Help in resurrecting my ZX81

Discussions about Sinclair ZX80 and ZX81 Hardware
User avatar
mrtinb
Posts: 1906
Joined: Fri Nov 06, 2015 5:44 pm
Location: Denmark
Contact:

Re: Help in resurrecting my ZX81

Post by mrtinb »

Wow. Thank you!
Martin
https://zx.rtin.be
ZX81, Lambda 8300, Commodore 64, Mac G4 Cube
User avatar
gammaray
Posts: 590
Joined: Sun Apr 17, 2016 2:44 am
Location: Texas

Re: Help in resurrecting my ZX81

Post by gammaray »

Did you need the back side (solder side)?
5-TS1000,UK ZX81<-Sheelagh, US ZX81, 2-TS1500/KDLX , 3-TS2040 printer, 2-TS2020 cassette decks, ZXPAND+AY, ZeddyNET, ZXBlast, UDG, ZX8CCB, AERCO, BUILDS/REPAIRS ZX Spectrum, ZX80 Minstrel, ZXMAX48 v1 v2, 2-TS-2068, ROM, 16kRAM
User avatar
mrtinb
Posts: 1906
Joined: Fri Nov 06, 2015 5:44 pm
Location: Denmark
Contact:

Re: Help in resurrecting my ZX81

Post by mrtinb »

Nope. That's easy to see on my board. But thank you.
Martin
https://zx.rtin.be
ZX81, Lambda 8300, Commodore 64, Mac G4 Cube
User avatar
gammaray
Posts: 590
Joined: Sun Apr 17, 2016 2:44 am
Location: Texas

Re: Help in resurrecting my ZX81

Post by gammaray »

:oops:
5-TS1000,UK ZX81<-Sheelagh, US ZX81, 2-TS1500/KDLX , 3-TS2040 printer, 2-TS2020 cassette decks, ZXPAND+AY, ZeddyNET, ZXBlast, UDG, ZX8CCB, AERCO, BUILDS/REPAIRS ZX Spectrum, ZX80 Minstrel, ZXMAX48 v1 v2, 2-TS-2068, ROM, 16kRAM
User avatar
mrtinb
Posts: 1906
Joined: Fri Nov 06, 2015 5:44 pm
Location: Denmark
Contact:

Re: Help in resurrecting my ZX81

Post by mrtinb »

Edit: Drawing is wrong! It's updated in later post.

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

Image
Last edited by mrtinb on Sun Sep 02, 2018 2:33 pm, edited 2 times in total.
Martin
https://zx.rtin.be
ZX81, Lambda 8300, Commodore 64, Mac G4 Cube
User avatar
mrtinb
Posts: 1906
Joined: Fri Nov 06, 2015 5:44 pm
Location: Denmark
Contact:

Re: Help in resurrecting my ZX81

Post by mrtinb »

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
https://zx.rtin.be
ZX81, Lambda 8300, Commodore 64, Mac G4 Cube
User avatar
Andy Rea
Posts: 1606
Joined: Fri May 09, 2008 2:48 pm
Location: Planet Earth
Contact:

Re: Help in resurrecting my ZX81

Post by Andy Rea »

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.
what's that Smell.... smells like fresh flux and solder fumes...
User avatar
mrtinb
Posts: 1906
Joined: Fri Nov 06, 2015 5:44 pm
Location: Denmark
Contact:

Re: Help in resurrecting my ZX81

Post by mrtinb »

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
https://zx.rtin.be
ZX81, Lambda 8300, Commodore 64, Mac G4 Cube
User avatar
Andy Rea
Posts: 1606
Joined: Fri May 09, 2008 2:48 pm
Location: Planet Earth
Contact:

Re: Help in resurrecting my ZX81

Post by Andy Rea »

Well i actually meant R2 oops... i would pull TR2 and then see what you get at the base connection.
what's that Smell.... smells like fresh flux and solder fumes...
User avatar
mrtinb
Posts: 1906
Joined: Fri Nov 06, 2015 5:44 pm
Location: Denmark
Contact:

Re: Help in resurrecting my ZX81

Post by mrtinb »

R2 is a totally different place in the circuit (RAM CS signal from ULA to RAM).
Martin
https://zx.rtin.be
ZX81, Lambda 8300, Commodore 64, Mac G4 Cube
Post Reply