A newly working Commodore 720 - many questions...

From: Rob Clarke <crock_at_clarke-family.org.uk>
Date: Thu, 16 Feb 2012 00:34:07 +0100
Message-ID: <4F3C40EF.60306@clarke-family.org.uk>
Following on from my success with the 610 two weeks ago, I have now got 
the 720 (mostly) working. Thanks again to all on CBM-H who've helped me out.

Glorious green monochrome pic here, running the burn-in tests:


The observant will notice that banks 3 & 4 are not populated. Although 
it's an immaculate 720 case, it's actually a 710 board. The board looks 
unused (there is not even any signs of scratches or oxidation on the 
IEEE edge connector) but as the case and monitor ar mint, I think I owe 
it to them to put another 128k on board.


Q1) As I understand it, I just need to fit the 16 missing drams and 
decoupling caps?

Q2) WRT to the differences in the PLA between the lo and high profile. I 
think I read somewhere that it was supposedly to be able to accomodate 
the coprocessor board. However, looking at the schematics, the PLA just 
routes RAS and CAS for the 4 memory banks, using all 8 outputs but just 
on different pins. Could someone explain where the differences are?

Q3) The burn-in tests correctly identify whether the machine is a high 
or low profile model. ANyone know how it programatically does this?

If you look at the pic below, I've had to jury rig the power, borrowing 
the PSU from my 610 to power the board while using the original PSU to 
power the monitor. As per my previous post, the PSU for the 720 is shot. 
Even adjusting the trimmer, it will only output 4.6v before cutting out. 
I'm happy to put a more modern PSU inside but this raises a couple more 


Q5) The case of the 720 PSU says it's rated for 80w but the back of the 
720 says 120w, can someone else with a high profile machine clarify 
this? Does mine have the "wrong" PSU?

Q6) I think modern ATX supplies have now dropped the -12V rail. From 
looking at the schematics this is only used for the RS232 port. As I'm 
not likely to use this any time soon, will I have any issues if I omit 
the -12V rail?

Q7) The CBM-II PSU's all supply a 50/60hz signal from the PSU, which I 
think is just used for the TOD clock on the CIA. As I'm unlikely to find 
this on a modern PSU, is there a really simple way of generating this 

Sorry for so many questions!


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Received on 2012-02-16 00:00:12

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