Re: PET 2001 fix Part 3 - RAM/ROM board etc.

From: Philip Lord <>
Date: Sat, 04 Jun 2011 10:30:53 +0900
Message-id: <>
Just wanted to let you ll know the 2001 board is now 100% fixed.

So the complete list of thanks and fixes are:

• Ethan Dicks - Replaced dead diodes in power circuit, and got the machine fired up.

• MikeS - For suggesting to replace the 74LS165 at B2 to fix the vertical line problem and for also suggesting to remove C8 (C8 was actually ok, but doing this allowed for much more detailed testing).

• dave_m - For finding the final fix for the offset characters. 74LS00 at D8.

• André - For following this fix from cbm-hackers to VCF, and for his brilliant timing diagnosis which pointed us all in the right direction for the final fix.

• Nicolas Welte - For correctly diagnosing a build issue with the RAM/ROM board I was using for testing. I had placed the resistor pack in the wrong way! The other issue with the RAM/ROM not booting BASIC4 was solved by me, as the Flash ROM which came with the board was corrupt. I also built one of Nicolas's 6550 video RAM replacement boards which was installed in the machine too.

• Anders - For sending me a keyboard from a PET 3000 series machine. I built up an adaptor to connect it to the 2001. Works great.

• Matthew D'Asaro - For making the PET composite video adaptor, which was invaluable for testing.

Hopefully I have remembered everything correctly. I'm sure there are more people to thank, but I think this list covers off on the top 7. Thanks everyone.

I learnt a lot of things during this fix, in particular I learnt how to use a scope for basic testing. 
If you want to read the entire fixit thread on VCF you can find it here ('s very long):

Thanks again to everyone


On May 24, 2011, at 2:20 AM, MikeS wrote:

> Could you show and compare what the +5Vcc looks like right at the various chips involved? 50mv/div might make things easier to see.
> ----- Original Message ----- From: "Philip Lord" <>
> To: <>
> Sent: Sunday, May 22, 2011 11:30 PM
> Subject: Re: PET 2001 fix Part 3 - RAM/ROM board etc.
> Hello again,
> I've tested the outputs from the 7805's and have put the pictures up on my site again.
> However, the first thing I did when I was trying to get this board working was to replace all the 7805's and the 47uf capacitor at G10, so they are all new and should be working fine. Please look at the results on my page.
> I also tested D5 pin 14 (5v) on both machines. I also checked with a digital voltmeter the voltage on pin 14 and I get 5.08VDC
> I had previously tried the broken board in the working board chassis running off the working computers transformer, but tried it again just incase. The results are still the same.
> Phil
> On May 23, 2011, at 1:39 AM, MikeS wrote:
>> Looks like we're thinking along the same path; I'd asked Philip to check the
>> Vcc of some of the chips in that area, but we had to solve some ground and
>> probe issues first. Looks like we may be getting there at last...
>> ----- Original Message ----- From: ""André Fachat"" <>
>> To: <>
>> Sent: Sunday, May 22, 2011 12:20 PM
>> Subject: Re: PET 2001 fix Part 3 - RAM/ROM board etc.
>>>> > Ok, C5 Pin 5 actually looks quite ok.
>>>> I took a closer look, but if I dropped down to 2us the low-high
>>>> transition
>>>> moved of the screen, and not being at all savvy in scope usage, I'm not
>>>> yet able to figure out how to do bring it back. I was able to get to 10us
>>>> though and couldn't see any spikes.
>>> So much for that theory. But I have another one looking at these signals.
>>> The overall quality of the signals does not look good on the broken PET.
>>> Stuff like that appears when bypass caps don't really do their work, or
>>> the supply voltage is not good enough... Can you measure the supply
>>> voltage of the chips (and I mean at the chip's pins, not at the power
>>> regulator)? Most notably D5? Use the 'scope, so you can see ripples on the
>>> supply voltage which can be the cause for this as well.
>>> Note that there are multiple voltage regulators on the board, one for its
>>> own part of the board. If I understand schematics sheet 2 correctly VR-3
>>> is responsible for D5. Maybe just this one starts to fail...
>>> André
>>> -- 
>>> NEU: FreePhone - kostenlos mobil telefonieren!
>>> Jetzt informieren:
>>>     Message was sent through the cbm-hackers mailing list
>>     Message was sent through the cbm-hackers mailing list
>      Message was sent through the cbm-hackers mailing list 
>      Message was sent through the cbm-hackers mailing list

       Message was sent through the cbm-hackers mailing list
Received on 2011-06-04 02:00:13

Archive generated by hypermail 2.2.0.