Dale,

About 7 years ago (was it really that long ago?) I had an opportunity to
purchase a number of 7475s for almost nothing.  I had them stacked up
like cordwood in my garage until the boss told me to get rid of them
(schools and students).  I had planned to make a 'photoplotter' with one
of them.  Build an LED/mask/lens insert for a pen body and signal the
LED on from the pen-down solenoid.  Write on Kodalith film.  Didn't have
blue LEDs back then, red won't work on Kodalith.  Project died when I
discovered how inexpensively I could get boards made by houses similar
to APC or Olimex.

There was no (smoked plastic) cover interlock on any of the units I
had.  I had several units which had no plastic cover at all (probably
broken and thrown away).  It's just a dust cover anyway.  There is a
little section of the plastic body which interferes with pens that are
longer than the standard HP pens.  This section can be hacksawed out
with no adverse effects.  Wouldn't do that on a new $1400 plotter but on
a $30 ebay special ...   One of my units came this way.  This allowed
the use of the longer HP pens (or the Koh-I-Noor replacements) which are
used in their DraftPro lines.

I only had two units, out of about a dozen, die.  One lost a servo drive
transistor.  The other had a flakey connection to the keypad.  Aside
from that, those were nice little plotters!  Thanks for the memories.

Best regards,

Dave

Dale Botkin wrote:
>
> Ah.  Stupid me, I hadn't even looked.  Last time I did look for one was
> several years ago, and they were hard to find then -- but eBay was barely
> a startup at that time.  You can do this with any of that series of HP
> plotters, I think I also used a 7470 at the time.
>
> I can't tell you guys how nice this setup works, though.  The pens I found
> are only a couple of bucks each, and I could go from layout to complete,
> etched & drilled board in about an hour for a simple project.  Lines were
> very clean, you just have to remember you simply can't do 8 mil lines with
> 4 mil spacing or anything crazy.  I did have traces between DIP pads with
> no problems.
>
> I didn't realize they were so common and cheap.  Wow.  This beats the hell
> out of trying to modify an ink jet, trust me.  The standard pen body can
> be used as a holder, just cut off the top, remove the guts, cut off the
> tip just at the end of the large taper and slide in an ultra-fine marker.
> All you have to be careful of is that you load the pen manually and bypass
> the cover interlock switch so you can plot with the flip-up smoked cover
> open to clear the tall pen.  Set the plot speed to 1IPS, plot a reverse
> image and Presto!  Nearly instant PCB.  Only difficulty might be large
> copper areas like ground planes, I seem to recall that doesn't work too
> well and will require some manual touching up prior to etching.
>
> Dale
>
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