochrome-only sources were common). - I think you might be right here! I have another older LCD which I could try.

You could try letting the fault clear and then try cooling various things including the regulator, electrolytic capacitors, etc, to see if the colour noise comes back when you do that. Don't blast the electrolytics too hard, they contain liquid inside which needs to remain liquid in order for them to work...

I will need to get some more 'poop spray' from B&M so might take a while! And, I hesitate to say this(!), but the machine switches off when I try to load in a program! Yes - Bermuda loading triangle again. I have tried a sound setting which works on my working ZX81 but this mchine simply switches off everytime I load something. I shall be patient though and try various volume settings.

I have had it running for over an hour now though and it is still working. The black characters remain a little grainy - though no colour now because I've switched it off! At least the replacement ULA works. Could this issue be caused by my soldering efforts!?

[SiriusHardware]

May I draw your attention to the paragraph "Mic Levels" on the first page of the manual?

The video and tape signals are somewhat interconnected on the ZX81 so any change to the video output circuit can mean that you may need to set the loading volume to a substantially different level to the one which worked previously.

[John Earland]

Originally Posted by SiriusHardware

May I draw your attention to the paragraph "Mic Levels" on the first page of the manual?

The video and tape signals are somewhat interconnected on the ZX81 so any change to the video output circuit can mean that you may need to set the loading volume to a substantially different level to the one which worked previously.

Yes - I saw that. I will keep trying. I'm sure something will happen. However, the ZX81 switches off now even if there is nothing plugged into the EAR. Just typing LOAD "" switches the machine off and I have to reset it. It wasn't doing this this morning but it is now.

[SiriusHardware]

Unless you have a specific reason not to, I would try putting switches 3 and 4 to OFF and leaving them there, since your '81 does not have anything unusually exotic in the way of memory arrangements. (The external RAM pack does not count as 'unusual'.).

[John Earland]

Originally Posted by SiriusHardware

Unless you have a specific reason not to, I would try putting switches 3 and 4 to OFF and leaving them there, since your '81 does not have anything unusually exotic in the way of memory arrangements. (The external RAM pack does not count as 'unusual'.).

Yes, I've tried all combinations. At the moment only the mod switch is on but it's still crashing when I type LOAD "". It is strange because it wasn't doing that this morning - hey ho. The joys of vintage computing!

[julie_m]

Originally Posted by John Earland

I have a manual desoldering device with a plunger which has to be reset everytime. I'm doing the mod now.

Then that's an even better job than I first thought!

[John Earland]

Originally Posted by julie_m

Then that's an even better job than I first thought!

Thank you

[John Earland]

Originally Posted by julie_m

Then that's an even better job than I first thought!

Here is the desoldering device!

[SiriusHardware]

Yes, fantastic job if that was your first time doing a major desoldering job, and with a manual solder sucker as well. I confess I thought you had bought an 'electric' one.

As always, try the simple stuff first. Carefully remove the new ULA from the socket and make sure none of the pins went heading off in the wrong direction, ie, got folded over, when the device was inserted. It's very easily done, still happens to me occasionally even after 40+ years of doing this kind of thing. Is the IC socket you fitted an 'ordinary' socket, or a turned-pin type? Turned-pin sockets are great but the IC pins all have to be arrow straight otherwise it can be very difficult to get the IC in.

If all looks OK we'll come up with a list of things to measure from and to to see if there are any missing connections from the ULA socket to anywhere else.

Sorry to have to ask: But you did solder that apparently unsoldered pin that I pointed out? Something like that would definitely cause a problem like this.

Also - I'm assuming you have already tried this - does it behave any differently when the RAM pack is not plugged in, so it's just the bare 1K machine?