ZX81 rebuild, take 2
Re: ZX81 rebuild, take 2
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.
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.
Re: ZX81 rebuild, take 2
These are the two versions, first ULA 2C158E -
and ULA 2C184E -
and ULA 2C184E -
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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Re: ZX81 rebuild, take 2
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).
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
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Amp
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ZX81 Chip Pin-outs
ZX81 Video Transistor Amp
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer being good this year.
Re: ZX81 rebuild, take 2
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
It seems they are part of the MIC circuit and have nothing to do with the modulator.
https://datassette.nyc3.cdn.digitalocea ... ects03.pdf
Re: ZX81 rebuild, take 2
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 .
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.
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 .
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.
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Re: ZX81 rebuild, take 2
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.
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
Re: ZX81 rebuild, take 2
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..."
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..."
Re: ZX81 rebuild, take 2
I meant that inadvertently touching a 9V power plug to the resistor wire might buzz the circuit.
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Re: ZX81 rebuild, take 2
Only if you manage to short the plug tip and ring at the same time
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 Amp
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer being good this year.
ZX81 Chip Pin-outs
ZX81 Video Transistor Amp
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer being good this year.
Re: ZX81 rebuild, take 2
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.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.