Photo-copier and laser printer repair notes


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Some observations on the operation and repair of electo-mechanical aspects of firstly photocopiers and then laser printers. The mechanical aspects of both are broadly similar varying on the manner of the latent image production.
Considering the function of copiers requires the use of high voltages,high temperatures,fine dusty carbon and the mechanical handling of paper ,it is amazing the things ever work at all. In the damp uk,a lot of problems stem from damp paper and/or damp toner. Outside of a heated (say,office) environment in the uk it may be necessary to keep paper in a sealed box with dried out silica jell crystals and maybe some low wattage heating in or around the toner transfer area of the copier.Some silica gel in a paper or cloth bag actually inside the toner transfer area will stop the toner from clumping and making the filings spill out into the machine.If paper make sure it is strong enough as spilt silica gel crystals could damage the OPC drum.Activate silica gel by placing in an oven at about 120 degree C until colour changes from off white to pure white or mix in some coloured self-indicating type.If using seived and recycled toner then store this toner in a container with a packet of activated silica gel.
Rarely is any copier malfunction due to failure of the electronics ,most commonly problems in the mechanics of toner or paper transfer,the nearest to electronics failure is the odd broken reed switch or microswitch in the exposure traverse path or paper progress path. A more detailed explanation of the function of the corona voltages is described at the end of this piece in the laser printer section, although voltage polarities vary and high voltage ac is also used.
Basic operation is :toner is fed from the hopper to the toner transfer section where it is mixed with iron filings,rotating cylinder with magnets rotate this filing/toner combination close to the OPC (opto-photoconductor)drum and toner transfers to the drum that has been charged-up via the main corona,exposure by bright light neutralises the charge on the white areas of the image and this toner is drawn back off the drum by another voltage,the latent image is then transfered by the transfer corona as the paper passes over. Finally there is a further light exposure of the OPC to loosen excess toner and the drum is cleared,also at the output the paper passes through the fuser drums (approx 180 degree c) to fuse the carbon into the paper. To check fuser temperature ( particularly over-temperature which can destroy the non-stick surface ). Using a thermocouple probe touch the heated fuser drum immediately after switching off the m/c. Place near the end of the drum to avoid the rough weld-point breaking the PTFE surface. Trying on a working / good machine gave a temperature of 145 degrees centigrade - temp nearer centre of roller may be higher.
I am pitching these notes to people in possession of ex-rental copiers that are no longer justifiable to repair and are obtainable fairly cheaply, ie well outside any warranty considerations. Assuming all parts are present in the machine but it basically doesnt work. To avoid flooding the machine with toner it is advisable initialy to disengage the toner hopper/transfer section especialy if using any old toner to check out the m/c. If a m/c is working in order then the use of any old toner will give some result but beware the output heated fuser roller may break down the non-stick surface if insufficient surfactant is not transfered from the toner and the OPC (optical photocunductor) drum may become coated with a film of toner that will not disappear on normal light and excess toner reclaim voltages. Better to add toner initialy in small quantities into the toner/iron filing transfer area. In the area of iron filings and toner there is a subcircuit that inductively monitors the presence/absence of toner to engage the transfer of toner from the hopper and is tuned specifically to the manufacturers toner formulation. Often unplugging this subcircuit does not stop function of whole m/c but check whether toner replenishment is permanently enabled or disenabled. Wrong formulations of toner for a particular machine may give a good image but gives false absent toner indication with continuous operation of the toner replenishment motor. Can usually be fooled into giving a "correct" toner signal level by connecting a resistor and preset between the signal line and 0V or +V (usually 24-28V). Toner consists of principally carbon black with chemicals such as styrene ,styrene acrylic copolymer, polypropylene,metal complex dyes, methylmethacrylate,styrene butylarylate, silicones and oleic acid so wear surgical gloves if coming into direct contact. Incidently if necessary at any time to separate iron filings from toner the only method i have found is using a 75 micron sieve vibrated with an engraving tool. To recycle excess toner warm over a low temperature source of about 60 degrees for a few hours (to dry out) and then sieve through net curtain material to remove any foreign matter like swarf or hair etc.Store the sieved toner in closed containers with a pouch of activated silica gel crystals (kept in a low oven till all the crystals turn to the dry state colour).On the safety front when in operation there are high voltages up to 10 Kilovolt AC and DC present and high temperatures on the exposure lamps and fuser roller also gear trains and belts that can trap fingers. It doesnt take much dust contamination of mirrors to reduce the exposure of the OPC and a consequent poor image in a vertical band of A4 paper. If the printed output has dark bckground with white silouetting around each letter this is indicative of dirty optics. Damp paper gives rise to random areas of feint image (check a pack of say 50 sheets with a 500 volt insulation tester with the prods close together). Or pass a sample sheet through the machine,through the heated fuser and then repeat process on reverse side ,if blotchiness goes then you have damp paper Discharging corona voltages ,due to dust/damp conduction paths can often be heard arcing and give rise to horizontal bands of poor image,randomly spaced . Remove any corona wire housings and check with a 500V insulation tester "Megger" for a resistance greater than 200 Mohm to chassis. Generaly any repeat pattern of corrupted image gives the area at fault ,measure the circumferences of the OPC,toner transfer drum and fuser rollers to see if pattern repeat lenghts equal any of these circumferences. Beware if part of the document bed is causing a problem, ie misalignment and part of it is affecting the image then the repeat pattern is after any optical scaling ie 71 percent say
Beware of losing any screws inside the m/c ,there is only a narrow space between toner transfer roller and the OPC and any object, swarf or paper clip/ staple trapped in this area leads to great damage to the Germanium/Selenium/Cadmium Sulphide layer on the OPC. Small chips off the OPC when dabbed with nail varnish will stop the printing of black dots but of course not transfer any image on this spot. For scratch lines , mask off either side of the line , as close as possible, with old or gum-weak tape, load the nail varnishin with a single long bristle or cocktail stick, after a few minute pare down the excess with a razor and then remove the masking pulling off away from the varnish. To check whether there is mis-alignment of the optical path,remove the OPC assembly and shine a small torch in the reverse optical path and the beam should be viewable the whole width of the document glass when moving the torch the width of the m/c. Some mirrors may not be easily accessable for cleaning but usually can be accessed using cotton wool on the end of a piece of thin wooden dowel or similar. Beware of build-up of toner dust getting to the exposure lamp and fusing into the silica glass leading to loss of exposure illumination.
Grubby/patchy feint image can be due to build up of toner on fuser roller/s, and check the spring tension between the rollers. There is often trouble due to build up on the temperature sensor that touches the heated roller,toner transfering to the roller/s rather than the paper can indicate too low a fuser temperature but beware when using non-manufacturers specified toner. The toner transfer drum usually consists of a set of static long magnets set inside a rotating roller that drags toner/filings around to a magnet positioned opposite the OPC. Problems with poor mixing/distribution of toner/filings is a common problem, often showing in the image as wavy or crescent shaped horizontal bands of repeat pattern smaller than the roller circumference due to its relatively fast rotation. Check for even distribution by removing the whole transfer section and rotating by hand over a sheet of white paper. Too little filings allows too much toner to transfer to remainder of m/c and too much filings reduces the flow. Irregular densities along the length of the roller gives vertical perterbation of image. Incidentaly in all this article when referring to vertical or horizontal i am describing a m/c with the paper fed through the m/c short edge first.
When part of the output copy lengthways goes feint/patchy in a strip with small deposits (iron filings) the toner/filings mix has got thin at that end of the developer chamber. Prop up the rear or front of the machine on the edge away from the corrupt image to let filings/toner move to the bad end. At first seriously prop up and tilt the machine to confirm the problem then leave permanently slightly propped to stop it recurring. It may be due to a loss of filings in the developer section so perhaps add some to top up.
Long vertical V shapes of poor image can be due to retraction problem/toner build-up on the OPC mechanical wiper blade.
Lowest part of letters on each line of text feint or absent compared to the upper parts maybe due to too low transfer voltage.
Paper jam at the input is often due to paper with wavy leading edge or the single sheet grabbing rollers being slippery after double-sided printing.
Paper jam at output can be due to toner build-up on the paper splitter wedges that rub against the fuser roller. Paper jam at the output is often due to the leading edge of the paper having a large component of black image which proceeds to glue itself to the upper output roller. Hence the common advice to close the cover over the original so there is not a dense black boarder around the copied image. Using photocopiers long ago before knowing how they work this advice always mystified me . I thought how could what appears on the document cover as light or dark have a direct mechanical consequence?
Crumpled paper when double sided printing. There is usually a set (curvature) on the leading edge of the first pass,make sure this edge is trailing on the second pass by turning the paper round the other way second time. Also beware of large areas of black image on first pass especially on what becomes the leading edge of the second pass or black stripe down one side or other.
Large splurges of toner is often due to a build up of toner in the excess toner handling system at the OPC often due to damp toner accumulating here.
For your future reference assuming you have in front of you a m/c that basically goes through its mechanical cycle try disconnecting in turn various lamps/heaters and note what error codes ensue. Similarly (BUT BEWARE have hand on the mains switch to switch off quickly)disable various reed switches and microswitches to find what error codes emerge.

Fuser drum problems.
The spring action on the paper splitter wedges should be just enough to keep contact with the drum. Too much and the high temperature coating (PTFE etc) of the drum will wear . Eventually or by accident (jammed up thermistor that monitors the fuser drum temperature and allows to go too high ) or other problems with paper splitters part of this drum coating will break away. A "get you home " fix is to wrap some plummer's PTFE tape around the bared section of drum. Wind on in the sense that the paper splitters will run with the outermost cut edge. Brush in some silicon oil to this PTFE (pehaps heat with the heater) until the white PTFE goes translucent. Periodic application of more silicon oil in this area should help the output paper appearance. Another possibility is a hank of wool touching the corresponding area of the pressure roller that backs up the heated fuser roller. This wool soaked in silicon oil and drapped into a small sump of silicon oil it was a technique used in older copiers that used silicon rubber coatings before Teflon/PTFE type materials became available. With fuser roller demounted. Small strip of absent teflon coating problem . Find a piece of metal tube a bit smaller in diameter than the fuser roller. Coat with some silicon oil. Wrap ,without tension about 4 turns of plummers' PTFE. Heat up with hot-air gun on maximum setting and roll across some paxolin sheet to "meld" though really needs higher temperature. When cold slide off and cut to reqired width and force/slide onto the fuser roller after coating with silicon oil. For a completely bare metal fuser roller the best "get-by" or where replacements are unavailable ,fix unfortunately uses some very expensive tape that is used in avionics or such. Scotch tape from 3M,Austen ,Texas smokey brown in colour (Teflon? tape that does not soften at fuser temperatures usually about 220 deg C )and high temperature gum. Make a spiral wrap the length of the fuser anchored with some thin strip tape off the active ends of the roller. Then some thin strip rolled perimeter wise in areas of paper splitters so they don't catch on the spiral gap. Accepting A3 machines or A4 machines fuser rollers from different machines are surprisingly similar. With a bit of bodging ,grinding back a flange or flat could be made to fit. Same goes for OPC drums it is often possible to rob and adapt an OPC from a completely different make of machine. High precision engineering is not required (applies in principle to all of the mechanics of a photocopier) just mechanical/thermal/wear integrity. Even different diameters are possible as the pressure roller is usually sprung loaded with no chain drive. As long as there is not continuing positive holding paper drive at the point the paper leading edge hits the fuser it should pass through. Other possibilities with dismounted fuser rollers with stripped non-stick coating. Some mains light bulbs for decorative effect are given an orange/red colouring by covering with silicone rubber sheet. Remove this covering and cut to form a cylinder and slide over the roller. Another possibility is use RTV silicone gasket sealant. Twist some fine copper wire onto the Ali cylinder in the damaged area,coat with the RTV paste. Unroll a condom onto the roller and then roll the RTV area on a flat surface to give a smooth finish to the RTV before allowing to cure.
Strong black image on one side of sheets graduating to grey on the other side of sheets may be unevenness in developer chamber - try propping up the machine frame under the appropriate (black) side.
Using auto exposure option gives a grey background can be due to build up of dust on the pick-off section of the lens. Long term requirement to set the exposure ever more higher until near the top of the scale can be due to dirt on the most hidden lenses, mirrors or the final cover slip glass prior to the OPC.
I cannot even find a way of testing toner to determine its characteristics outside of a machine, eg dustiness, oiliness/silicones (affects the amount of corona/transfer voltage required , as far as i know only added to make sure the fuser and associated pressure roller stay clean and the rotating developer cylinder does not throw out the toner ), fuser/melt temperature ,conductivity (for triggering refil and full surplus bottle switches) - those are just my guesses for some of the characteristics. Anyone know of how to do such testing especially dustiness/oiliness/claginess test, I don't even know what its called - If you blow on a small pile of such toner it will blow away but not as easily as if it was totally dry dust of the same particle sizes.
Laser Printers
Similar to a photocopier mechanism,corona discharge from a main charger gives a static coating to an OPC drum or belt. The OPC is exposed by a laser beam reflected off a facetted rotating mirror to form an electrostatic latent image. This image then attracts toner and then transfered to the paper by the transfer corona,fixed into the paper at the output between heated fuser rollers. The OPC then goes through a second rotation for discharging and cleaning. Typical print cycle voltages etc. Main charger 5KV ,200 micro -amp ,corona wire gives a negative charge to the OPC. The data modulated laser beam ,synchronised to the rotating mirror writes data line by line (CRT) fashion to the OPC. Also like a picture tube there needs to be correction for varying path lenghths between centre of image and periphery but with a laser printer the optical data stream is read at a varying clock rate per position in each line. The negative charges on the OPC image are irradiated by the laser and neutralised where the required latent image should be. The toner developer drum is charged to -300V and toner statically transfered to the OPC,or not where there is negative charge on the OPC. The laser exposed portions of the OPC attract the toner forming a visible image on the surface. A +5KV, 60 micro-amp transfer corona attracts the toner from the reverse side of the paper while the paper progresses through the m/c. A -4 KV ,100 micro amp corona then applies a cleaning voltage to the OPC to weaken toner adhesion,to assist in this the OPC is further exposed to a lamp or series of LEDs. A cleaning voltage of 100V is applied to the developing drum to collect residual toner. The above voltages and processes must be considered as representative only as there is no consistency between models and manufacturers. Printed output fault indications.
Unlike a photocopier where absence of a main charging corona voltage gives a no black output,in a laser printer the opposite is the case and an all black sheet is printed. A black sheet with just odd patches of white visible indicates the absence of the developer voltage. For a mainly black sheet with some white and text indicates low main charger voltage. All white can be a lack of the final transfer corona voltage. White sheet with some broken text can be due to low transfer voltage. Sheet having mixed black and white background with patchy text can be low developer bias voltage.
Good text but with splodges of black or vertical black lines can be due to a dirty main corona wire,vanishingly small pieces of dirt can cause this. Text with precise width but random horizontal lines can indicate the start sensor being out of adjustment or noise in this part of the system.
Inconsistent distortion of text can indicate inconstant deflection mirror rotation. Text ok but a large part is black can be due to overexposure of the OPC with the m/c open to bright light,leave in closed position to clear over 10 minutes.
Single thick black bar on the page can be the OPC and paper being out of sync. Poor quality output with vague printing (in reverse) of the printing on the reverse side of the original can be due to poor exposure due to exposure lamp misalignment within the reflector assembly. Consistent,page to page, grey horizontal strips can be due to pick up of toner lying on bed or ledges around the OPC.
Some very useful hints/tips on this file http://www.smarka.com/members/copygen.htm

More specific copier faults and repairs on repair brief file http://www.divdev.fsnet.co.uk/repair4.htm

Diverse Devices,Southampton,England;

Telephone number - the same number as it has been since 1988 but email is now the preferred method of contact so number deliberately not placed here.
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(Obscure/obsolete components,second hand test equipment, schematics etc) Postal:
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SO17 2JN.

There is no point in contacting me about any of the above. I am not a copier repair engineer, I do not sell copiers or copier parts . I have no contacts in the copier industry, I don't repair other people's copiers and cannot advice others on their repair. You may be able to cultivate a relationship with a local copier repair engineer/shop where they have a graveyard of old copiers out the back that could be robbed for parts for a consideration. 

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More hints & tips and repair briefs on homepage http://homepages.co.uk/~diverse/