HP2631B printer

[Rik Bos]

> -----Oorspronkelijk bericht-----
> Van: cctalk-bounces at classiccmp.org 
> [mailto:cctalk-bounces at classiccmp.org] Namens Tony Duell
> Verzonden: donderdag 4 maart 2010 21:33
> Aan: cctalk at classiccmp.org
> Onderwerp: HP2631B printer
> 
> 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
> 
NICE work !

-Rik