You may rememebr that a few weeks ago I asked ofr help on the encoder of
an HP2631B printer. I've not sorted it all out, and will describe what I
did...
The initial problem was a defective 2114 RAM (!) on the procesosr board.
After replacign that, the printer rammed the carriage into the side of
the mechanmism, blowing the 2 motor driver transistors and an overvoltage
zenenr on the -20V line. Furtehr tests showd there were no signals coming
from the shaft encoder on the carriage movement
leadscrew.
I took the machine apart. Really apart. Not having the special tools to
remove the encoder form the leadscrew, I found I could take them out
together, You loosen the cap screw in the collar on the carriage motor
shafter, free the ribbon cable from the cips onthe rear paper guide, and
tkae out the 3 screws from the encoder mounting plate. The whole lot this
comes out of the right hand side of the mechamism. I could then remove
the end cover from the encoder (carring the IR emitter) and the little
cover over the cable connections.
The HP service manual warns you not t oremove the carriage rails or
platten bar, since 'an important adjustment will be upset'. I reckoned
that my pritner was in a pretty bad state already, and little could make
it any worde, so I took it all apart. I am wondering what the HP manaul
is talking about. The 2 carriage rails have their ends turned down to fit
in holes i nthe side plate, so theres no way they can be moved by more
than a few thou. And these ends are not eccentric, I checked. The platten
bar is locared by dowel pins to each side plate. It can't move either. As
far as I can see there is no problem at all with completely stripping the
mechanism. This meant I could deal with that substance well-know to all
classic computer people -- sound-deadening foam that turns to dust.
Back to the enconder. The person who said 'sounds like the IR emitter is
out' was vey close. The IR emitter was indeed not glowing. The reason was
that it's conencted to the rest of the encoder by 2 little pin sockets on
the emeltter assembly (encoder end cover) which fit onto wires coming out
of the enocderr boddy -- one of them seems to be the end of a current
limiting resistor... Anyway, one of those wires was bent and not
connecting to the emitter assembly. Easy to fix when you know where to
look!
I crelaced the shorted zneer diode on the PSU board, and did some simple
checks on the rest of the electronics. The DIP switches on the priner
logic PCB and HPIB conenctor PCB were not reliable. Since an HPIB address
swithc which doesn't set the address you expect is going to be a curse, I
replaced them. I managed to re-stake the puchbutton assemblies on the
control panel PCB --
http://www.parts.agilent.com indicates that type of
switch (plastic housing heat-staked to a PCB with gold contact pads) is
still avaialbe, but only if you return the instrument to Agilent for
repair. Sorry, but no way...
With everything back together (apart from the motor driver transistors),
it was time to give it a go. I could now see that the encoder was
producing pulses, that the end sensors worked, and that it wasn't trying
to turn on both motor driver transsitors at the same time. Powered down,
fitted the (expensive, 25A) motor driver transistors, and tried again with
the lesdscrew nut unscrewed from the carriage. The idea was that if the
motor 'ran away', it wouldn't slam the carriage into the side plate.
Tjhis time on power-up the mtoor ran -- but at a sensible speed, there
were plento of pulses from the encoder, and by just touching the
leadscrew nut I found it was indeed trying to drive the carriage to the left.
Time fore the real test. I ffittd the 3 screws holding the leadscrew nut
to the carriage, and tired again. The carriage went to the home position,
and the macjhine gave a long beep. The frontpanel buttons did nothing
apart from reset (which repeated the initialisation) and On-Line which
caused it to beep again. I tried frobbing the paper-out switch, it made
no difference, so I guessed I had a real fault.
I spend 2 hours looking at signals. The peocessor was clearly running.
The end sensor signals were fine. The enocder, position counters,
direction flip-flop, and so on all seemed to be doing the right things. I
was beginning to think i had a nasty fault in the custom HP procesosr
chip. And yet, it was running the firmware, at least enough to run the
carriage to the home position, sound the beeper, and so on. Checking what
the processor ws trying to do to the pritner logic PCB indicated it was
reading the sensors and writng to the carriage motor register, which made
sense. It wasn't randoming acccessing all the ports.
What had I missed? I went back to the paper out signal. It was high
(indicating out-of-paprr) at the input pin of the 3-state buffer on the
printer logic PCB. It was low on the motor harness pin on the PSU board
(that makes sense, there's a NOT gate on the PSU board which inverts
this sigal). But it didn't change state when I frobbed the microswitch.
Aha...
Although the microswitch is hidden inside the printer mechanism, I
managed to disconenct one of the faston terminals from it (the switch is
closed when out of paper). This time when I powered the machine up, it
homed the carriage and didn't beep. I could do linefeeds and formfeeds
from the panel, the on-line button worked, and the
self-test seemed to be
trying to print something (I'd not fitted the printhead
at this stage.
So the microswtich was faulty. Strangely it was stuck closed (most switch
porblems cause them to not make contact). I removed the printer mechanism
again, turned it over, and removed the rear paper guide (4 screws). 2
mores screws released the microswitch from the guide. And it didn't
'click' when I pressed the actuating lever. Unfortunately, although it's
a standard V3 size switch, the actuator is unusual, so getting a
replacement would be nnon-trivial. With nothing to lose, I drilled out
the rivet holding the swtich together, took off the cover and remvoed the
contacts. I then fount that other substance well-known to classic
computer types -- grease that turns to cement. Cleaned it off, cleanded
thee contacts (well, while I had it apart) and resassmbled it. Now it
clicked. And an ohmmeter showet it was woring electrically too.
Put it all back together again. Now it will initialise and respond to the
control paenl -- provided there's paper in it. Time to fit the printhead
(trivial), and it now makes that well-known buzzing that everybody who's
ever been near a dot-matrix printer would recognise. Will it print
anyhting sensible? Well, let's try the ribbon. Which is jammed. The
ribbon cartridge is heat-staked together, but the hold trick of pulling htr
ribbon out and widing it back in got it free enough to work. Now the
self-test prints a character set -- and it looks quite sensible,
Tiem to try it with a computer. I grab my HPIB test set-up (HP71 + HPIL
module + HP82169 HPIL-HPIB interface) and cable it all up. Set the
printer address to 4 (yes, I used a PET in the old days...) type PRINTER
IS 4 and then PRINT. The darn thing does a formfeed (!). Then try
PRINT"0123456789". It prints "0000444488" andanotehr formfeed. Clearly
the 2 least significat bits (bits 1 and 2 in HPIB terminology) weren't
gettign through (an unconnected HPIB line is high, which corresponds to
logic 0 on this bus). Hence the CR character was becoming a formfeed...
Fortunately I'd picked an HPIB address where this wasn't a problem,
I hoped the HP custom PHI HPIB chip hadn't failed. I disconnected the
HPIB cabel from the HP82159 and removed the HPIB interface PCB together
with the connector PCB and HPIB cable from the printer/And then did
continuity checks from the free end of the HPIB cable to the pins on the
3448 buffer chips on the HPIB PCB. Fortuneately bits 1 and 2 were indeed
open.And a few morre detaild test showed it was nothing more than dirty
contacts on the H{PIB socket. A cotton bud and propn-2-ol cured that.
And then it printed properly -- at last. The last job was to fit the
cover and platten knob, which was trivial -- at least after everything else.
-tony