HP19c Printer Disassembly

[jjohnson873]

Since there appears to be no suitable replacement for the HP19c printhead, I'm in the process of disassembling the HP19c printer to measure and view the printhead and ribbon cable assembly. I've removed the printer from the calculator and I'm down to the actual mechanism with the motor removed. I have yet to find any photos from anyone who has gone beyond this point. There appears to be a circlip holding the screw drive shaft that moves the printhead. There's also a circlip that holds the steel rod where the printhead assembly moves back and forth. There is an outer shell to the printer housing and there is an inner printer assembly that contains the printhead, the printhead guide, the screw drive shaft that moves the printhead and the paper movement mechanism that applies pressure to move the paper. There are no other screws holding this mechanism together, so I'm assuming that the circlips keep the inner print assembly inside of the outer print housing. Contained in the inner printer assembly, there is a shaft with two small gears on it that appears to be the paper advancement mechanism. I can see some small teeth that are behind the small gears that must be attached to a paper feed mechanism, but you cannot see it without taking the inner printer assembly apart. I'll likely remove the circlips as the next step. It makes me wonder if HP just replaced the whole printer assembly if the printhead went bad. I've attached a picture of the current disassembly.

[PANAMATIK]

(07-21-2021 04:46 PM)jjohnson873 Wrote:  I have yet to find any photos from anyone who has gone beyond this point. ...

It is easy to go beyond this point. You have to remove only one black plastic wheel gear on one side by pulling it from the shaft and remove all the inner and outer circlips of the shafts, then you can remove the shafts and the printhead with flex cable can be freed.

Never take the printhead apart from the flex cable, you will not be able to restore connections again.

Bernhard

[jjohnson873]

Thanks Bernhard. I also found a 2002 reprint of Michael Meyer's description of disassembling the HP-19c printer, overhauling it and reassembling it. I intend to take photos and post them so others will have a roadmap of what to look for. Michael specifically mentioned that the brass pins hold in the paper guide mechanism need to be removed (he thought the pins were a shaft, versus pins). Then there are several springs, including one that presses the top paper guide against the printhead pressure bar. Knowing that springs fly out at the worst possible moment, during disassembly, and then land and roll to a location that you'd never expect to find them, I will take some extra precautions to contain them.
~ Jim J. ~

[jjohnson873]

I figured out how to disassemble the HP-19c printer (more details later). I've removed the thermal printhead and the measurements for the head are as follows, in case anyone has access to a similar 7-wire, thermal printhead. Fundamentally, the HP-19c thermal printhead is 6.37mm x 14.00mm. The 7-lead ribbon cable is 135.85mm long, including the gold contacts.

This particular HP-19c thermal printhead has 3 of the 7 thermal print pins are dead. Average resistance of a good thermal pin is 11.8 ohms from the pin lead to the ground lead, including the ribbon cable leads.

Here is a picture of the printhead.
   

[teenix]

It would be interesting to see if the connection between the two ribbon types has broken inside the grey plastic. Seeing as it's broken, pressing a pin into the ribbons could help serve as a contact for the multimeter.

cheers

Tony

[PANAMATIK]

(07-26-2021 04:57 AM)teenix Wrote:  It would be interesting to see if the connection between the two ribbon types has broken inside the grey plastic. Seeing as it's broken, pressing a pin into the ribbons could help serve as a contact for the multimeter.

cheers

Tony

Advice: Never open the protection grey plastic on a fully functional print head. the bond wires within are very small and sensible.

You can measure the resistance behind the grey plastic, directly on the aluminium stripes, although they are covered with isolation further up. This verifies damaged printer pixels and releases you from opening the grey plastic. Anyway You would need a microscope to examine the tiny bond wires.

Bernhard

[jjohnson873]

I was able to completely disassemble the HP-19c printer and reassemble it again. One word of caution, there is an extremely small spring used to apply pressure to the printhead bar. This spring is extremely difficult to reassemble and it will certainly fly off to parts of your room unknown... more than once. I spent considerable time looking for it when reassembling the HP-19c printer. There is really no reason to disassemble the printer down to this level. If you've removed the ribbon cable from it's connector, you can remove the thermal printhead without removing anything else by releasing the small plastic clip holding the ribbon cable to the printhead metal bracket. Then you can just pull out the printhead from the printhead assembly. If you need to, you can remove the printhead unit by removing two circlips, but it's unlikely you'll need to do that if the printhead is moving smoothly.
I had measured this printhead from the ends of the ribbon cable. It only showed 4 of the 7 thermal print pins with a resistance. But when I pulled out the printhead and measured near the bottom of the printhead, on the frontside, per Bernhard's suggestion, I found that the printhead measures 10.8 ohms on every thermal print pin to ground! This indicates that the printhead itself is good. I'm going to have to figure out how to check the ribbon cable for breaks.

HP-19c printer parts shown in attached pic.
   

[jjohnson873]

Further investigation into the HP-19c printer flex cable revealed cracks in the copper traces where they meet the gold plated fingers. The cracks are on the end of the flex cable where it plugs into the connector on the pc board. You might assume they are where the cable is folded. While this too appears to be a crack in the copper traces, this wouldn't affect the operation of the printer. However, the cracks on the picture shown below is where the flex cable traces connect to the gold plated fingers. Now I'll need to try to repair the flex cable with exposing a small part of the copper trace and use some solder paste to reconnect the traces to the gold plated fingers.