ZX81 rebuild, take 2

Discussions about Sinclair ZX80 and ZX81 Hardware
Moggy
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Re: ZX81 rebuild, take 2

Post by Moggy »

To the best of my knowledge I know of no standard mod resistor/capacitor wise that was ever mooted for the 3 different ULA's a mixture of which I have had on several of the 2 issues of board over the years all working perfectly and as far as I'm aware the 2 items in question are just a crude tone shaper for the EAR/MIC circuit and have nothing to do with the modulators output so moving them wouldn't make any difference I would have thought.

I assume that all the chips were in place when you tried this and you have removed them afterwards? If not I'm not surprised there is no output and without an heatsink I don't think that regulator would last long. Sorry for any incorrect assumptions but I can only go by the pictures as presented.

Best wait for one of the techies to answer this more fully.
sP1d3r
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Re: ZX81 rebuild, take 2

Post by sP1d3r »

These are the two versions, first ULA 2C158E -
Resistor R27 1K & capacitor 47nf C11
Resistor R27 1K & capacitor 47nf C11
and ULA 2C184E -
Same components by Ear socket
Same components by Ear socket
Moggy wrote: Tue Jun 21, 2022 9:18 pm without an heatsink I don't think that regulator would last long.
I only power the boards on to see if they boot up, I plan to replace the 7805 regulator with a later type when the boards are working.
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1024MAK
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Re: ZX81 rebuild, take 2

Post by 1024MAK »

sP1d3r wrote: Tue Jun 21, 2022 9:47 am I've noticed that my ZX81 has odd looking 'mod' in the ear socket, a capacitor and resistor.
This pcb outputs nothing via the modulator, I'd expect something but there's nothing whatsoever.
You mean like the vast majority of issue one boards?

Here’s photos of two of mine (ignore the pink wires, they are part of a test set-up for switching the video output between the modulator and a composite video circuit).
6885A0BE-A1CE-443F-8D87-DAC46BCDD5DB.jpeg
B6AAA78A-EAC1-4059-AFEB-4B5E396F23DA.jpeg
2C87FBCF-90B0-4002-91E8-8FBFB6E59C90.jpeg
We don’t know for certain why these boards were manufactured with these components in this position, it could well be as Moggy speculates.

I do know that this arrangement does not affect the video output.

If you have access to an oscilloscope, the quickest way to try to work out what’s happening is to see what signal you have on pin 16 of the ULA. For U.K. specification machines, the video signal goes directly from pin 16 of the ULA to the modulator video input pin. The only other connection is to R29 (which should be a 1MΩ resistor), which is the feed to the Mic socket circuitry. The high value of R29 prevents the Mic circuitry from affecting the video output.

Are the ground pins of the modulator on the bottom of the board soldered? Does the modulator get +5V power?
If you don’t have a ‘scope, what DC voltage do you get on pin 16 of the ULA?

By the way, all ULA chips on a ZX81 are backwards and forwards compatible (apart from only the last version having the back porch functionality). This applies to both the issue one boards (all variations) and issue three boards (all variations). It probably also applies to TS1000 boards (but I only have one example),

Mark
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ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

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Moggy
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Re: ZX81 rebuild, take 2

Post by Moggy »

An article in this magazine on page 20 sheds some light on the use of aforesaid res/caps.

It seems they are part of the MIC circuit and have nothing to do with the modulator.

https://datassette.nyc3.cdn.digitalocea ... ects03.pdf
sP1d3r
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Re: ZX81 rebuild, take 2

Post by sP1d3r »

Thanks for all the info, much appreciated.

It would appear that by the time that issue one motherboards with ULA 2C184E were being sold, Sinclair were pushed for time because the only apparent advantage to putting the 47nf capacitor and 1K resistor between the power and ear socket is that it's quicker.
Which is fine unless you are in the habit of putting the power plug in without looking .
1024MAK wrote: Wed Jun 22, 2022 11:38 am
Are the ground pins of the modulator on the bottom of the board soldered? Does the modulator get +5V power?
If you don’t have a ‘scope, what DC voltage do you get on pin 16 of the ULA?
I measured from the left hand modulator wire on the ULA2C158E motherboard with my scope and there was a signal, it was about 1.9V peak to peak square wave type signal with a short min and a longer max, the right hand wire which I think is the modulator power was 0V.
I couldn't upload a jpeg image of the signal because it said the resolution couldn't be determined.

I'll do some more checking and post back about it.
Lardo Boffin
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Re: ZX81 rebuild, take 2

Post by Lardo Boffin »

sP1d3r wrote: Thu Jun 23, 2022 10:55 am I couldn't upload a jpeg image of the signal because it said the resolution couldn't be determined.
I get that sometimes with photos from my iPhone. All I do is edit the photo slightly by either making it narrower or shorter and then it uploads ok.
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Moggy
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Re: ZX81 rebuild, take 2

Post by Moggy »

Don't worry about accidental inserting the power plug into the wrong socket as the caps in the circuit block any DC from causing any damage which is mentioned in the manual.

QUOTE.

"Chapter 1 - Setting up the ZX81
Unpack the ZX81, you will have found
1. This manual.
2. The computer. This has three jack sockets (marked 9V DC IN, EAR & MIC), one aerial socket, &
an exposed part of its circuit board where you can plug extra equipment. It has no switches - to turn it
on you just connect it to the power supply.
3. A power supply.
This converts mains electricity into the form that the ZX81 uses.if by accident you plug it into the wrong socket in the computer you will do no damage. If you want to use your own power supply, it
should give 9 volts DC at 700mA unregulated, & end in a 3.5mm jack plug with positive tip..."
sP1d3r
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Re: ZX81 rebuild, take 2

Post by sP1d3r »

Moggy wrote: Thu Jun 23, 2022 1:42 pm Don't worry about accidental inserting the power plug into the wrong socket as the caps in the circuit block any DC from causing any damage which is mentioned in the manual.
I meant that inadvertently touching a 9V power plug to the resistor wire might buzz the circuit. :o
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1024MAK
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Re: ZX81 rebuild, take 2

Post by 1024MAK »

sP1d3r wrote: Thu Jun 23, 2022 11:07 pm
Moggy wrote: Thu Jun 23, 2022 1:42 pm Don't worry about accidental inserting the power plug into the wrong socket as the caps in the circuit block any DC from causing any damage which is mentioned in the manual.
I meant that inadvertently touching a 9V power plug to the resistor wire might buzz the circuit. :o
Only if you manage to short the plug tip and ring at the same time :shock:

If only one part of the plug is touching any part of the board or a component on the board, there is no return path, hence no current will flow.

Unless we are misunderstanding what you mean…

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
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sP1d3r
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Re: ZX81 rebuild, take 2

Post by sP1d3r »

Lardo Boffin wrote: Thu Jun 23, 2022 12:39 pm
I get that sometimes with photos from my iPhone. All I do is edit the photo slightly by either making it narrower or shorter and then it uploads ok.
Thanks for the tip, the resolution is a strange 1248 x 758 and resizing it didn't work unfortunately but I got a picture of it.
Modulator signal
Modulator signal
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