Tandy DMP-203 dot matrix printer

 I came across a very old printer at the thrift store, and got it for a good enough price that it could serve as a distraction so that I wouldn't buy anything else for a few days.

 

It's a Tandy DMP-203 dot matrix printer.  It was pretty dirty, but seemed somewhat well boxed (not original).  It had an attached power cord.  Initial power-on testing at the store showed it to be quite dead.  The ink cartridge was still there.

I was interested in reviving this one.  Initial removal of the cartridge was easy enough.  

My first concern was seeing why it wasn't powering on.  To determine that, I'd have to get inside.

After a brief inspection, I found there was one obvious external screw that could be removed at the top, rear.   

 After that's been removed, there are two visible "tabs" that are press-fit / snap-on latches, the tabs attached to the bottom frame.  If you press those gently inward, and/or pry the upper case away, each gently lifts up.  Here, I'm using a flathead screwdriver to press.

Then, I found there were three more case screws.  All three are just behind the front "vent" lines on the upper frame.

As it turns out, most of the case screws are the same.

With the screws removed, I pried the front and sides up.  Thankfully, the case did not have additional plastic press-fit clips around the sides or front.  But it took additional force to pull up.  I'll show why later.

 This is the kind of edge connection that's going on inside the front and sides.

After doing more prying, the top part popped off with a bit of a snap, and this is why.  There's one more press-fit clip underneath the left, middle of the top part.  It's about 4.5" away from the left side.  In the future, it's probably better to lift up the edge, stick a screwdriver in the gap, and push that clip a bit to release it.

The top was off, but not fully disconnected yet, because there's a little control panel board screwed to the top.  Remove that and set the screws aside.  Note its position.

Interestingly, the top panel has slots that would allow for a larger control board.  There's room for three more buttons, and maybe a few LED indicators.  The board, itself, has room for another button, and appears to have unused pads.

This is what it looked like when I had the top flipped over, and the control panel was not yet disconnected.

With the control panel removed, you can see there's room for another button and indicator light.  To avoid losing them, I put the screws back into where they originated in the top panel.

Now that the top panel was removed, I could see the power supply board (beige) and a part of the main PCB (green).  Both were pretty dirty.  The power board has four screws holding it down.  Two were blackened somehow, perhaps by corrosion or stray ink.

I removed the four screws holding the power board down.  

Then, to remove the power board from the case, it turns out that you have to pry upwards where the on/off switch is.  There's a cutout in the plastic of the case that otherwise would appear to be cracked plastic.  But, it's there to let the switch clear the case as it's inserted.

Connected to the mains is a fuse.  I removed it and tested it, and it's still ok.  But if your board isn't powering on, this could be the cause (and could indicate a bigger problem further inside).

This is what it looked like with the power board loosened.

The main power cord is held by small clips on the right side of the printer.  

The cord goes under the platen roller knob.  That knob, as it turns out, is press-fit onto its rod.  It has a cut-out that goes around a tough pin that then goes through the rod.  You can just wiggle the knob firmly to pull it away to the right.

You can then gently loosen the main power line around its case clips, and snake it around the rod and pin, and then loosen the piece on the back to provides strain relief.

With the power cord released from its clips, I could now see the underside of the power board.  It was really messy, but it seemed like there weren't any clear burn marks or bad solder joints.

The power board was connected to the PCB with a 5-wire ribbon and a shrouded connector.  (The ribbon also isn't color coded in any way.  You kind of have to trust that the bend of the ribbon will help you remember how to put it back together again.)  The connector shroud was clipped pretty firmly in place.  This is old school stuff, and seems designed to prevent removal.  That makes sense, given it's a rattling device.

After some prying with a very small, flathead screwdriver, the shroud popped off like this.  The trick is to get the screw under one corner of the shroud and pop it upward.  This really only can be done if you have room to get to it, which is why it was necessary to pull the power board away from the bottom frame.

The ribbon wires then are held down with "biting" connectors.  You can't just pull them up at this point, and since there are multiple of them, it'd strain things to try to pull them out individually.

To disconnect these wires, I simply used five pointed toothpicks.  I pushed those in alongside the original wires, effectively releasing the biting metal pieces.  You can kind of feel when you've pushed the toothpick in enough to release each wire.

Once the toothpicks were in place, I could remove the ribbon's wires.

Not shown here, but... now you have toothpicks that are stuck.  What do you do?  Use a small, flathead screwdriver and push each socket tooth.  The toothpick will come out, and the socket tooth won't bite into the screwdriver, so it can come out, too.

Now for a bit of power board inspection.  Just as they say on the internet, it looked bad and actually smelled a bit like rotten fish.

Here you can see one of the electrolytic capacitors leaked badly...

and another one did, too.

There are a few other electrolytics that look ok, but I'll replace them anyway just because of age.

I took plenty of pictures to try to remember the orientation.  But it turns out that the board has good markings on it, showing capacitor polarity.

Off to de-solder.  Here are the caps that were removed.

And here are the printed specs and rough measurements.

C11 (smallest blueish)

• 4.7 uF
• 6.3V
• 5.16mm diameter
• 11.63mm height
• 5mm lead spacing

C9 (small blackish)

• 330 uF
• 10V ELNA (M) (20%)
• 8.18mm diameter
• 12.36mm height
• 5mm lead spacing

C2 (Nichicon, blue)

• 220 uF 
• 200V
• 18.4mm diameter
• 40mm height
• 7.4mm lead spacing (but also can take 10mm!)

C13 (Nichicon, brown/black)

• 4700 uF (4.7F) 
• 25V
• 18.3mm diameter
• 34.5mm height
• 7.5mm lead spacing 

Armed with that info, I found replacements at both Mouser and Digikey.

One nice surprise on the board is that the board design for C2 could accept either the original through-hole leads at 7.5mm lead spacing, or a snap-in style capacitor with 10mm lead spacing (and 1.5mm-wide, stubby leads).  The original PCB had solder flatly laid over the larger holes seen here, since they were disused.

To remove the bad capacitors, I had to cut through a bit of the leaked gunk, and then de-solder.  After de-soldering, I did some overall board cleaning.  Here are the before and after shots.

At this point, I researched and ordered replacement caps.  In the case of C2, I opted to get a similar replacement with 7.5mm lead spacing, only because that's what Digikey was serving up.  I'd looked at Mouser earlier, and found that they could have provided snap-in form-factor capacitors only, and at a cheaper price.  I went with Digikey in the end, just so I could have all the parts on a single list there.

Onward with diagnosing the (seemingly) frozen print head.

I tried moving the platen roller, and it rolled as expected, if begrudgingly.  However, I couldn't move the printer head side to side (along the X axis) without a lot of force.

I figured the next best thing to do would be to remove the print carriage assembly from the PCB to take a closer look at it.  I thought maybe by de-tensioning the belt, and removing the X axis motor, I could rule out motor issues and avoid pressure on the motor pinion gear.

The print carriage assembly was held in place by four plastic clips in the lower frame.  I thought that a tensioning pin also was getting in the way, but that's actually part of the print carriage assembly and could have been removed later.

To release these pins, I used a flathead screwdriver, and gently pushed each one away from the center, while lifting the carriage from a corner point initially, or from the center subsequently.

After that has come up, the carriage assembly is still entangled by the ribbon cable (be very careful with that), a four-wire ribbon cable, and the two 6-pin motor wire harnesses.

There is a DIP switch panel that I should have removed first.  With the print carriage assembly in its original place, you can slide the panel left, up, and out.  This untraps the print head ribbon cable and makes subsequent moves strain that ribbon less.  This is the view from below.

 The four-wire pin assembly is another of those that has a hard-to-remove shroud, and has socket points that "bite" into each wire.  I'm not wholly sure what those do.  Note that for this one, there is a color coding on one side, so mind that as you reassemble later on.

Here again I used the toothpick trick.

I also marked the header to remember where the gray wire would go.

The print head ribbon cable remained firmly in place.  I have not found a way to remove that, so you have to work around it and be very careful to avoid breaking or pinching it.

The six-wire harness to the control panel can also be removed from the PCB using the toothpick trick.  The bends in the cable help remember how to put it back in later on.  Unclip shroud first.

Exposing teeth. 

 Toothpicks and removal.

 

The 6-wire motor harnesses each have their own header, and those can be pried up gently with a small, flathead screwdriver.  This is just a picture for color orientation purposes.

 The red header came off first (Y axis, or roller axis).  I just used a normal screwdriver to twist-pry it up.

Then, the white one (X axis, or print head axis).

It's nice that the headers and sockets are matched by color so I can remember which goes where.

This is a close-up of the print head ribbon connection on the PCB end.  I don't see an obvious way to detach it.  More modern ribbon connectors have a disconnect mechanism.  At some point above, I gained access to, and removed the screws holding the PCB to the lower frame.  This allowed me a lot more freedom of movement, and helped avoid messing up the print head ribbon cable.  At least, I think it did.  I won't know until I reassemble and test.

I wonder now if I the ribbon is held in by the same "biting" tooth mechanism as the others.  Those work, because they're used tinned, stranded wire that metal tabs can hold onto.  Once presented with something more solid, like a screwdriver tip, the teeth can't bite.

Ribbon cable ends would be soft copper.  If I were to be able to slide a thin, metal edge into there (perhaps a trimmed aluminum can?) maybe the "teeth" would release from the ribbon.

Here's where things stood at this point.  The PCB and carriage assembly are out of the lower frame.  In this picture, they're upside-down.  The two are held together by the print head ribbon cable.  The print head still doesn't move cleanly along the X axis.

In order to remove the PCB, I had to unscrew the screws that were holding it to the lower frame.  I also took off the screws that were holding the parallel port (Centronics?) clips, and associated outer flange.  I went further than needed, removing the inner mounting brackets.  All those parts are seen at the top, right.

The next part of the disassembly was to take off the X axis stepper motor.  It's seen here, mounted to a metal plate.  The plate slides and tension is provided by the spring.  In that way, on the opposite side, the motor gear talks to a drive gear that pulls the belt that moves the print head.

The motor is labeled 42SIN-15D6YA, but if you search for that on the internet, you'll likely end up looking at NEMA 17 stepper motors.  This is not that.  This is a pancake, unipolar, 6-wire stepper.  I'm not sure what voltage it runs at and I don't have any other specs on it.

I removed the spring, and the two screws holding the motor in place.  I also removed a tensioning wire on the side of the carriage.  With that, the motor came out, and the belt was left loose.

At this point, with the motor completely out, I should have been able to slide the print head freely from side to side, barring there being any "parking" mechanisms.  But the print head was still very tough to move.  I could move it a bit with quite a bit of force.

I tried adding some sewing machine oil to the X axis rod, and that didn't really help.

So, I started thinking that the X axis could have a sleeve bearing that was frozen, so I started looking at ways to remove the rod entirely from the frame.

This is the blue lever mechanism on the right side of the rod.  Looking at it closely, it didn't provide any clues as to how I could remove it.

Looking at it from the inside didn't provide clues, either.  I tried unclipping the blue lever away from the frame so it could rotate beyond its normal detente range.  I hoped that would get it to a point where it would be released, but it didn't come free, and it felt like I was putting too much strain on it.

So, I started to remove more from the back of the assembly.  This is the rearmost assembly that holds the sprockets for tractor-feed paper.  I removed it by first taking off the C clip and then sliding off the associated gear.  With those off, I could slide the rod to the right, allowing the left end of the rod to come off.  Then, I could pull the rod out of the frame, inward from the right end.

The next task was careful and organized removal of the gears.  Some are held in by screws.

 

With the first set of screws removed, I could slide the beige handle outward from the frame, but could not remove it.

 

Once the handle has been moved away from the frame, it can rotate and dangle.  On reassembly, remember to rotate it back upward.

I tried to remove the pin that is used for the roller knob, but that wasn't a good move.  (I was able to move it almost halfway out, and then got stuck, so I had to cleverly use a C clamp to gently get it back to where it came from without bending it.)

The next nylon gear was accessible, now that the beige lever was out of the way.  This one has a beveled top, and behind it there is a washer and spring, pushing the gear away from the frame.  The non-beveled part of this gear (the larger diameter portion) can engage its teeth with the sprocket axle's gear.  When the beige gear's "c" arm rotates over the beveled gear, it pushes the gear inward, compressing the spring, and causing the larger diameter gear to disengage from the sprocket axle gear.  Here, I remove the beveled gear, washer, and spring.

Going back to the roller axle, right side, there's a C clip holding a gear in place.  I removed that.

And now I could slide that gear away from the frame.

That revealed another of those through-the-rod pins, like the one seen under the roller knob.  The thing to note here is that the white gear has slots that line up with the pin, so it's important to align that when reassembling.

This is a close-up of one of those through-the-rod pins (photo through loupe).  It looks like maybe some kind of spring metal, crimped, and then inserted.   If I were to take it out, I might not get it back in again.

In general, I think the through-rod pin mechanism was a cheap way to avoid using set screws.  In some ways, given the environment, it may have also been more reliable than set screws.

I still had not freed up the roller from the frame.

It took me a while to figure this part out.  It turns out that there are white bushings that have levers on each end of the roller.  But, if you look closely, there's a tiny kind of stiff glue holding each lever bushing in place.

The blue stuff is a glue, and its purpose it to keep the lever from rotating around the axle.  I used an X-acto knife to cut off the blue and scrape it a bit.

 

But even with that glue removed, the lever would rotate.  It turns out that the lever also has a nib that sets into a hidden hole in the frame. You have to pry the lever tip away from the frame a little in order to release the nib, and then then lever can rotate (assuming you got the glue out).

On the left side of the roller axis, the lever bushing holds the axle in place.  Behind it, there's a washer and spring under tension, so those can pop out.

I got the levers away and rotated them to a vertical position.  As you do this, you can see that the inner part of the levered bushing has flats on the edges, allowing it to slide upward once in position.

Even with the levers set up properly, the roller wasn't completely free, because of the position of the print head.  I forced the print head all the way to the right.

 

Finally, I could get the roller and lever bushings off.  (And, pop! the spring and washer came loose on the left side.)

 

Here's the right side of the roller assembly.  If you look closely, you can see the nib on the lever.

Ah, finally with the roller assembly out, I could move the print head along the X axis, and it moved pretty smoothly.  But, why?  If it was moving cleanly, that meant that the problem was not some kind of frozen bushing, or dirtiness of the X axis rod, though it certainly could use cleaning and oiling.

That's when I finally saw this:

I noticed that the print head was prone to tipping forward and backward as I moved it left and right along the rod.  It should have been more rigid.

As it turns out, there's little L-shaped clip on the front of the print head carriage, and it was riding on top of  the plastic rail at the front of the carriage frame.

 

That meant that the print head was tipped inward and binding against the roller.

There was no adjustment mechanism for this, so I just pushed that part of the frame inward a bit, and that allowed the L to pop over into its proper position.

Once that was done, I could slide the print head left and right with light finger pressure.

 

So as it turns out, much of the disassembly done earlier wasn't really necessary, and put the print head ribbon at risk.  It did allow me access to a lot more of the machinery, which I could clean.

After that, I could put it all back together again.

A few points on reassembly:

1. Be super careful to avoid damaging the print head ribbon.  That might be impossible to replace. 
2. Remember to align the roller's right side gear with the pin.
3. It seems like it shouldn't really be necessary to glue the levers in place, but maybe I'll do it.
4. The beige lever on the right side has to be rotated upward so that it engages the beveled gear correctly.
5. The four-wire ribbon can get trapped in the wrong place under the carriage, preventing proper carriage landing.
6. Be sure that the print head ribbon is pulled through the DIP switch panel hole before dropping the carriage in place. 
7. Before setting the carriage down, there's a lever at its front.  That lever has to be held up before the carriage goes down.  The lever is part of the X axis homing switch assembly.  If the lever isn't moving properly, the print head probably will move left as it tries to home, and will crash into the rigid lever, and might break it.  So, make sure it's properly oriented before dropping the carriage back in place, and test manually afterward to make sure you can hear it clicking the tactile switch underneath.
8. After the carriage is in place, you should be able to put the DIP switch panel back on.  Double check to make sure the print head ribbon is positioned properly before putting the panel on. 
9. Probably, it's best to keep the motors disconnected (i.e., don't reconnect the red and white connectors to their headers) as much as possible.  You don't really want motor movement to provide some kind of flyback current to the board. 

With this all reassembled, but with motors still disconnected, there are few other interesting things to note.

1. The X axis does move properly it takes more pressure than just a finger push.  That makes sense, because the belt is now under tension, and you're pushing against the gearing and the stepper motor.  But, it moves without an extraordinary amount of force.
2. As the print head moves, you'll see that a gearing mechanism rotates the gears and pin that would drive belt movement.  I don't quite understand how that works.  As well, it moves them bidirectionally, suggesting it'll keep hitting the same section of the ribbon over and over again, unless something else controls the ribbon drive gear.

Now that all that is done, I'm awaiting the replacement caps so I can get the power board working.  I'm hoping that will provide insight into the normal motor voltages.  I could try to drive them independently using some kind of microcontroller (Arduion, Polulu motor boards, e.g.) but can't really safely do that without knowing the typical voltage.