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

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" <random6000@mac.com>
To: <cbm-hackers@musoftware.de>
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"" <afachat@gmx.de>
> To: <cbm-hackers@musoftware.de>
> 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
>>
>>
>> -- 
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>
>
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