Re: 251101-01 Kernal ROM chip for SX-64 (UD3)

From: Nicolas Welte (
Date: 2007-01-09 21:59:24

Marko Mäkelä wrote:
>> I made some pictures of the modification:
> Neat! You could have saved me some time by telling where you added the
> circuit.  Am I right that you installed a socket and a chip to the lower
> right corner, right of UB2?  It's neat that all required signals were

Yes, that is UA2 (but unmarked). I installed an 74LS138 there. Also, I had 
to clip the output pins of the original 74LS42 decoder, where most 
connections go. I preferred this to cutting copper traces on the 4040 
board. I had one trace cut to test another alternative, injecting Phi2 
directly into an unused input of the LS42. That didn't work unfortunately. 
Maybe the 6532 didn't like that signal. That's the extra wire you see on 
the LS42.

> available nearby.  Was it a completely empty socket (only Vcc and GND
> connected)?  It's a pity that the consumer-grade Commodores did away
> with this practice of having some prototyping area (or place for fixing
> hardware bugs).

Yes, that socket was completely empty, except for the power supply.

I also made a circuit diagram of my modification:


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