by Drulli » Sun Dec 28, 2014 1:40 pm
Good point, Garth.
My assumption was that the printer and cassette drive use the same power regulator circuit as it is in the rechargeable battery pack for the hp-41.
I looked for the printer service manual (or cassette drive) but unfortunately I could not find it in the web. (I know it is existent, in a post in the hp-museum someone stated he has it. I will send him a request but there is no valid email available. If you or anybody else can give me a hint where to find it, would be great.)
What I found is a hand drawn schematic of the printer modules, incl. PSU (power supply unit).
If it is complete or correct I don't know. But it looks reasonable.
It shows that the normal hp charger AC voltage is bridge-rectified (without load it should be around 22Vdc) and smoothed by a 100uF Elko. After that a pull down resistor (8.2 Ohm, really so small, or kOhms?) provides around 6.5Vdc to the positive batteries contact(I measured it on my printer. There is quite a AC ripple on the dc voltage, I don't know how the NiMH handle that, I assume it is ok.)
In the data sheet of the NiMH subC batteries the max. charge voltage is 1.5V. Thus 1.625V (6.5V/4 batteries) is a bit too much, especially if it is provided permanently. For single NiMH I measure a charging voltage of 1.32V on a standard battery charger.
Furthermore there is no kind of current regulation / limitation, it is difficult to predict how high the charging current of the NiMH will be. Trickle charging current is 30..150, max overcharging is 300mA. Comparable NiCd can handle the same amount of current or even more. The old NiCd suck about 10mA.
Conclusion:
Most easiest way to solve that issue would be to modify the pull down resistor in a way that only 5, 5.5 V is provided to the battery contacts.
Most elegant way would be to put a little 78L05 power regulator circuit (like the one in the rechargeable calculator battery packs) into the PSU which provide the right voltage for the NiMH.
I will think about it and put it on my project list.
Ok, the post is far too long. But anyhow, may helpful in some kind.
Kind regards,
Ulrich
HP-41 CX, HP-41CV, Printer 82143A, Cassette Drive 82161A, PILbox, 82104A Card Reader, MAth I, Circuits I