1	Run a print sample and identify the print positions that need work. It can be helpful to use a repetitive pattern or black plot test for this purpose. If you slowly raise the platen lever while the patterns are printing you will see weaker columns dropping out first. Using a worn ribbon also helps identify any weak positions.
2	Power down the printer and unplug the power cord. Open the front cover and remove the ground connector on the left side casting directly above the forms thickness lever. Push in the locking tabs on the rear of the printer (left and right) and gently pry the rear of the cover up using a flat blade screwdriver.
3	If there is someone around who can give you a hand, this next step is much easier with two people. Raise the cover far enough to remove the control panel connector (P801). There are two locking tabs on the connector top and bottom, press both to remove.
4	Turn the cover upside down and set it down on a surface where it won't get scratched up. Remove the control panel by gently pressing on the locking tab as shown and pushing the panel through the front of the cover.
5	Re-connect the control panel to the printer at P801 and lay the panel in the base pan. You will need this to test the printer in later steps.
6	Note that front cover latches (left and right) are removable. Use caution so that they don't get lost during the repair process.
7	Remove the paper and ribbon cartridge. Close the tractor doors. Remove the ribbon deck hold down screws. (2) on the left and right side of the deck.
8	Remove the two screws securing the hammerbank cable clamp on the left side as shown.
9	Unplug ribbon motor at J502
10	GENTLY release locking tabs on the underside of the ribbon deck (left and right side) and slide the deck forward and out of the printer.
11	Disconnect hammerbank cables from hammerbank PCBA
12	Remove the hammerbank cable bracket (5 screws) leaving the center screw for last.
13	When you remove the last screw gently lower the bracket and let it hang below the printer.
14	Remove the six screws securing the black air plenum. Close the ejector levers on the lower hammerbank cable connector. Disconnect the air hose from the fitting shown. Remove the air plenum by tilting the lower edge forward and gently pulling it out.
15	Remove left side upper and lower bridge screws. DO NOT REMOVE RIGHT SIDE BRIDGE SCREWS! Note that the top screw is longer than the lower one.
16	Remove two screws holding the bearing block support to the printer frame.
17	Rotate the shuttle drive gears until the hammerbank shaft bolt is accessible. Using a 5/32 allen blade, loosen the bolt. Continue to unscrew the bolt keeping pressure applied so that the hammerbank moves to the left instead of the screw moving to the right.
18	Fully raise the platen lever and remove the hammerbank by gently sliding to the left. TAKE GREAT CARE NOT TO DAMAGE THE THIN RIBBON SHIELD. Once the hammerbank is removed, you should strongly consider doing a thorough cleaning and inspection of the printer. It will never be easier than it is right now!
19	Place the hammerbank on a clean flat surface with the ribbon shield up and the left end of the hammerbank on your left. Remember that this assembly is magnetic and WILL ATTRACT small bits of metallic debris which can disappear inside assembly and cause all sorts of frustrating problems. Be sure that your work area is clean.  Remove the ribbon shield (4 screws) and set it in a safe place where it will not get bent.
20	Align your test printout with the first print column centered over the first hammer as shown.
21	Identify and mark the hammer centered on the offending position(s). The only procedure that can be done without removing the hammer module is to clean the underside of the hammer. Debris under the hammer can act to keep the hammer from fully retracting causing it to drag as shown. Another condition called "Morning Sickness" can be caused by ink migrating to the hammer and working its way to the back. This "glues" the hammer to its post until things warm up and the hammer is released.
22	These problems can usually be corrected by GENTLY lifting the hammer with a small screwdriver just enough to slide a narrow strip of non buffed business card under the hammer, let the hammer drop and slowly pull the strip out. Repeat until the strips come out clean. Inspect the hammer and make sure that it is resting at the same level as its neighbors.
23	If the problem you are correcting is a non printing position, do the following. Inspect the hammer tip and insure that it has not broken off. If it has the module will need to be replaced. Check the resistance of the coil, for an MT-691 the coils should measure about 23 OHMS. An open coil also requires module replacement. When checking coil resistance, the 691 uses a unique coil contact pin out. Moving from left to right across each pin, the sequence is CC123BCC456B. Where C=COMMON, B=BLANK, and the numbers represent each coil position. You should measure between the common point and each numbered position and get about 23 ohms. The BLANKS are not connected.
24	To remove the hammer module, turn the hammerbank over so that the hammers are facing down. BE CAREFUL NOT TO DAMAGE HAMMER TIPS. Remove the two screws securing the module. Turn the hammerbank over and gently work the module up and out.
25	It is important to identify whether you are replacing an upper or lower module. They look identical but are NOT interchangeable! This shows the orientation of an upper module removed from the hammerbank. The part# for the uppers is 078371 and they are in even print positions. The part# for the lower modules is 078370 and they cover odd positions.
26	There is also a color code placed on the modules at the factory (nearly impossible to detect). They place an orange stripe on the plastic coil potting on the uppers, and a black stripe on the lowers. The difference between the modules is their magnetic polarity. If you get them mixed up it will affect the print on adjacent modules as well as the one you are working on.
27	When replacing a module you may need to add shims to adjust the relative module height. New replacement modules are generally "thinner" so that they can be adjusted "out" with shims. Make sure that all surfaces are clean before re-assembly.
28	Install new module with no shims and install but do not tighten the mounting screws. Place the alignment tool over the hammer tips insuring that the small holes are on the tips you are replacing. Note that there is a slight vertical offset between the tips on the upper and lower modules. The larger holes in the tool are to accommodate this offset.
29	While holding the tool on the tips, snug up the mounting screws.
30	Gently lay the alignment tool on the hammer tips bridging the module being installed. If there is any gap between the tips and the tool, shims will need to be added to compensate. Remove the module and add shims as needed to make the tips flush. repeat previous two steps. Re-install the ribbon shield. There is some play in the position so insure that the holes in the shield are centered over the hammer tips before tightening the screws.
31	Fully open the platen lever and re-install the hammerbank into the printer by gently sliding the right hand shaft into the bearing assembly. Be careful not to push the shaft bolt out of the bearing housing. Snug but do not tighten the bolt. If you do push the bolt out, slowly rotate the drive gears until the bolt becomes visible. place the bolt on the end of your magnetized allen wrench and try again.
32	Fully close the platen lever and secure the left side bearing support (2 screws). Snug but do not tighten. Install two screws into the left side of the bridge (right side of the printer). Remember the long screw goes on top. Snug but do not tighten. Set platen gap on left side by placing .014" feeler gauge between platen and ribbon shield. Adjust the gap by carefully changing the front to back position of the left hand bearing support. Tighten the two screws and the right hand shaft bolt (between the drive gears)
33	Check the platen gap on the right side with the .014" gauge. If it needs adjusting, slightly loosen the right hand bearing position screw.
34	Slightly loosen the lower (larger) flex arm mounting screw. Slowly rotate the drive gear until you can get straight access to the screw from the right side of the printer as shown.
35	Move the right side of the hammerbank front/back as needed to achieve .014" gap. Once set, tighten all screws and recheck gap to insure it is.014" across the entire hammerbank. This step can take a few tries to get it set correctly. Pry the hammerbank away as shown to increase the gap, tap it in to tighten. Manually spin the drive gears to insure there is no binding.
36	Re-install cable bracket and  re-connect hammerbank cables, The printer can run for short periods without the air plenum installed so you can leave it off for now to save time during testing. Re-install the ribbon deck, you can leave the screws out for now and let the clips hold it in place. connect J502 from the deck. Load ribbon and paper into the printer.
37	Before setting paper into the tractors, pull it through the printer by hand to check for smooth resistance. With the platen fully closed there should be slight, even resistance across the page. Apply power to the printer and run your test prints. If the print looks good pat yourself on the back, your almost done. Remove power and remove ribbon deck. Disconnect top hammerbank cables at P417 and P713 and re-install air plenum. Be careful here, a lost screw into the hammerbank means removal of the entire assembly! Connect air hose fitting. Re-connect P417 and P713. Re-install "L" bracket on left side of hammerbank cable bracket. Re-install hammerbank cable retainer clips. Re-install ribbon deck, put the screws in this time. Re-install hammerbank cable retaining bracket onto left side of ribbon deck.
38	If you want to run some extended tests (recommended) now is a good time before you re-install the cover. If you're ready to proceed, disconnect the control panel and set the printer cover in place. Reach in through the control panel opening and re-connect the cable. Drop the control panel into place right side first. make sure that the cable is not interfering with the shuttle drive gears and drop the left side of the control panel into the cover.
39	Congratulations! You have successfully completed the most difficult repair procedure on this printer. If you have any comments on this procedure or anything to contribute please contact us at feedback@chainc.com CHA can provide you with all supplies, spares, and documentation you may need to support the entire  line Tally of printer products. Introducing Mr. Hands (Mike Newkirk) a master technician and 9 year veteran at CHA.