Hardware Feature Extra
Name . Source
Servicing Your ZX Printer . Your Computer - March 1984

Servicing Your ZX Printer

The ZX Printer is a very simple design which should provide years of service if it is properly maintained. Under no circumstances should servicing of a ZX Printer take place if it is still under warranty, any faults should be repaired by Sinclair under guarantee.

The symptoms of a printer in need of maintenance ate:
Excessive noise shown as drive screeching, and loss of print synchronisation displayed as badly formed characters.

There are two further problems with the quality of print output from the ZX Printer which apply to new printers as well as old:
The non-uniformity of print, shown as light and dark bands of printing, caused by irregularities in the metalisation of the paper. If it is very bad, there will always be the odd patch here and there, return the roll for replacement.

Roller deforms the paper
The paper transport mechanism does not always feed the paper through after prolonged periods of inactivity. The roller deforms the paper and cannot grip sufficiently to overcome the initial paper roll turning torque. Changing the ridiculous hubs to a solid bar will, to a large extent, remove this problem. A cheap improvement maybe obtained by taking the hubs and pushing a biro outer case 4.75inch (97mm) long into them. It is essential for the fit to be tight otherwise it will not work, and be careful not to damage the hubs.

This modification - figure 1 - provides effectively one long continuous holder which will provide paper feed in most, if not all, conditions.

The only problem incurred by the above procedure is encountered when separating the hub from the biro case in order to install a new roll of paper, it is necessary to prise the hub and biro casing apart. If you find paper feed a problem then it may well he worth the trouble.

Noise from plain gears
The noise is generated by the plain gears which drive the optical reader and the helical gears which provide the paper feed through two worm wheel drives.
Sinclair designs show very high quality innovative thinking in system design, but do tend to have a basic lack of technique in the more mundane engineering fields. There are several inexpensive ways of producing low noise drives and this design would have benefited enormously if some way could have been found to incorporate such techniques.

The printer may be serviced in two stages, the first stage hopefully will remove the distortion of printed characters and the second stage, which is more complex, should reduce the noise level.
This service should only be attempted if you are reasonably dextrous with your hands and feel confident that you can cope with the very delicate task. Failure to take great care will inevitably result in an unserviceable printer.

First obtain the following tools:
Fine paint brush
Small cross head screwdriver

And for stage two:
A pair of pointed pliers
A 1/8 flat bladed screwdriver may be used in place of the cross recess head screwdriver.

Find a flat clean surface to work on which should be covered with newspaper, this is not going to be a clean job. Remove the paper roll carrier from the printer and turn the printer upside down in the workspace in front of you.

Access may be gained to the optical disc by undoing the four ‘a’ screws in figure 2a. Note that there are two long Screws.

Use the rubber pads to lift the base plate and be careful as the base is lifted away not to strain the two wires which connect to the reader. Remove all the accumulated dust and metalized paper particles from the cavity using either a soft toothbrush or a small paint brush.

It is not advisable to oil the gears as besides giving dust somewhere to stick, it can be spread onto the optical surface and cause future print distortion.

A very small dab of butter may be placed in the shaft bearings, but remember no oil. The gear teeth my be lubricated with a little graphite, rub a 2B pencil lead lightly over the gear teeth, remove excess with the paint brush.
Replace the base plate taking care to ensure that all three shafts are correctly replaced in their respective bearings and then secure the plate in position with the two small screws in the middle.

If you have found the above task difficult then replace the two longer screws and do not go any further. Successful servicing should produce printer output that is correctly aligned.

Test the printer
The printer should now be tested to ensure that it is in fact functioning correctly. If the printer fails to operate, take the base off again and recheck that the reader wires have not been broken and that the optical disc is clean and undamaged.

Assuming that you wish to continue and that the printer still works, we can now attempt stage two.

Remove the paper carrier and turn the printer upside down. Remove the long ‘b’ screws shown in figure 3a and stand the printer back upright with the stylus aperture facing you.



Lift the top cover a 6mm away from the base, inside the stylus aperture gap will be the push fit connector shown in figure 3b. This has to be disconnected by pulling the connector with pliers away from the lug in the lid.
A flat small screwdriver may he used to lever the connector apart if in difficulty but do please exercise restraint and do not use excessive force.

When the connection has been broken, I hope not literally, then take the top casing away. The whole of the printer mechanism should be in full view now so a few more words of warning before we continue.

The toothed rubber band carrying the stylii should not be dismantled; clean the band in place. The stylii are small and very loseable so keep a close watch on them, notice that the top of the stylus is shiny where it rubs the power return strip in the lid.

Use the brush to clean the inside of the lid and pay particular attention to the power return strip, clean thoroughly and make sure that there are no hairs left behind from the brush. Apply a very small dab of butter to the shaft bearings and put the lid safely to one side.

Take careful note of the way the paper feed switch is positioned, if you are going to make a mistake on reassembly it is likely to be this piece round the wrong way.
Carefully brush all the dirt away from each component, take your time and do not hurry and remember that to remove most of the dirt is about all you can hope for, many parts are inaccessible.

I must admit that I have very lightly greased the gears in the upper compartment, but this is not strictly necessary and may speed up the time for the next service.
Carefully replace the push button lever, ensure the stylii are in a trailing position on the drive band spigot - figure 3c - and replace the covet to within 6mm.

Now we have reached the last difficult task, to replace the pushfit connector. Widen the gap slightly and using the sharp nose pliers, push the two halves of the connector together using a lot of care and patience.
The lid must now be pushed forward to ensure clearance of the mica paper cover on the right hand side of the stylus aperture. Replace the plug cable lead into its slot, it will probably have fallen out by now, and looking down over the top of the lid use a pencil point to maneuver the paper feed button into alignment with its location hole in the lid.

The two halves of the assembly can now be eased together, again we are at a delicate stage, do not force it, if there is an obstruction simply pull apart by 6mm and restart the finial assembly procedure.

The four long screws can now be replaced, do not overtighten, and the unit tested. Once a technique has been developed for servicing the printer it becomes quite easy, but it still needs care if the printer is not to be permanently damaged.

WARNING: The following issue of Your Computer contained a warning about using butter as a lubricant. It is better to use vasaline if lubrication is required, butter will attract dirt (and probably smell too!).

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