RE: PET 2001 Fix....WAS: Will pay good money for NON working PET 2001 motherboard.

From: Bil Herd <>
Date: Mon, 9 Nov 2009 09:31:40 -0500
Message-ID: <>
I would be hard pressed to draw any conclusions as to the health of a single chip in a complex system by looking at a single chip.  The way I would might reach a conclusion about a chip such as character rom with minimal equipment would require a scope: Trigger the scope on the /OE pulse and observe the data output during the time that the charom is supposed to be active. (Some charoms are wired active all of the time).

Then what else the databus is connected to becomes involved: is there contention or is something else dragging the bus hi or lo?  Another chip on the line with it's VCC at ground can drag db's to ground+.6 through their protection diodes, etc. Doesn't hurt to feel every chip and check for overly hot any time starting a troubleshooting session.

In the case of a "always on" char rom you have a few more games you can play if you are convinced you are in the right area, nicely forcing databits to set conditions should result in changing patterns on the screen, I.E. you can use the screen as a limited probe in this area.

While troubleshooting without a scope is possible to some extent (chip swapping does work), I recommend the use of one for serious troubleshooting efforts. One procedure is to check the basics of all of the parts in the localized area looking for the one that is the "most upset" either through clashing outputs, ground pins being dragged high, no activity, or missing signals altogether. You don't want to get caught focusing on the chip next to the broken chip, which will itself be acting strangely, but only because it's neighbor is truly having a bigger problem.

I know this all doesn't help if you don't have the equipment, I would just urge caution against too much shotgunning if there is the chance that you actually install a 2nd issue preventing you from ever really fixing the first.

What happens when you lick your finger and run up and down the datalines on the character rom? (Mother nature's logic probe)


-----Original Message-----
From: [] On Behalf Of Philip Lord
Sent: Monday, November 09, 2009 8:53 AM
Subject: PET 2001 Fix....WAS: Will pay good money for NON working PET 2001 motherboard.

Thanks for your advice. I'll look into the cassette transistors after
I figure out the character issue.
I also came across a page by Dave Gostelow that shows some pictures of
his keyboard dismantling which will also help when I get around to
fixing the keyboard.

I had only a few spare moments this evening to probe around the
Character ROM, so I though I'd post the results in the hopes that it
may help someone help me ;-)
Firstly, let me start by saying my logic probe is totally crap, and
I've been meaning to get a good one.
The ROM adaptor I previously spoke about made it quite difficult to
actually get to the pins of the 6540 socket, so instead I probed the
2316 that was mounted to the top. Here are the results:

Seems that all the inputs are pulsing except 9 and 10 which are at 0V.
All the outputs seem to be at 0V.
I downloaded the 2316 datasheet and looking that the truth table:

  if CE = Low, WE = Hi and OE = Low, then the mode is READ, Output is
ACTIVE, and power is ACTIVE.

Unfortunately I don't actually know what all that means :-(

Thanks again

On Nov 9, 2009, at 2:39 AM, William Levak wrote:

> On Sun, 8 Nov 2009, Philip Lord wrote:
>> Hi William,
>> Thanks for he reply.
>> For my own education, I was wondering why you feel the the
>> transistors are the issue?
> Experience!
>> I'm also wondering how exactly would you test for a short in a
>> transistor??? Is that something that can be done in circuit  with a
>> multimeter?>
> Since it causes the computer to hang up, you want to check for any
> changes other than putting power on the cassette motor line.  I once
> found one that had a short between two traces on the circuit board.
>> I'll take your advice on dismantling the keyboard and cleaning with
>> alcohol. I'll need to be especially careful as this keyboard is
>> actually a Japanese variant with Japanese characters also printed
>> on the keys.
> Cleaning the keyboard contacts involves removing the keyboard from
> the computer and removing the back of the keyboard.  It does not
> involve the front of the keyboard at all.
> SDF Public Access UNIX System -
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