LONG post alert. >
Might be instructive though, or at least, amusing.
I just joined the group today. I noticed that you are searching for a
power adapter (or mains
adapter depending on which side of the pond you're on) for an HP7440a
plotter or at least info on
power requirements.
Here is the link for the service manual from the very helpful and
useful HP Museum (mentioned in
Curt Carpenter's article in Circuit Cellar #202 about using the 7440a
to do direct PC board resist
layouts):
http://www.hpmuseum.net/exhibit.php?hwdoc=80
I'm glad to have looked at this documentation since I was about to
hook up a 20 vac transformer
paralleling the inner and outer pair of pins as suggested by some
forum on the net. This would have
shorted the secondary winding(s) with unpleasant results. Actually
paralleling any two pairs would
guarantee shorting some part of the secondary.
The nitty gritty of the power supply:
--the transformer comes in many models to match the rated 100, 120,
220 and 240 input range. The
secondary is 20 volts AC around 2A with a center tap of 10 volts AC--
and it must be a true center
tap. If asymmetrical, one half of the winding will shoulder all of
the digital circuitry load and the other will loaf along.
--the 20 vac is fed to a full wave bridge to C33 a 2200 uf filter
capacitor to produce approx 26 vdc unregulated to drive the motors.
This voltage is also monitored by support ic U6 which adjusts the gain
of the plotter drive circuits to compensate for line voltage
variations.
--the center tap produces a nominal 10 vdc through a center tap full
wave arrangements (ie using two
diodes) to power the digital logic. The circuit is a departure from
the conventional (and more familiar) one which grounds thecenter tap
and places a diode in each leg of the transformer secondary joining up
at the filter cap. HP chose cleverly to connect the center tap
directly to C32 a 3300 uf filter cap and ground each leg of the
secondary through its own diode. The cleverness emerges when it
becomes clear that these grounding diodes are already part of the full
wave bridge! All of the necessary free-wheeling is already present to
isolate the two full wave supplies and to prevent discharging any
filter cap through a secondary winding.
--the on-board power supply has a charge pump to generate -9 vdc used
by the encoders and RS232
circuitry.
To understand all of this more easily I would recommend printing out
the schematics. They are complete but because of their size they are
broken up into sections. To prevent serious migraines I would
recommend trimming and taping together the sections so that the entire
power supply circuit paths can be viewed at once. Note that there are
two versions of the 7440a. Option 001 is RS232 and Option 002 is
HPIB. The RS232 version's power supply simply adds one current limited
+9v supply for the serial interface. I sincerely hope you don't have
the HPIB version unless your computer speaks this protocol!
The Bottom Line:
--find an original adapter. PRO: guaranteed compatibility CON:
limited availability and
unlimited price!
--find a suitable transformer with appropriate primary voltage and 20
vac center tapped with a 2A
rating. PRO: certainly cheaper and will likely work especially when
care is used to make proper
connector pin hookups CON: not a common voltage/current rating
especially center tapped
--find two identical transformers each with a single 10 vac winding at
2A and wire in series (wire
the primaries in parallel of course and check the phase of the
secondaries) PRO: more likely to
find these transformers (you may even have them in your junk box or
spares bin) CON: care must be
taken to turn both transformers on and off simultaneously so that
digital and motor drive circuits
will do likewise, additionally there is the temptation to use 12.6 vac
transformers which may
produce excessive voltage from the full wave bridge especially since
this voltage is continually
monitored by U6. I would avoid this temptation and I decline to take
responsibility for any
information in this post -- use at own risk!
--find a non-center tapped 20 vac 2A secondary and use to power the
full wave bridge while using an
independent 10 vdc (must be DC!) at approx 0.5A source connected to
the positive end of C32 the 3300 uf cap (with the negative end of the
source connected to circuit ground of course) PRO: presents another
availability and reasonable-cost option CON: the same caution about
powering everyhing up and down at the same time.
The choice is yours. I have deliberately left out pin number
references. Powering up this device
in any do-it-yourself manner is not a cookbook proposition. Complete
understanding of what you are
doing and a healthy measure of experience are required. Good luck and
proceed at your own risk!