224 Restoration
Soemtron 224 Restoration
All images ©soemtron.org 2007-
Our 224 in it's original state
Our machine is very, very dirty (it apparently was used in a meat processing factory !), and would seem to have been stored on it's back for many years judging by the way all the vertical surfaces on and inside the machine are dirtiest, we also think that when it was in use it was on top of some other piece of equipment which would seem to have forced dirty air up into the 224 through the base ventilation. This Soemtron 224 has also been dropped at some time in the past as the main card cage is bent and one of the reservoir capacitors has some very bad scrapes (picture #28) from the motor fan. On close inspection these appear to have actually punctured the capacitor case although no leakage is evident. Connections to the digit detector photocell on the printer mechanism are also broken, presumably from continued flexing of the wiring. Some of the electronics boards show that extensive repairs have been carried out in the past, backed up by visible damage to the top edge of the main card cage.
We believe that there are only three 224's left in the world. 526 Soemtron 224's were manufactured by V.E.B. Büromaschinenwerk Sömmerda during 1968 to 1972.
Click any image for a larger version.
This page has photos of our Soemtron 224 before restoration began.
Printer mechanism - mechanical (S# D104)
From the markings on several parts of our printer mechanism and the way it is pinned together it would appear each printer is a custom build. The printer mechanism consists of, and can be conveniently be broken down into, several smaller mechanical sub-assemblies or modules consisting of -
Print wheel layshaft assembly
- Left side plate, drive clutch, mains switch /indicator, manual paper advance gearing.
- Print roller, paper feed pinch roller, single / double line spacing mechanism.
- Front print wheel position selector assembly, eight selectors.
- Rear print wheel position selector assembly, ten selectors.
- Ink ribbon feed, reversal and colour selection assembly.
- Print wheels lay shaft assembly. (See picture right)
- 8:1 gearbox, main shaft and various drive cams assembly.
- Drive motor and coupling with a right angle friction drive to the electronics cage fan.
- Decimal point selection switch assembly.
Printer mechanism - operation
In the printer mechanism a print cycle takes one complete revolution of the main shaft, which driven through an 8:1 worm and wheel gear train from the drive motor conveniently allows us to describe the operation of the printer in terms of drive motor revolutions, or eight stages.
| Stage | Operation |
| -3 to -1 | Slack time required to bring the drive clutch to full engagement. Following activation of the drive clutch solenoid it can take upto three revolutions of the motor to bring the drive to the correct position for printing. |
| 1 to 4 | During these four motor revolutions the eighteen individual print wheels are rotated via selector racks and intermediate gears on a common lay-shaft. As each print wheel rotates forward from it's rest position, each digit to be printed is compared with a position count from an encoder disc on the print mechanism main shaft. An equality in the count and a required digit activates the respective print wheel solenoid, stopping the print wheel in the correct position. At the end of this stage each print wheel will be at rest or stopped in its correct position (0123456789,+-*/). |
| 5 | The eighteen print wheels with each wheel stopped in position, are then all rotated forward on another lay-shaft, a small pawl automatically locks each wheel in position which then contact the paper on the print roller through the ink ribbon. |
| 6 | The print wheels are returned on their lay-shaft back to their rest position in contact with the intermediate gears. |
| 6 to 7 | Advance the ink ribbon and print roller for the next print operation, also reset the ink ribbon back to it's black setting if required. Also if required the print roller will advance two lines during this time if the paper advance solenoid has been tripped, otherwise the paper roll automatically advance one line only. |
| 7 to 8 | Spin the eighteen print wheels back to their rest position, this action automatically resets the print wheel stop solenoids. Miscellaneous other latches are also reset as is the drive clutch. This operation only takes two revolutions of the motor as opposed to the four revolutions it took to set up the print wheels, as there is no requirement to select digits for printing. A microswitch operated off another cam and pin on the main shaft encoder wheel, signals completion of the current print operation back to the control electronics. |
Other controls on the printer mechanism allow the user to manually advance the paper roll and to release the paper feed pinch roller so that the paper roll can be changed. Positive results are printed in black, and negative results are printed in red, under control of a release solenoid that allows the ribbon to pop up presenting the red half of the ribbon, it is mechanically reset during period 6/7 of a print operation.
The motor is a two phase induction motor of an external rotor design, I.E. the squirrel cage rotor spins on the outside of the stator, this gives the motor a higher inertia value as the rotor also acts like a flywheel. This higher inertia would appear to be required for the 224 printer mechanism to get over the high initial torque of engaging the clutch and rotating the main shaft off its stopped position. The speed of the motor calculates to 1500rpm and actually measures at 1490rpm, so through the 8:1 drive this results in a single line print timing of 322mS. With the possible three revolution latency in the drive clutch, a one line print can take between 322mS and 442mS, giving a maximum print speed of three lines per second. A fan on the back of the rotor provides motor cooling, whilst a rudimentary Oldham coupling on the motor shaft tries to cater for any motor misalignment. A right angle friction drive from the back of the Oldham coupling powers the fan in the electronics card cage, pushing air from inside of the unit through the printed circuit cards.
 |
 |
 |
 |
Printer mechanism - Digit position photosensor amplifier - circuit
Damage to the wiring loom of the printer mechanism's digit position photo sensor amplifier had rendered the amplifier non operational, possibly from a previous repair to the cable screen or faulty production, either way the screen wiring had too much solder in the joint and the screen parted company at the point where the screen joins the cable.
We replaced the complete wiring loom, the screened cable is wired in a similar manner to the original with two cores in parallel, the other two wires were replaced in the same size but in differing colours rather than cause any confusion with the original two Green wires.
 Broken sensor cable |
 New photo sensor wiring |
Printer mechanism - Solenoid driver board and sub assembly- circuit
The twenty small solenoids on the 224 printer mechanism are powered by drivers formed from a monostable circuit with a driver stage. The twenty drivers are split into two boards with ten drivers on each, with each board plugged into a sub-chassis with integral wiring harness and connectors for the harnesses from the printer mechanism and digit detection circuits. A small adjustable bracket mounted on the back of the printer mechanism clamps the boards in place preventing them from working loose in their connectors.
The solenoids trip or latch escapements in the printer mechanism which perform the following tasks -
- Sixteen solenoids stopping the position of the sixteen left-most print wheels in the printer mechanism. Each wheel prints 0-9 and comma.
- Two solenoids stopping the position of the two right-most print wheels of the printer mechanism which show the current arithmetical operation.
- One solenoid releasing a drive clutch which engages the continuously spinning motor to the main drive shaft of the printer through an 8:1 worm and wheel reduction gear train.
- Lastly a solenoid engaging a double space paper feed mechanism.
All the solenoids automatically reset or dis-engage during completion of the current printing operation.
Initially it might seem a little strange to have sixteen print wheels for a machine only capable of fifteen digits, however the extra wheel ensures that the current position of the decimal point "comma" can be inserted into a printed result and still show the full fifteen digits.
 Solenoid driver board |
 Sub assembly panel |
 Sub assembly panel |
 Complete driver assembly |
Site design ©2007-  -  [ |