RE: A newly working Commodore 720 - many questions...

From: Rob Clarke <>
Date: Thu, 16 Feb 2012 18:37:14 +0100
Message-ID: <>
Thankyou all so much for the replies.

>> Q1) As I understand it, I just need to fit the 16 missing drams and
>> decoupling caps?
>You also need the BASIC 256 ROMs, or the added 128k will not be used by
>On startup, the kernal tests the installed RAM, but there is no feedback
>how many RAM was found.
>Even if RAM is faulty, there is no display.

Thanks Christian. Makes the test rather pointless doesn't it...

>> 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?
> Or you look for an old AT supply.

MikeS was correct, it was the -5V rail that had been dropped from the ATX
specification. The -12V still exists. I've ordered a 150W ATX supply used
for powering mini-itx boards. It'll supply about 4.2 watts on the 12V,
which should be enough for the monitor.

>> 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 signal?
> Yes, use an optocoupler.

Wow - so many options. I like the option of a single chip solution from
Gerrit as it doesn't require opening the PSU or attaching or hooking up to
mains voltages. Incidently, the MAX machine used a 555 to generate the TOD
signal, as Uz speculated.


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Received on 2012-02-16 18:00:12

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