Tuesday, January 15, 2019

Commodore MPS 803 - Fixing Printing Head

Some time ago I bought old, dirty, untested MPS 803 printer on my local ebay like portal. After replacing ink tape cartridge I noticed that only 4 from 7 printing head pins are working fine. Also motors was not able to automatically move the tape. So printer require some repair work to be done. I decide to give it a try and make this printer work again.

MPS 803 inside.

At start I open the case and investigate inside elements. Everything was in really good condition. There was no visible corrosion which was good information since the outside of the case was really dirty. I believe this printer have long life in office or workshop where every couple of days it was cleaned by "fresh" wet cloth. This make case really dirty after some time.

Carriage motor is also powering ink ribbon feeding mechanism. For some reason motor is to weak to drive this which cause the carriage to stop randomly with printer error light on.
To solve ink ribbon feeding issue I disconnect gear that was powering the mechanism and fead it by hand using small knob on ink tape cartridge. Not the perfect solution but works.

After further checking I notice some broken tracks in ribbon going to printing head but unfortunately this was not the only one issue.

I desolder ribbon and check coils. Some of them were broken. After looking at printing head I decided that I will try to dismount it and maybe I will be able to fix the coils. This was a little like crazy idea but worked fine at the end.

Disassembled printing head.

I removed plastic cover and open printing head. I remove electro magnet blades. I carefully remove plastic part that keeps springy tappets pins that hit ink ribbon. This part also need some cleaning since two of pins was stuck by dry ink. I left with metal corpus and coils inside it. I removed coils and investigate them.

Removed coils, the case and first fixed coil.

The place were coils broke was output pins. So this bring me to conclusion that some driver outputs can be broken(shorted) or basically this was weak point of coil since there is only one wire there and heat can't transfer to anything else like in middle of coil. After checking driver signals using my simple Chinese digital oscilloscope I confirm that driver is working fine and broken coils was the fault here.

Driver signals for single coil.

Now started hard precise part. I resolder all coils leads and bring them outside the case using short wires. To my surprise I was able to fix all coils. I mount coils inside the case and carefully assemble the printing head. I melt part of plastic case to keep it in the place and my printing head was one piece again. I glue wires to metal case to not break them of by accident.  

Assembled printing head.

The last part to fix was the flexible ribbon wire. I decided to remove already broken ribbon and exchange it with bunch of twisted wrap wires. Maybe it is not the cleanest solution but will work for now. I solder it to cut plug of the flexible ribbon and from other side I solder it to printing head wires. 

Plug used to connect to the driver board.

New flexible wire "ribbon".

I bundle the wires on head and print some small cover to make it look better. Unfortunately I need to remove transparent part from case cover to allow printing head to move freely.
My printer was working again.  After cleaning the case it looks great. Maybe it is not 100% original device but at least it is functional and I can print something with it.

First prints in action.

Printing result.
It was pleasure to see how such printers works and how simple they are in general. I can imagine that I should be able to easy control such printer using Arduino only. Maybe this is an idea for another project.