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

From: André Fachat <>
Date: Sun, 22 May 2011 18:20:28 +0200
Message-ID: <>
> > 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...


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Received on 2011-05-22 17:00:07

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