Re: Static PET PSU questions

On 10/6/09, William Levak <wlevak@sdf.lonestar.org> wrote:
> On Fri, 2 Oct 2009, Ethan Dicks wrote:
>
>> Well if there were display frequency changes between the 1977 PET and
>> the 1978 PET, then I can entirely understand what I'm seeing on the
>> display.  Obviously there were changes between the 4032 and the 8032.
>> I had no idea there were changes between 40col boards.
>
> Some 40 col. PETs work at the same frequency as the 8032, usually 12 inch
> displays, but also a few 9 inch ones.  The determining factor is the 6545
> CRTC chip.  The frequency is determined by how the 6545 is programmed.  It
> can be programmed to run at the frequency of the older PETs.\

I knew about "Fat Forties" having different video than older PETs, but
not those in the old 9" CRT cases.

> If you are placing a board that does not use a 6545 in a PET that also
> does not use the 6545, then the frequency should be the same.

That is what I'm doing - a 2001-8 in a 2001N/3032 case - no CRTC
anywhere to be found.

On the bright side, I am now certain of the old 5-pin vs newer 8-pin
transformer harness - if you just plug the 8-pin plug over the Static
PET's 5-pin power connector, aligning the two brown AC lines over pins
1 and 5, with a black GND over pin 3, you can safely plug in a newer
transformer into an older PET.  There's no need for the other set of
transformer leads to be plugged in because the Static PET doesn't use
-5VDC anywhere.  It may seem obvious, but I wanted to be extra
cautious before trying out things that were never intended to be
combined.

> In this
> case, the symptoms you describe indicate a defective timing circuit for
> the video horizontal and vertical drive lines.

The visual symptoms are consistent with both the horizontal and the
vertical sync pulses being twice as slow as designed.  I have the
schematic here and am considering which flip-flops could be damaged
and generate these specific symptoms.  The chars are rock-steady and
well-formed, just not filling the entire CRT.

>  Also, these lines depend
> on the 5V power supply.  If this is low, it can also cause these symptoms.

My +5VDC is, at present, coming from an external regulated +5VDC power
supply since I have yet to reinstall the four 7805s that were removed
from this board prior to it being shipped to me (to save the box size
by removing the tall heat sinks.

Speaking of those old heat sinks, they aren't exactly like others I've
seen in photos and in the CRT housing.  Those all had a rounded-corner
rectangular hole machined in the base for the regulator leads - plenty
of clearance.  The four heat sinks I have for this board have a
rounded-corner *triangular* hole, but the size is such that it pretty
much touches the outer edge of the three pads for the 7805s.  I didn't
see any sort of insulator for under the heat sinks, but there does
happen to be plastic washers (not shoulder washers!) for the tab on
the regulators.  Perhaps I'm just being paranoid, but it looks to me
to be too easy to end up with a short from +9V unregulated to +5V
regulated across the heat sink.  Am I missing some essential lifter or
insulator?

Thanks,

-ethan

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