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

The power supply may still be causing the same issue, if you look at D5 pin 9, which is the clock input to the address counter and of considerably lower quality on the broken PET. If, after switching DIS.ON, the power supply is kind of "drained", the counter may simply "miss" the first pulse, causing the exact syndrome (as with the previous assumption that a spike would clear the counter after DIS.ON going on).

Now for the cause of the power supply, peek around the power supply voltage regulators with the scope. All four regulators have the same input. Compare the output of the regulators with each other and with the good PET. If all regulators show the same pattern on the broken PET, look at the inputs, maybe the capacitors are going away (there are some in the PET's case, right? I can't see any of the big ones usually used for power regulation on the board picture you uploaded - you could probably swap boards with the good PET then, if you haven't done so yet to check that). If only one voltage regulator shows that problem, it's most likely that regulator.

André


-------- Original-Nachricht --------
> Datum: Sun, 22 May 2011 18:20:28 +0200
> Von: "André Fachat" <afachat@gmx.de>
> An: cbm-hackers@musoftware.de
> Betreff: 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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