Service manual for plotter, HP/Agilent 'ColorPro' 7440A.

[Lostgallifreyan]

Does anyone have the service manual for the HP/Agilent 'ColorPro' 7440A
plotter? Also, the programming manual? I'll pay for either those, or good
scans of them. eBay sellers aren't forcoming right now on that model.

The most important bit of info I want is the details of power supply to the
four pins on the connector. The label on the base suggests 20VAC at 2A, but
that seems unlikely in extreme! There are no rectifying and smoothing
components inside, and each of the four pins seems to have its own track.
Two of them have a capacitance across them, too big to safely put directly
across 20VAC. The conflict between label and reality is too great to trust
to anything but original info. I don't want to risk reverse engineering
unless no info can be had at all.

 

[Michael A. Terrell]

Lostgallifreyan wrote:

Does anyone have the service manual for the HP/Agilent 'ColorPro' 7440A
plotter? Also, the programming manual? I'll pay for either those, or good
scans of them. eBay sellers aren't forcoming right now on that model.

The most important bit of info I want is the details of power supply to the
four pins on the connector. The label on the base suggests 20VAC at 2A, but
that seems unlikely in extreme! There are no rectifying and smoothing
components inside, and each of the four pins seems to have its own track.
Two of them have a capacitance across them, too big to safely put directly
across 20VAC. The conflict between label and reality is too great to trust
to anything but original info. I don't want to risk reverse engineering
unless no info can be had at all.

Here is some 7440 info:

<http://www.home.agilent.com/agilent/facet.jspx?cc=US&lc=eng&k=7440&sm=g&t=80039.k.1>


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[CLFURENT]

"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message
news:46C323D7.7BC77BEA@earthlink.net...

Lostgallifreyan wrote:

Does anyone have the service manual for the HP/Agilent 'ColorPro' 7440A
plotter? Also, the programming manual? I'll pay for either those, or good
scans of them. eBay sellers aren't forcoming right now on that model.

The most important bit of info I want is the details of power supply to
the
four pins on the connector. The label on the base suggests 20VAC at 2A,
but
that seems unlikely in extreme! There are no rectifying and smoothing
components inside, and each of the four pins seems to have its own track.
Two of them have a capacitance across them, too big to safely put
directly
across 20VAC. The conflict between label and reality is too great to
trust
to anything but original info. I don't want to risk reverse engineering
unless no info can be had at all.


Here is some 7440 info:

http://www.home.agilent.com/agilent/facet.jspx?cc=US&lc=eng&k=7440&sm=g&t=80039.k.1


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

You can list your needs on "E-Bay WANTED". I did that for a manual I
couldn't find - it took a few months but I hit the jackpot. My "listing"
there is about 5 months old or more - and I still get hits from it for
equipment with the same model number.

 

[Lostgallifreyan]

"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in
news:46C323D7.7BC77BEA@earthlink.net:

Here is some 7440 info:

http://www.home.agilent.com/agilent/facet.jspx?cc=US&lc=eng&k=7440&sm=
g&t=80039.k.1

Thankyou. No good though, that plotter is 22 years old, they don't keep
anything on it, except a passing reference in some other document for
something else.

 

[Lostgallifreyan]

"CLFURENT" <Sales.............@Clfuren.............com> wrote in
news:46c326a5$0$22488$ecde5a14@news.coretel.net:

You can list your needs on "E-Bay WANTED". I did that for a manual I
couldn't find - it took a few months but I hit the jackpot. My "listing"
there is about 5 months old or more - and I still get hits from it for
equipment with the same model number.

I'm considering it, but as you say, it's possible to get mails indefinitely
if the request is too braod so I want to try to get the information some
other way first, even if just to narrow down what I ask for.


One small breakthrough:
http://repc.stores.yahoo.net/hppowsupus.html

That describes the right power supply, and it's the same for many devices.
I'm trawling eBay for adaters/psu's for those, without much success, I
found one in Canada (I'm in the UK).

If anyone has any of those HP printers listed there, and is willing to get
the PSU and let me know what the four pins are doing, i.e. which if any are
commoned, or not connected, and especially, which are outputting 20VAC,
that will be a great help to me.

 

[Michael A. Terrell]

Lostgallifreyan wrote:

"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in
news:46C323D7.7BC77BEA@earthlink.net:

Here is some 7440 info:

http://www.home.agilent.com/agilent/facet.jspx?cc=US&lc=eng&k=7440&sm=
g&t=80039.k.1


Thankyou. No good though, that plotter is 22 years old, they don't keep
anything on it, except a passing reference in some other document for
something else.

I've had about a 50% sucess rate with their online manuals, but it
never hurts to check, and check back, because they keep adding more old
manuals. They have some a lot older than 22 years on there.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Lostgallifreyan]

"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in
news:46C369B7.672D4491@earthlink.net:

I've had about a 50% sucess rate with their online manuals, but it
never hurts to check, and check back, because they keep adding more old
manuals. They have some a lot older than 22 years on there.

Cool, I grant it's a nice way to look for stuff so I'll do it. Still hoping
someone's got one of those PSU's to get me a pinout, but for the
programming manual I can watch and wait, first thing is to get the thing
started... Direct drawn PCB's. SMT, single sided and such. A cheap and
effective way to make them, it seems. Good to learn with, if nothing else.

 

[Bob Pownall]

Lostgallifreyan wrote:

Does anyone have the service manual for the HP/Agilent 'ColorPro' 7440A
plotter? Also, the programming manual? I'll pay for either those, or good
scans of them. eBay sellers aren't forcoming right now on that model.
snip

I have some contacts in HP's printing divisions. I'll ping them and see
what I come up with.

In another post, you indicate the plotter is a mid-1980's vintage. Is
that correct?

Bob Pownall

 

[Lostgallifreyan]

Bob Pownall <repownall@netscape.net> wrote in
news:13c7iqmho48q766@corp.supernews.com:

Lostgallifreyan wrote:
Does anyone have the service manual for the HP/Agilent 'ColorPro'
7440A plotter? Also, the programming manual? I'll pay for either
those, or good scans of them. eBay sellers aren't forcoming right now
on that model.
snip

I have some contacts in HP's printing divisions. I'll ping them and
see
what I come up with.

In another post, you indicate the plotter is a mid-1980's vintage. Is
that correct?

Bob Pownall

Nice one. Thankyou. And yes, it is. I had a look inside yesterday, awesome
thing, looks like it was made last week, not 22 years ago. No brittle
plastics, wire sheaths are still flexibly plastic, no oxidised metal that I
can see, belts are taut but not exerting strong constant strain, it looks
like it could wait a century and still look good as new. Even the
electrolytic caps look fine, but I won't know till I get power to it
whether they are. I bet they are though.

 

[Michael A. Terrell]

Lostgallifreyan wrote:

Does anyone have the service manual for the HP/Agilent 'ColorPro' 7440A
plotter? Also, the programming manual? I'll pay for either those, or good
scans of them. eBay sellers aren't forcoming right now on that model.

The most important bit of info I want is the details of power supply to the
four pins on the connector. The label on the base suggests 20VAC at 2A, but
that seems unlikely in extreme! There are no rectifying and smoothing
components inside, and each of the four pins seems to have its own track.
Two of them have a capacitance across them, too big to safely put directly
across 20VAC. The conflict between label and reality is too great to trust
to anything but original info. I don't want to risk reverse engineering
unless no info can be had at all.

First of all, do you need 120 or 240 VAC input?

I find several different supplies listed online:

http://repc.stores.yahoo.net/hppowsupus.html says 2V 2A, but doesn't
say AC or DC. It also lists some HP printers that used the same supply,
and I MIGHT have a 120 VAC input supply in my pile of dead printers. I
also have some old HP plotters in storage that might use the same
supply.


http://forums.driverguide.com/showthread.php?t=3266 says:
Power connector:

|o -> gnd
|o -> ac1
|o -> vcc
|o -> ac2


vcc and gnd -> 10Vdc @400mA

ac1 and ac2 -> 20Vac @1A

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Lostgallifreyan]

"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in
news:46C3E61B.EB13FB53@earthlink.net:

Lostgallifreyan wrote:

Does anyone have the service manual for the HP/Agilent 'ColorPro'
7440A plotter? Also, the programming manual? I'll pay for either
those, or good scans of them. eBay sellers aren't forcoming right now
on that model.

The most important bit of info I want is the details of power supply
to the four pins on the connector. The label on the base suggests
20VAC at 2A, but that seems unlikely in extreme! There are no
rectifying and smoothing components inside, and each of the four pins
seems to have its own track. Two of them have a capacitance across
them, too big to safely put directly across 20VAC. The conflict
between label and reality is too great to trust to anything but
original info. I don't want to risk reverse engineering unless no
info can be had at all.


First of all, do you need 120 or 240 VAC input?

I find several different supplies listed online:

http://repc.stores.yahoo.net/hppowsupus.html says 20V 2A, but doesn't
say AC or DC. It also lists some HP printers that used the same
supply, and I MIGHT have a 120 VAC input supply in my pile of dead
printers. I also have some old HP plotters in storage that might use
the same supply.


http://forums.driverguide.com/showthread.php?t=3266 says:
Power connector:

|o -> gnd
|o -> ac1
|o -> vcc
|o -> ac2


vcc and gnd -> 10Vdc @400mA

ac1 and ac2 -> 20Vac @1A

Excellent find, that second info, it sort of confirms what I suspected, two
power busses. Hard to see why 4 pins otherwise. Also, that capacitance I
detected would likely be extra smoothing for the DC buss.

I'm after a 240V input PSU but I suspect cracking open the case might show
a dual-primary transformer.

 

[Lostgallifreyan]

"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in
news:46C3E61B.EB13FB53@earthlink.net:

http://forums.driverguide.com/showthread.php?t=3266 says:
Power connector:

|o -> gnd
|o -> ac1
|o -> vcc
|o -> ac2


vcc and gnd -> 10Vdc @400mA

ac1 and ac2 -> 20Vac @1A

Been inside. That info checks out, pretty much, except that's 2A as claimed
by the other source, there's a bridge rectifier made of 4 diodes, and a 50V
2200ľF cap before a 2A fusible resistor protecting what comes next. The
other cap is a 3300ľF so I guess the PSU is unusual in rectifying a 10V or
12V winding but not smoothing it.

I guess this is part of some common standard HP wanted so they could use
the same PSU on so many devices.

I could probably wire something up now but this HP 7440A plotter is so
awesomely perfect I want to treat it nice, so I'll try to get service and
programming manuals and original PSU if I can. I'll try the eBay wanted ad
soon if I don't get lucky here.

 

[Bob Pownall]

Lostgallifreyan wrote:
<snip>

The first response I got back listed these (non-HP) sources:
http://www.ambry.com/page/category_hp.asp?strHpCategory=printer&strHpKeyword=Manual&text=Printer%20%3E%20Manual&cat=printers

http://www.jpcparts.com/page/category_hp.asp?strHpCategory=printer&strHpKeyword=Manual&text=Printer%20%3E%20Manual&cat=printers

http://www.everprint.com/online/simdetail.asp?partno=07440-90000

http://www.partshere.com/online/detail.asp?partno=07440-90000

Since you've already said you're willing to pay for manuals, these (or
something like them) might be your best option.

Bob Pownall

 

[Lostgallifreyan]

Bob Pownall <repownall@netscape.net> wrote in
news:13c8q567a43t871@corp.supernews.com:

Lostgallifreyan wrote:
snip

The first response I got back listed these (non-HP) sources:
http://www.ambry.com/page/category_hp.asp?strHpCategory=printer&strHpKe
yword=Manual&text=Printer%20%3E%20Manual&cat=printers

http://www.jpcparts.com/page/category_hp.asp?strHpCategory=printer&strH
pKeyword=Manual&text=Printer%20%3E%20Manual&cat=printers

http://www.everprint.com/online/simdetail.asp?partno=07440-90000

http://www.partshere.com/online/detail.asp?partno=07440-90000

Since you've already said you're willing to pay for manuals, these (or
something like them) might be your best option.

Bob Pownall

Thankyou. I'm willing to pay, but I'll pass on those, having looked at
them. They might not have any. Even if they do, they'll set the price to as
high as they like the momemt I request a quote, then add all the surcharges
and handling charges and brokerage charges they can think of.

I'll wait till someone on eBay offers one. Can't negotiate with people who
run vast virtual carts. Maybe on eBay, I can. eBay also contractually binds
a seller to have the item before trying to sell it for a start.

 

[Lostgallifreyan]

Lostgallifreyan <no-one@nowhere.net> wrote in
news:Xns998EAA8D3B45zoodlewurdle@140.99.99.130:

LONG post alert. > Might be instructive though, or at least, amusing.

Bob Pownall <repownall@netscape.net> wrote in
news:13c8q567a43t871@corp.supernews.com:

Lostgallifreyan wrote:
snip

The first response I got back listed these (non-HP) sources:
http://www.ambry.com/page/category_hp.asp?strHpCategory=printer&strHpK
e yword=Manual&text=Printer%20%3E%20Manual&cat=printers

http://www.jpcparts.com/page/category_hp.asp?strHpCategory=printer&str
H pKeyword=Manual&text=Printer%20%3E%20Manual&cat=printers

http://www.everprint.com/online/simdetail.asp?partno=07440-90000

http://www.partshere.com/online/detail.asp?partno=07440-90000

Since you've already said you're willing to pay for manuals, these
(or something like them) might be your best option.

Bob Pownall


Thankyou. I'm willing to pay, but I'll pass on those, having looked at
them. They might not have any. Even if they do, they'll set the price
to as high as they like the momemt I request a quote, then add all the
surcharges and handling charges and brokerage charges they can think
of.

I'll wait till someone on eBay offers one. Can't negotiate with people
who run vast virtual carts. Maybe on eBay, I can. eBay also
contractually binds a seller to have the item before trying to sell it
for a start.

I just tried one of these strange outfits that say they have, or can get
parts. First one I tried for the part in question, and sure enough, it
really IS that bad! Read on if you're bored enough.....


Hi, I saw this on your site:
Caritronics K-3000 TRANSFORMER PLY HV 3000VDC FS $87.00
Can you tell me any specs for it? 3000VDC is nice but how much current? I'd
need to know before deciding on anything.
Any info at all is welcome.

Hello!



I have received your request for a quote. I am working on getting your
prices but in the meantime I would love to have you set up in our
system. Can you please email me the following information:



Name

Company name

Address



Phone

Fax



Thank you!



[Company details omitted, this is a fun post, not investigative journalism]



Creating Business Excellence Through Customer Communication

No, you don't need any of that. If you have the part, AND information to
help me choose, I might be able to decide to buy. If you ask me for
information that doesn't help me find the part or information about it, I
won't go there, it's a waste of time, and a potential annoyance.

I am working on your quote and waiting for the additional information
you requested. I just figured we could save time by putting your
information in the system already. As soon as I receive the information
I am waiting on, I will send your quote through email.

If you're able to get info on that part, just reply by mail as you already
did, that's all you need. None of that other stuff is needed, no-one buys
parts that way, they just buy parts. IF the information about them is
forthcoming without obstacles, otherwise they go elsewhere.

I can quote you the price on the part K3000 to be $1763.00. If you are
interested in me sending you a formal quote please let me know.

Awesome! I found a couple for less than $50 including postage. I knew you
weren't for real, and you just confirmed it MAGNIFICENTLY. What kind of
outfit are you, really?
You don't have the parts, do you, or even any access to them at all? You
don't know a thing about them, do you? They don't weigh 3 lb each either,
so you might want to correct that little detail on your site.
Let me guess, you're one of these middlemen outfits who try to profit by
inserting themselves between people and the things they are looking for, a
bounty hunter for stupid people who don't really know what they want and
have far too much of other people's money to spend. I'd have the same
change trying to buy this part from a Nigerian email scammer, I imagine.

-------------------------------------------------

Ok, maybe not all of them are like that but this should be a warning not to
go to people like that, they'll slap you silly if you're daft enough to let
them think you really want something from them.

 

[PopMed]

On Aug 29, 1:16 pm, Lostgallifreyan <no-...@nowhere.net> wrote:

Lostgallifreyan <no-...@nowhere.net> wrote innews:Xns998EAA8D3B45zoodlewurdle@140.99.99.130:

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!

 

[Lostgallifreyan]

PopMed <popmedkoh@gmail.com> wrote in
news:1188584148.224710.287910@k79g2000hse.googlegroups.com:

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

My hero! And I don't even beleive in them. Thankyou, that is very awesome,
an amazing stash.

I agree, finding the original adapter seemed wise once I learned of the
unusual arrangement of 4 terminals. Even with the centre tap the fourth
terminal is not yet clear to me, but it will be with those schematics.

I got a supply, WITH a deskjet printer. Won it on eBay for the cost of
postage, I added a couple of quid because no-one else bid and the seller
was willing to change his postage conditions to send it to me.

If I do have to make a second supply, I'd probably use a 12V 2A toroidal
and take out enough turns to drop the voltage. As they have two separate
secondaries, they are ideal for this.

I got lucky btw, NEW (mothballed) ex-MoD (Navy) device with RS-232. It
looks and works like it was built last week, not over 20 years ago. Got it
for price of postage, again, only thing wrong was no power supply.

With 8 Staedtler Lumocolor refills and a few Lumocolur pens to raid various
width tips from to refit plotter pens with, I'll be making boards soon.

 

[Dave Platt]

In article <Xns999DCE2B04719zoodlewurdle@140.99.99.130>,
Lostgallifreyan <no-one@nowhere.net> wrote:

I got lucky btw, NEW (mothballed) ex-MoD (Navy) device with RS-232. It
looks and works like it was built last week, not over 20 years ago. Got it
for price of postage, again, only thing wrong was no power supply.

With 8 Staedtler Lumocolor refills and a few Lumocolur pens to raid various
width tips from to refit plotter pens with, I'll be making boards soon.

There's a service tweak I discovered/invented for these plotters which
you might possibly find useful.

The paper motion (in the "Y" direction) uses several cylindrical
rubber wheels, which hold the paper against individual rollers on the
other side the paper. On the used plotter I bought, the plotter had
been stored with the paper drive mechanism in the "engaged" position,
and the pressure from the rollers had flattened the rubber on the
wheels enough to create a distinct "dimple". This caused paper
motion to be uneven and unreliable - the roller/wheel interface would
lose pressure when the roller hit the dimple in the rubber.

Trying to buy replacement wheels from HP seemed out of the question.

I found a solution at a local hobby store - a soft, flexible silicone-
rubber tubing (translucent blue in color) which is used as a fuel line
in radio-controlled airplanes. I was able to dismount the existing
rubber-coated wheels and scrape off all of the old rubber (it's
pressure-molded onto a ridged core/axle), cut off a short length of
the silicone rubber tubing, and force the tubing over the core of the
wheel.

When re-mounted on the plotter, the new silicone-rubber wheel surface
proved to work very well.

--
Dave Platt <dplatt@radagast.org> AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

 

[Lostgallifreyan]

dplatt@radagast.org (Dave Platt) wrote in news:2qfmq4-q8t.ln1@radagast.org:

On the used plotter I bought, the plotter had
been stored with the paper drive mechanism in the "engaged" position,
and the pressure from the rollers had flattened the rubber on the
wheels enough to create a distinct "dimple". This caused paper
motion to be uneven and unreliable - the roller/wheel interface would
lose pressure when the roller hit the dimple in the rubber.

Good cautionary point. I caught myself reading a manual the other day while
leaving the paper engaged, and disengaged it thinking of exactly that
problem. I'm slightly conditioned to thinking of this risk and preventing
it, having a bandsaw, those too can go agly if you leave them under tension
and static for a significant period. I also used to repair tape decks...

Trying to buy replacement wheels from HP seemed out of the question.

I found a solution at a local hobby store - a soft, flexible silicone-
rubber tubing (translucent blue in color) which is used as a fuel line
in radio-controlled airplanes. I was able to dismount the existing
rubber-coated wheels and scrape off all of the old rubber (it's
pressure-molded onto a ridged core/axle), cut off a short length of
the silicone rubber tubing, and force the tubing over the core of the
wheel.

When re-mounted on the plotter, the new silicone-rubber wheel surface
proved to work very well.

Nice idea. I think polyurethane might also be good, or that kryptonic stuff
used for skateboard wheels, but not so easy to find in a tube, perhaps.