Looking for (idler?) gear wheel of HP-46 printer unit

[Duane Hess]

Background:
This all started over a year ago with a Canon F-20P (print only) which had a broken 17/68 gear. Visually, a previous owner must of stored it in a steam sauna, it has many issues. Upon goofing around it showed some functional ability. Borrowed the 17/68 gear from a F-10P (print & display). Saw the nub on the drum & 2 on the motor shaft. Figured they were timed speed of rotation of drum and motor. Exponentiation e(x) produced a response. The drum was obviously out of sync, but produced consistently produced the same inaccurate symbols. e(1) should produce the value of e. Upon examination noticed the 2 was off by ... some number of rows+partial row. The decimal point was off by the same number of rows + partial row; likewise with the rest of the value.

Thought about loosening the gear plate & rotating the drum until aligned. Then thought why bother? The unit has so many issues, potentially from tarnished connections figured after clean up, the printer may be OK or need realignment. Assumed, at this point, the 3 nubs allowed rotational speed calculations for timing regulation. Stealing the gear from one unit to the other is all the examination I had ever done on a EP-102 or EP-104 in my life to that point. Didn't have everything needed to even try cleaning it up, so placed it on the shelf.

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For various reasons started cleaning up a EP-102 from a HP-46. It is reassembled minus the 17/68 gear & mounting plate and the ribbon lift shaft.

After your note about timing started thinking while driving around & at afternoon coffee.

A brass collar on the motor shaft has 2 rotation sensor nubs. Kept wondering why 2? And why 120 degrees apart? Better speed control? Were their purpose speed control or something else? If 2 for speed control, why not 180 degrees apart? If the motor PCB had built-in speed control imply the collar/nubs perform another purpose?

What do the nubs mean? Cleaning the printer made me aware of the ridge on the motor shaft, which physically powers the hammer striking mechanism. Pondered if the first nub indicates start of the print hammer activation. And the 2nd number indicates the print hammer has struck? i.e. hammer firing duration timing?

It just bugged me. The only obvious indication of "where is the drum & when should the hammers strike?" seems to be the drum & motor sensors. Without other feedback how would printer timing circuits know the 1's row is coming under the hammers, for example? Does some geometry between the two shafts indicate "starting point?" If not geometry, do the drum/motor nubs provide a timing base? i.e. using the sensed drum rotation rate and "hammer time" of the motor shaft for hammer fire calibration? That is why I ensured orienting the drum to its exact rotation position on the shaft during reassembly.

Further pondering made me feel hammer timing is not resultant of time delay calculations. Replacing the drum in its original position should assure correct operation upon full printer reassembly. Also considered whether timing requires "must be close" geometric orientation; i.e. the each shaft nub must be within a small angular boundary to its sensor. Still unsure; realized the drum nub must pass the sensor twice to time one full rotation. In terms of early 70's electronic circuit response speeds such a large rotational delay should lend itself to easily calculating hammer firing delays for the 13 rows on the drum. So, why a need to be close? Just simply get up to speed and calculate from there. Meaning reassemble and all is OK.

Purchased a Canon F-10P while in Jr. High school (70's). Never disassembled it. It has a printer and display with an ON/OFF print switch like the 46. Being adolescent, I needed to try things. As I recall turning the printer switch ON consistently required (more/less) 2 drum rotations before being able to print. It has minimal print buffering but trial and error seemed to produce that result. This might imply a delay timing synchronization method; i.e. not geometric.

Being naturally "tense" came home after coffee today and looked at the printer; which does not have the idler gear & mounting plate installed yet.

The ridge on the motor shaft strikes a forward protrusion of hammer actuation "picks" on the back side of the printer, which in turn shove into the lever end of the print hammers. What determines when to energize the selenoids that push these picks into the path of the motor shaft ridge?

Realized I never bothered to count the number of teeth on the drum nor motor shaft gears. There are 39 and 12, respectively. Feared the positional orientation method may be a hair-puller! 39 teeth running a 17 tooth gear which drives 68 teeth on a 12 tooth gear. The integral rotations for repeatable boundary alignment of the drum/motor sounds like "OH HECK!" Lacking other ideas decided to examine what happens upon one drum rotation:

- how does one drum rotation compare to the 17 tooth gear? 39 - 17 - 17 leaves 5. So 2 rotations + a 5 "tooth rotation" arc.
- 2 rotations + 5 teeth of the 17 gear causes how many tooth displacements of the 68 gear? 2 rotations = 136 tooth displacements. 5 teeth on the 17 gear produce:
2 * 68 / 17 = 20 teeth
so 136 + 20 = 156 tooth displacements for both the 68 gear and driven 12 toothed motor shaft.
- 156 displacements on the 12 tooth motor shaft is:
156/12 = 13 motor rotations

WHEW! Feared it would be way more difficult.

For whatever reason, noticed there is 6 equally spaced resting positions of the motor. i.e. turning the motor by hand you can feel six "chunks" (detents?) during its movement. Let go of the shaft and it turns to the closest of these six positions. Happened to notice two of the detents produced motor shaft nub alignment its positional sensor. Nub (on a detent), detent, nub (on a detent), detent, detent, detent. This explains the 120 degree nub separation. The mid-nub detent alignment was centered between nubs. Have no idea why these 6 locations occur. Something in the motor? The motor shaft bearings?

Checked the motor shaft ridge location relative to sensor/detent alignment. At mid-nub detent the ridge appears to begin striking the forward surface of the hammer actuation picks. The 2nd nub detent reflects when the ridge just clears the pick striking surface; i.e. the hammer has fired. .... Decided the 1st detent (leading nub) has two possibile purposes. To indicate "energize needed selenoids." Or is an inhibit to signify insufficient time left to energize a selenoid and provide correct operation. The latter should mean energizing occurs after the 2nd nub detent and before the 1st nub detent returns.

Learning towards geometry orientated timing and not time-delay synchronization, looked at the drum. The drum nub was (nearly) centered between the 1's and 2's row. Rotating the drum, noticed the machined surface of the hammers was very close to 1/3rd taller than the tallest symbol. Interestingly, rotating the motor one detent moves the drum 1/2 tooth of the drum. And the drum nub width (rotationally) appears almost exactly 1 tooth.

Turns out 6 detents, being one motor rotation moves the drum one row (3 drum teeth). DUH! finally hit me. 13 motor rotations produces 1 drum rotation and there are 13 rows/drum.
3 teeth * 13 rows = 39, the number of teeth of the drum gear.

Having a 2nd assembled 46, took off the calc top cover to check drum/nub alignment. Left the rest of the calc fully assembled. Moved metal tear bar up out of the way. Was able to flex the right black plastic shield enough to see the drum gear. Rotated the motor with needle nose pliers inserted into a hole in the left plastic shield. This verified the nub and 1's/2's row alignment, albeit a tad under 1 tooth lower on the drum than the printer I'm refurbishing. Unfortunately did not think of checking the motor shaft collar alignment before replacing the top over & storing the calc.

Interesting potential that drum nub location might tolerate being + OR - one tooth position off relative to the drum. But clearly not either +/- one tooth; the machined hammer surface is not tall enough.

Was in a quandary at this point. Cleared the head then wondered how was this device assembled and calibrated. The EP-101 was introduced in '68 (for Olympics). The EP-102 was introduced afterwards (year?) then replaced by the minimal/modestly refined EP-104. From memory of my Jr/Sr High school youth the EP-104 was used into mid-70's. After that calcs usually had an individual spinning plastic wheel per column. Yes, I was such a loser as to regularly scope out office equipment stores.

In hindsight, during the '68-'75 era, robotic assembly of this printer was likely minimal if at all. That will be my premise. Possibly some components were resultant of mechanical assembly (a cog & gear machine), but still few parts. Should the hammer firing calibration be due to geometric synchronization, undoubtedly assembly alignment was manual. Further, to have reasonable assembly speed and "getting it right the first time" this alignment would require "eyeball it" precision.

Back into a quandary again. Ashamedly, after some TV, "DOH!" hit. I have an EP-102 printer with an intact gear for a different calc in a box:
- the drum nub aligned with the 1's/2's row and was also nearly 1 tooth lower relative to the drum of my refurb printer
- both motor shaft nub detents were exactly aligned with the sensor
- makes sense, detent locations are very precise and repeatable, hence allowing easy alignment
- width of motor sensor pickup and rotational width of nubs look identical
- with drum nub aligned exactly to sensor, 1st drum nub aligned to its sensor
- width of drum sensor pickup and rotational width of nub looks identical

The refurb printer motor shaft nubs were off just under a nub width. Both were offset the same amount and rotational direction.

One wonders if any amount of drum or motor shaft nub overlap with their pickup sensors or are abut to it, is close enough for alignment. Closeness of motor shaft nub positioning should be easy. One detent position would be way off. Drum nub/sensor pickup width should allow 1 tooth mispositioning. Consistent with variances of the 3 printers examined and machined surface height of the print hammers.

Haven't tried this yet. The 17/68 gear mounting plate is held on by 3 screws. Top right, bottom right and bottom left corners of the plate. Looking from the plate side, through the plate hole revealing the motor shaft makes me feel an easy 17/68 gear alignment scenario is probable. Remove the top right and bottom left screws, loosen the lower right screw. It looks like the plate might easily rotate away from the gears without displacing either the drum or motor. Like-wise rotating back for assembly without displacement. Possibly barely wiggling of drum, mounting plate or motor shaft might allow all parts to fall together correctly.

Motor detents should hold the shaft alignment. The toothed lever which flops forward against the metal portion of the drum gear to actuate a paper feed might be useful also. Aligning the drum with its sensor and pulling that lever forward should lock the drum in place. Then rotating the mounting plate back should be easy. This allows wiggling of the motor, if needed. Would take some force, but minor vertical movement of the advance lever should allow wiggling of the drum without skipping a tooth. Seems the same applies if the mount plate is free of all screws and is simply placed into position. That's what I'll guess anyways.

Need to resolve some property maintenance in another town, likely taking most of a week. Will have my laptop but will not be messing with the F-20P, returning gear to F-10P or the refurbed 46 printer until I'm back.

You probably don't care about my personal items. The point of mentioning it is I need way too much therapy to wait a week before trying out today's messing around and posting about it.

Evidently the coffee was not strong enough!