Re: Help in resurrecting my ZX81
Posted: Wed Aug 08, 2018 9:10 pm
Wow. Thank you!
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Help in resurrecting my ZX81
Page 5 of 7
Re: Help in resurrecting my ZX81
Posted: Wed Aug 08, 2018 10:06 pm
Did you need the back side (solder side)?
Re: Help in resurrecting my ZX81
Posted: Wed Aug 08, 2018 10:10 pm
Nope. That's easy to see on my board. But thank you.
Re: Help in resurrecting my ZX81
Posted: Wed Aug 08, 2018 11:57 pm
Re: Help in resurrecting my ZX81
Posted: Wed Aug 15, 2018 10:15 pm
Edit: Drawing is wrong! It's updated in later post.
Thanks for the scans. Now I know where to measure.
Thanks for the scans. Now I know where to measure.
Re: Help in resurrecting my ZX81
Posted: 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.
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 :
:............:..............:............................:Re: Help in resurrecting my ZX81
Posted: 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
Re: Help in resurrecting my ZX81
Posted: 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.
Re: Help in resurrecting my ZX81
Posted: 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.
Re: Help in resurrecting my ZX81
Posted: Sun Aug 19, 2018 5:05 pm
R2 is a totally different place in the circuit (RAM CS signal from ULA to RAM).