First, I just looked at the schematic I sketched of my HP41 rechargeable pack PCB and I had overlooked the diode in series with the 365 ohm resistor, so the current I calculate is more like 14 mA. That might actually be high for the original NiCads, weren't they 110 mA-H? Plus, when good NiCads are in the circuit, dropping 4.8V, the zener is not conducting and the voltage on the 365 ohm resistor is higher and so the current is about 18 mA. Even when the cells have risen to 1.4V each, the current is about 16 mA.
Yes, I appreciate Norm's advice. I'm sorry to hear that Motorola doesn't make zeners anymore and that nobody has taken up the slack (maybe somebody like Oliver Whats-his-name of Germanium Power Devices will come forward.) Motorola's zener databook is what I've always used as a reference and I guess I didn't know how good I had it. If the TL431 came in a TO-220 package I'd feel a lot better! You could use it as is (with two resistors to set the voltage) for at least a 1W zener or heat sink it and use it where you would put a 7800 series (series) regulator. I can't think of any reason why you couldn't turn it upside down and use it instead of a 7900 series (series regulator)(a message from your department of redundancy department). I like shunt regulators because they get warm. Please don't tell the EPA I said that.
My rechargable pack was in pretty good shape and I can read the markings on the parts. The zener is marked "M[Motorola logo]K/020/057/503". The two diodes on the negative side of the bridge are marked "MK/020/965/426". The two diodes on the positive side of the bridge, and the diode in series with the 365 ohm resistor, are 1N4002. The three terminal regulator is marked "M26/-0275". I wonder why they used two different kinds of diodes in the bridge? Maybe that will become evident if we can find out what the two private labeled ones are. I looked in the service manual for the HP3421A and in it, HP consistently used 1901 as a prefix for rectifiers and switching diodes, and 1902 for zeners. It's probably safe to assume that the third line on these diodes is a datecode and they only used the unique digits from the part number, making the 6.2V zener a "1902-0057" and the the diodes in the bridge "1902-0965". Looked at separately, these two zener diodes are connected back-to-back across the transformer secondary where they can clip high voltage transients of either polarity that come through the transformer from the line. Normally, the highest voltage across each diode is twice the peak voltage (capacitor charges to one peak, then transformer lead swings to the opposite peak) so any zener higher than 40 or 50 volts wouldn't normally conduct.
I'd like to read more about the idea that pulsating current is better for charging batteries. I have been studying a fairly old (1988) Sanyo NiCad databook lately (when I was having trouble soldering to the straps on my drill batteries) and it has no mention of the subject. There is a discussion of full wave vs. half wave rectification but the purpose of the discussion seems to be minimizing the weight of the transformer. BTW, this databook has a section on battery packs and they show photos of some examples, one of them is the HP Classic battery pack, 82001(might be A, the opening in the plastic for the terminals is a little shorter than my B).
Also, I looked up a PPC Journal article I had run into and bookmarked: V7N2P25 (Feb.-Mar. 1980), how to modify a Classic recharger (82002A) to use it as an AC adapter for a 41C. It includes a schematic of the Classic recharger as it comes and it shows both supplies - the constant voltage for the calculator and the constant current for the battery - originating from the same rectified and filtered source. BTW, the constant current source compares the voltage drop across a 13 ohm resistor to a transistor Vbe. 0.6/13=46 mA, 0.7/13=54 mA.
The modification for the 41C consists of changing the zener on the base of an emitter follower transistor in the constant voltage supply to a 1N755A (7.5V 1/2W), adding a protective zener (1N754A, 6.8V 1/2W) after the emitter follower, adding a 1N4001 rectifier diode in series with the load, and then building a plug to mate with the little gold contacts that were accessible on the side of the early 41's.
The 6.8V protective zener looks to me like cutting things pretty close. If the 7.5V zener is high and the 6.8V zener is low, it is going to be drawing current and there isn't anything to limit it. The text says the 1N4001 is to prevent discharging the battery on the 41 but I think they also needed another junction voltage drop since they started with 7.5V.
I tried to find an article about the HP41 rechargable pack that might have schematics and part numbers but so far, no luck. However, I did find this: in V13N9P8 there is an article about batteries in general. First there is a lot of discussion from someone at HP about the batteries for the earlier calculators (before the 41). Then starting on page 11 there is a shorter article from a battery shop in California about the special problems of the 41, which have been discussed here before - people who got the card reader found their batteries had a very short life. The introduction to the set of articles states that the 41 article will help people cope until the rechargeable pack is available from HP, so it must have already been announced at that time (Dec. 1986).
The 41 article says the 41C in ' "computing" state ' draws about 6 mA and the card reader draws 250 mA. The article also has general instructions for building an AC adapter for the 41. It does not reference V7N2. It does suggest using a "6.3 or 6.8 V" zener to protect the calculator. I wonder if 6.3 was a typo or if the writer was thinking of the common vacuum tube heater voltage?
I think the function of the 6.2V zener in the 41C rechargeable pack is the same as we have been discussing for the 25, except in the 41, 6.2V is a better fit since the battery has twice as many cells as the 25. The 12V pre-regulator and the low charging current makes it possible to use a small zener.
I have "extracted" the two articles as GIF files and will email them on request. The whole bunch is about 500KB. The Classic recharger schematic alone is about 10KB. I guess I could scan my 41 rechargeable schematic too, it's just hen scratchings on a post-it note.