ZX80 Gerber files

Discussions about Sinclair ZX80 and ZX81 Hardware
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1024MAK
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Re: ZX80 Gerber files

Post by 1024MAK »

MaFli wrote: Tue May 16, 2017 6:52 amI've also traced down the components that are not populated (R36, R37, C3, TR1, D1, D2). Looks like they tried to fix the back porch of the video signal. Does anyone have any idea? Even more weird is the D1, D2 divider. Were these intended to be zeners or other clamping devices in case something went wrong with the power rails? And why is the center point between the two connected to a dummy piece of copper in the middle of the top layer? That is the long rectangular bar in the middle. Clue anyone....????
The two diodes, D1 and D2 and the PCB "bar" track were apparently provided to connect to a shield to protect the circuitry on the board, or maybe to protect the user...
There are/were a number of television video systems around the world, some use positive sync signals and some use negative sync signals. Same as in some television video systems, the video signal is a positive going voltage for peak white, while in others, positive is total black. So R36, R37, C3 and TR1 may have been provided to allow for one of these other television video systems.

I don't think it's anything to do with providing a back porch, as at the time ZX80's were expected to be used with a black and white / monochrome TV. So the missing back porch did not matter. Further, I don't think that circuitry would have the intended effect.

Mark
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MaFli
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Re: ZX80 Gerber files

Post by MaFli »

1024MAK wrote: Tue May 16, 2017 11:58 am The two diodes, D1 and D2 and the PCB "bar" track were apparently provided to connect to a shield to protect the circuitry on the board, or maybe to protect the user...
There are/were a number of television video systems around the world, some use positive sync signals and some use negative sync signals. Same as in some television video systems, the video signal is a positive going voltage for peak white, while in others, positive is total black. So R36, R37, C3 and TR1 may have been provided to allow for one of these other television video systems.
I don't think it's anything to do with providing a back porch, as at the time ZX80's were expected to be used with a black and white / monochrome TV. So the missing back porch did not matter. Further, I don't think that circuitry would have the intended effect.
Mark
Thanks, that makes sense! I'm still a newbee when it comes to the ZX80 :-)
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PokeMon
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Re: ZX80 Gerber files

Post by PokeMon »

1024MAK wrote: Tue May 16, 2017 11:58 am I don't think it's anything to do with providing a back porch, as at the time ZX80's were expected to be used with a black and white / monochrome TV. So the missing back porch did not matter. Further, I don't think that circuitry would have the intended effect.
Image

I think it is a kind of back porch generation and is indeed required even for older TVs. If the black level can not be determined resp. is wrong with a picture with white background it results in a bad contrast of the picture regardless if the signal is provided via RF or directly as composite video. There may be some enhancements in TVs trying to optimize it but in my opinion the result over the modulator was not the best and a missing back porch is an additional mess.

I think the circuit is an alternative video former with R33/R31 giving a DC offset, leaving R30 unused and providing the video signal over C3 and R37 (added to the DC offset) while the inverted (!) /SYNC signal will switch transistor on and the collector base capacity with the miller effect and R36 will form a kind of delay to leave the transistor switched even if sync signal is removed. As TVs often take the average of the black period it wouldn't harm if it is not a just digital signal and moving a bit.

This can be evaluated with carefully chosen values for all parts and will probably do the job (more or less good). Maybe Sinclair wanted the option to have a composite signal too if there would be more TVs with a separate video in and decided never to use this option as there was no need for the market at this time. Just a guess.

About the two diodes - yes I think it would be kind of static discharge protection as the keyboard membrane was metallized and could be maybe touched with the finger from the side while the top was quite good isolated with the print.
MaFli
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Re: ZX80 Gerber files

Post by MaFli »

Finally got my first ZX80 today. Just opened it up and what do you know... Mine has TR1 installed and some of the resistors that are normally not there. I must note that mine is a US model with a UM1082 modulator and D11 installed. I will scavenge a UM1233 from a ZX81 and remove D11 to turn it into a UK one. But before I do that I will take some measurements to see what the signal looks like (if this beauty is still working...)
I have updated the schematic. Some components have 2 values. The one on the left is according to the official schematic, on the right it is the value that is populated in my US version. DNP means "Do Not Populate" by the way.
I was surprised to see quite a bit of bodge work on the bottom side near the connectors. Probably to comply with US regulation.
Anyway, I'm going to hook it up to my powersupply (with current limit ON :D) to see if there's any life in this baby...
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Bodged components added to schematic
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Bodge work on the bottom side
Bodge work on the bottom side
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CompVid.jpg
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CompVid.PNG
Updated schematic with new values
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sirmorris
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Re: ZX80 Gerber files

Post by sirmorris »

The mods look like ear/mic conditioning, perhaps to improve load/save reliability.
Moggy
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Re: ZX80 Gerber files

Post by Moggy »

MaFli wrote: Wed May 24, 2017 2:57 pm I will scavenge a UM1233 from a ZX81 and remove D11 to turn it into a UK one.
The out put socket on the UK modulator is positioned to the left side as opposed to the right side as shown in your picture so the case hole may not line up correctly, better to composite mod I would suggest and retain or remove completely the original modulator.
Moggy
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Re: ZX80 Gerber files

Post by Moggy »

sirmorris wrote: Wed May 24, 2017 3:43 pm The mods look like ear/mic conditioning, perhaps to improve load/save reliability.
Agreed Charlie it looks like some kind of filtering.

I've seen a bleed resistor fitted to a few 80's (mine included)to knock the out-put signal strength down as the out-put is a damn sight stronger than the 81 and I can still load with half volume when using a cassette player whereas my 81's need the volume flat out..
MaFli
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Re: ZX80 Gerber files

Post by MaFli »

sirmorris wrote: Wed May 24, 2017 3:43 pm The mods look like ear/mic conditioning, perhaps to improve load/save reliability.
Could be indeed. The 47nF caps (BODGE3,4 and 5) are all soldered across the inputs of each connector (MIC, EAR, PWR), so these are probably for ESD/transient protection. Although I just noticed that BODGE3 is the same as C13. And that one is not populated on my board :-) Going to resolder that to the top now :-)

And there also an additional wire from 0V (GND) from all connectors straight to the chassis of the modulator. If you look at the original layout, 0v goes all the way around the board. 0V is highlighted in yellow. It has a more straight path to the 7805.
The other added resistors seem to lift the signal slightly. I guess you're right, that probably helps in better load/save reliability.

Out of curiosity, do all ZX80s have these metal springs on the bottom to connect GND to the enclosure? I have an additional one from my (now removed) UM1082 to the top shell. Both shells have a metal coating on the inside. From the pictures I've seen only the bottom part had a conductive coating? Or was that changed for the US version as well?

Anyway, just finished changing all resistors to the UK values and removed D11. UM1233 installed instead of a UM1082. I do see see a composite signal now!!! Woohoo! It might be alive... Going to install the ZX8-CCB and hook it up to my TV. Wish me luck...
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MaFli
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Re: ZX80 Gerber files

Post by MaFli »

Moggy wrote: Wed May 24, 2017 4:24 pm
MaFli wrote: Wed May 24, 2017 2:57 pm I will scavenge a UM1233 from a ZX81 and remove D11 to turn it into a UK one.
The out put socket on the UK modulator is positioned to the left side as opposed to the right side as shown in your picture so the case hole may not line up correctly, better to composite mod I would suggest and retain or remove completely the original modulator.
Darn... You are right! And a bit thicker as well. Hmmm... What should I do? Drill a hole? Or leave it original?
I have a ZX8-CCB that I could install into the UM1082. No use for that anyway. And I can keep my UM1233 in the ZX81 it came out of.
Moggy
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Re: ZX80 Gerber files

Post by Moggy »

MaFli wrote: Wed May 24, 2017 4:59 pm
Moggy wrote: Wed May 24, 2017 4:24 pm
MaFli wrote: Wed May 24, 2017 2:57 pm I will scavenge a UM1233 from a ZX81 and remove D11 to turn it into a UK one.
The out put socket on the UK modulator is positioned to the left side as opposed to the right side as shown in your picture so the case hole may not line up correctly, better to composite mod I would suggest and retain or remove completely the original modulator.
Darn... You are right! And a bit thicker as well. Hmmm... What should I do? Drill a hole? Or leave it original?
I have a ZX8-CCB that I could install into the UM1082. No use for that anyway. And I can keep my UM1233 in the ZX81 it came out of.
In my case left the modulator in place and fitted the CCB (great picture) also you might want to fit one of these...

http://uk.rs-online.com/web/b/tracopowe ... onverters/

...it makes the whole shebang run a damn sight cooler, not sure where in the world you are so this link is for illustration only but I'm sure a local supplier will be at hand.
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