Help in resurrecting my ZX81

[mrtinb]

Wow. Thank you!

[gammaray]

Did you need the back side (solder side)?

[mrtinb]

Nope. That's easy to see on my board. But thank you.

[gammaray]

[mrtinb]

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

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

[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 :
:............:..............:............................:

[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

[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.

[Andy Rea]

Well i actually meant R2 oops... i would pull TR2 and then see what you get at the base connection.

[mrtinb]

R2 is a totally different place in the circuit (RAM CS signal from ULA to RAM).