Hey, Silent One!
I didn't remember a lamp. I no longer have any 1/21's but have a service manual for Honeywell Pentax Product No. 293 and Asahi Pentax Product No. 62931.
There are at least three schematics, one of them showing a 'LAMP', that does not have a 'Parts No.', and perhaps that's why it doesn't appear in the parts list(s) under a sequential numeric designator.
The H-P version with no lamp has 5 pots (R1-R5) wired as rheostats (one lead not used). All of them are Parts No. 82, 'semi-fixed resistor'. They appear to be used to set meter deflection (range) for B.C. (battery check), and high/low. There are two rheostats for high and low, which doesn't make a lot of sense to me. Maybe it's to adjust the bottom and top deflection...either the meter doesn't go to zero, or it's for the overlap between high & low where the two scales overlapped.
Most of the ones I bought had bad battery corrosion that got into the meter movement. The one that worked well fell off a stack of items being carried to the car, and that jammed the meter. I only recall seeing tubular fixed resistors in mine...never adjustable...might vary with vintage...or they went to selected resistors when built.
The manual is better than nothing, but it's pretty lame. On the A-P version that does show the lamp, all the resistors R1-R5 are numbered #282 instead of #82. The parts list(s) only goes as high as #115. I imagine 282 means a different part value than 82 in the similar schematic. The lamp is fed by an adjustable (dimming/current-limiting) resistor #382 from the 1.3 V battery. That one is R6 and there is notation 5W crammed together near notation R6 & #382. Only sense I can make of that is the practice of using a W for ohms where a Greek capital OMEGA (you know, the horseshoe shaped letter) can't be typed. That would make more sense if they used a lower-case w, because lower case Greek omega looks like a cursive w. This W practice still appears in engineering magazines, but engineers aren't known for their spelling or grammar.
I'd suggest an LED & current-limiting resistor, but 1.3-1.65 V available from a range of coin cells is too low for LED's. If the lithium battery you adapted is in the 3-3.9 V range, an LED and resistor is a possibility. But you won't know how much backlight is too much without experimentation. Maybe a yellow or amber or orange LED (easy on the eyes). The white ones might still be in the 4 V range like blue (I haven't check LED forward voltage evolution 'in years).
Anyway, with this lousy sales pitch I'm making for the manual, I was looking it up tonight in preparation to put it on eBay. But I surely owe Donald Qualls something in good will...IIRC, you might have given me a G-Claron 150/9 lens in a Zeiss Ikon shutter (I still have it...& maybe it came from Ole, the (Norwegian?) oil rig guy who is also a LF photographer). & you've certainly given me plenty of answers over the years (much more useful than just telling me where to go!).
Murray
