Help in resurrecting my ZX81

Discussions about Sinclair ZX80 and ZX81 Hardware
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mrtinb
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Re: Help in resurrecting my ZX81

Post by mrtinb » Wed Aug 08, 2018 9:10 pm

Wow. Thank you!
Martin
ZX81, Lambda 8300, Commodore 64, Mac G4 Cube

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gammaray
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Re: Help in resurrecting my ZX81

Post by gammaray » Wed Aug 08, 2018 10:06 pm

Did you need the back side (solder side)?
Finish what you start. Truth is over the mountaintop or six feet under.
4 x TS1000, 2 x TS1500 , 2 x timex 2040 printer, 1/2 x timex 2020 cassette deck, ZXPAND+AY, ZeddyNET, ZXBlast..., unfinished Spectrum...Still no life!

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mrtinb
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Re: Help in resurrecting my ZX81

Post by mrtinb » Wed Aug 08, 2018 10:10 pm

Nope. That's easy to see on my board. But thank you.
Martin
ZX81, Lambda 8300, Commodore 64, Mac G4 Cube

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gammaray
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Re: Help in resurrecting my ZX81

Post by gammaray » Wed Aug 08, 2018 11:57 pm

:oops:
Finish what you start. Truth is over the mountaintop or six feet under.
4 x TS1000, 2 x TS1500 , 2 x timex 2040 printer, 1/2 x timex 2020 cassette deck, ZXPAND+AY, ZeddyNET, ZXBlast..., unfinished Spectrum...Still no life!

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mrtinb
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Re: Help in resurrecting my ZX81

Post by mrtinb » Wed Aug 15, 2018 10:15 pm

Thanks for the scans. Now I know where to measure.

Image
Martin
ZX81, Lambda 8300, Commodore 64, Mac G4 Cube

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mrtinb
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Re: Help in resurrecting my ZX81

Post by mrtinb » Sat Aug 18, 2018 10:11 pm

I have now measured a working and a broken board.

It seems the output of ULA pin 14 is a bit lower, and the signal is lost after going through 2K2 resistor.

There seems still to be a connection as the signal is amplified again with transistor TR2 but only to 0.01V.

Code: Select all

..........................................................
:            : Working ZX81 : Broken ZX81 with Andys ULA :
:............:..............:............................:
: ULA pin 14 :        1.70V :                      1.51V :
: R6 2K2 in  :        1.70V :                      1.51V :
: R6 2K2 out :        0.19V :                      0.00V :
: TR2 in     :        0.19V :                      0.00V :
: TR2 out    :        1.95V :                      0.01V :
:............:..............:............................:
Martin
ZX81, Lambda 8300, Commodore 64, Mac G4 Cube

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Andy Rea
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Re: Help in resurrecting my ZX81

Post by Andy Rea » Sun Aug 19, 2018 1:20 pm

that would suggest that a short exsists somewhere on the transotor side of R6... maybe a short on the circuit or even base-emmiter shorted on tr2
Last edited by Andy Rea on Sun Aug 19, 2018 4:42 pm, edited 1 time in total.
6 x ZX81, 1 x TS1500 , 1 x +3e, 1 x timex 2040 printer, 1 x timex 2020 cassette deck, siclair printer and some spectrum

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mrtinb
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Re: Help in resurrecting my ZX81

Post by mrtinb » Sun Aug 19, 2018 3:24 pm

I can't find any shortings. What if I connect a smaller resistor in parallel with R6 (on the bottom side) with a smaller value than 2K2? Then the value on the other side of R6 will be higher, and TR2 will amplify the signal to TTL level.
Martin
ZX81, Lambda 8300, Commodore 64, Mac G4 Cube

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Andy Rea
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Re: Help in resurrecting my ZX81

Post by Andy Rea » Sun Aug 19, 2018 4:42 pm

Well i actually meant R2 oops... i would pull TR2 and then see what you get at the base connection.
6 x ZX81, 1 x TS1500 , 1 x +3e, 1 x timex 2040 printer, 1 x timex 2020 cassette deck, siclair printer and some spectrum

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mrtinb
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Re: Help in resurrecting my ZX81

Post by mrtinb » Sun Aug 19, 2018 5:05 pm

R2 is a totally different place in the circuit (RAM CS signal from ULA to RAM).
Martin
ZX81, Lambda 8300, Commodore 64, Mac G4 Cube

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