--- Kevin Andres <kandres_at_epssecurity.com> wrote:
> Hello Peoples,
> I am reading the the mail starting fairly recently. I have a
> Commodore Pet vintage 1975... currently not running has
> a screen full of random characters, seems to me I
> remember this as indicative of a 6550 MOSTEK ram failure.
That's one possibility, but typically only if it's in the zero
page area. One way to test that is to swap out the zero page
RAM with a higher page and see if the problem persists. If
the machine comes up with less RAM for BASIC, that's probably
what the problem is. OTOH, those sockets are really crappy.
It's possible that removing and reinserting the RAM will make
a difference. Just pressing down on the socketed parts to reseat
them can revive a "dead" PET.
On a couple of later units, I've had to resort to removing sockets
and replacing them with machined-pin ones. No chips were faulty,
but there was intermittent contact causing all sorts of flaky
behavior.
Using a test instrument (pulse stretcher/logic probe, oscilloscope,
etc.) to monitor the reset line is another thing to check. IIRC,
they use a 555 timer to generate the reset pulse (common in the
mid-70s) and I've seen those circuits fail for a variety of reasons.
You can also build a NOP generator (use a second CPU chip) by bending
the appropriate data bus leads up and hardwiring the databus to the
NOP instruction (0xEA?) - You can then use a logic analyzer to monitor
the address bus for stuck bits and the select lines on the MSI chips
to see that they respond at the appropriate times.
> An Ohio Scientific Superboard, used as a development tool
> by a firm just starting in computer control of the real world.
A buddy of mine had one of those when we were kids. He built his
into a robot chassis (his second - the first was run by a Quest
Elf with 256 bytes of RAM). Fun little guys, as I recall.
> A DEC something that hasn't been run since it was shut down.
> The DEC had the BIG 10m removable drive packs, drives and packs since
> departed this earth in one lost shipment or other.
RL02s. Not rare, but that means that you have the drive controller,
either an RL11 or RLV11, probably (unless it's a PDP-8/a, in which case
it's an RL8A controller, but that's less likely).
There are plenty of people who could help you with that one if you
provide a description of what you have.
Welcome and good luck,
-ethan
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Received on Tue Jan 07 2003 - 10:53:00 GMT