HP 19C carriage return

[Geoff Quickfall]

Okay, here we have a 19C, fully functional including the printer.

The problem is, the printer head will not park. It continually moves even when not printing.

I can physically place the head where it belongs at the far right in the parked position. This after a semi dissemble. The head remains in position but as soon as I hit print off it goes printing correctly then returning to the park position, bouncing off that and back to the left and then right and then...

it seems that it bounces off the parked position due to momentum and then ends up in past the park and off it goes.

under the printer the contact points work correctly opening and closing. it would seem that there is a brake function on return that is not functioning.

any ideas?


p.s. I have this ocurr on two machines that were working. could it be over voltage from 4 alkaline (6,0 volts) versus 4.8 volts NiCad. would it require a shotsky diode to reduce the voltage.

HP 19C challenged!

Geoff

here is the parked position for the head:



what are the trimming caps for (white pots)?



the whole damn thing apart looking for the fault!

[Katie Wasserman]

(09-08-2014 11:41 PM)Geoff Quickfall Wrote:  p.s. I have this ocurr on two machines that were working. could it be over voltage from 4 alkaline (6,0 volts) versus 4.8 volts NiCad. would it require a shotsky diode to reduce the voltage.

That's a good bet. The 19C is particularly sensitive to voltage, but 6 volts is probably not too high. More likely you have too low a voltage getting to the motor drive circuit. Make sure you have good, clean battery contacts on the PCB (one of them looks sort of yucky to me).

[robert rozee]

is the carriage moving too freely?

try placing a finger lightly on the end of the plastic drive shaft (with helix), so as to add a little friction to slow it quicker when power to the motor is cut. if this works, have a close look at the pin that engages in the helix - i'm picking this pin is spring loaded and that either the pin has worn down over time or the spring behind it has snapped.

rob :-)

[Geoff Quickfall]

Hello Katie,

I will check the current and voltage again. The problem is the motor momentum and run time is too fast and long. As though the parked signal is not being accepted. The motor correctly de-powers but the momentum from the RPMs carries it through the parked position and right back into the run mode.

Is there a feedback current that brakes the motor once it starts its return? Is paper thickness part of the braking concept using friction?

Battery contact has been cleaned and tested, perfect. I am going to jury rig a three battery pack and see what happens.

Rob,

Thanks, but the mech, contact switch, spring for the head guide in the helix, in fact the whole damn thing is in perfect mechanical condition. Maybe I greased the helix and shouldn't have?

The guide mech. works perfectly, the motor too perfectly as it is so frictionless the motor passes through the parked position and into the run position again by momentum and not current, alone.

Perplexed me!

[Katie Wasserman]

(09-09-2014 03:53 PM)Geoff Quickfall Wrote:  Hello Katie,

I will check the current and voltage again. The problem is the motor momentum and run time is too fast and long. As though the parked signal is not being accepted. The motor correctly de-powers but the momentum from the RPMs carries it through the parked position and right back into the run mode.

Is there a feedback current that brakes the motor once it starts its return? Is paper thickness part of the braking concept using friction?

Looking at Tony's hand drawn schematics it looks like there is a braking circuit, similar to the one described in the HP-97 service manual. Pin 10 on the PIK chip drives a single NPN transistor that shorts out the motor winding.

[Geoff Quickfall]

I will check the 97 manual!

Maybe a cold solder joint as the removal and subsequent rebuilds trying to troubleshoot this finicky system has broken that contact.

Thanks Katie!!

Will let you know later.

[ElectroDuende]

(09-09-2014 05:11 PM)Katie Wasserman Wrote:  Looking at Tony's hand drawn schematics it looks like there is a braking circuit, similar to the one described in the HP-97 service manual. Pin 10 on the PIK chip drives a single NPN transistor that shorts out the motor winding.

I bet for that. I've not worked on hp printers, but on other brands, and all of them have this braking circuit that shorts the motor winding to stop it in virtually no space. When this circuit (usually a single FET) fails, you have misalignment problems.

[Geoff Quickfall]

Okay, re soldered all the main IC joints as well as many others. The printer board flexes pretty much when removed so hopefully that might iphave fixed it. There were three non transistors in the board and the exist at board flex points so they too got re soldered.

I then took apart the printer bracket completely. Found out why it did not epd any to load correctly or paper feed. There was a TINY dried wad of paper jammed in the paper feed out if site. I removed it and the paper now slips through into place where it should be picked up by the paper advance gears. These gears appear to be solid plastic, not the gummy variety.

A bunch of disassembly photos and reassembly to add to the 19c chapter!

Cheers, Geoff

Will reassemble after this flight and Reno and let you. Know how it turns out.

[Giancarlo]

Hello,
are these threads marketing ad for the book? :+)

You already have my commitment to buy the Book Geoff.

I am going to start purchasing some old calculators with the 'sold as is' info in the description but there is the risk they works like it happened to me in the past.

Do you think i could ask to the seller to verify that the calc really don't work before bidding? they will think i am crazy :+)

Giancarlo

[Geoff Quickfall]

Not really,

It has been a while in the promise list of things to do. I wanted to let people now that it is not a pipe dream, and that extra info is being put in, maybe a bit of a teaser!

Ebay is ebay, the caveat is to question, question, queues tin. Some buyers plead ignorance, some lie, some don't know but at least if you have asked the correct questions, have a copy of the answers and it runs out hey did committ a falsehood through ignorance or intent, you have a better chance if the dispute ruling in your favour.

Good luck on the purchases and repairs!

Geoff