Repairing HP82066B charger


Dear forum members,

after helping me so well with my request regarding charging times of the HP41 battery pack here, I now came across a different problem when I tried to actually charge the pack.

It seems that the charger is broken down. Indications:
- No reaction of the HP41 after inserting the "charged" pack,
- charger does not warm up during charging and
- I can not measure an output voltage when the charger is inserted into a power receptacle.

My question is:
Has anybody ever repaired an HP82066B charger? For example, my first problem is to get past the casing whithout damaging it too much (I can not find any screws).
Another idea would be to build a new charger on my own. As far as I know, all I have to do is provide the battery pack rectifier circuit with 3VA, 8VAC. Does anybody know where I can find a circuit diagram of the original charger that I could use to identify the required components?

Kind thanks for your help in advance!

Best regards,

Edited: 7 Jan 2011, 11:41 a.m.


Hi again. :-)

My question is:
Has anybody ever repaired an HP82066B charger?

For my part: no, and I'd say: don't even think about it. The AC adapter (no, that's not a charger) cannot be opened without breaking it. I cannot be reassembled in a way that ensures safe operation.

As far as I know, all I have to do is provide the battery pack rectifier circuit with 3VA, 8VAC.

The original 82066A provides something like 14 Volts AC while the battery pack is charged, and the charging current is something like 15 or maybe 20 mA. There's nothing even close to 3 Watts. Remember: the 82066A also had to power other devices like the printer and its charging ciruit for its much bigger batteries.

In the meantime you know the electronics inside the 82120A battery pack. You immediately realize that the whole thing essentially runs on the 12 V provided by the 78L12 voltage regulator. This IC performs best with an input voltage a few Volts higher, e.g. 15 V. Taking into accout the four rectifier diodes and the 100 Microfarad capacitor, this means that the whole thing should be able to run from a simple AC-adapter with 12 V output - AC as well as DC. If a cheap AC-adapter without voltage regulation is used, its nominal output voltage can be as low as 9 Volts since the effective (!) voltage is significantly higher than the nominal value: the whole charging circuit requires only a few mA, way below the adapter's max. rating like, say, 300 or 500 mA. That's also the reason why the 82066A is rated at 8 Volts: it provides this voltage *if* it is connected to a 375 mA load. With just 20 mA load the output voltage is much higher.

However, I'm not an electronics expert. So beware and correct me if I'm wrong. ;-)



Two remarks:

The original 82066A ...

Of course it's the 82066B. Sorry. ;-)
...this means that the whole thing should be able to run from a simple AC-adapter with 12 V output - AC as well as DC

With a usual AC adapter (i.e. constant (smoothed) direct current) a higher voltage like the mentioned 15 V is required. As Paul said, the minimum is 12 + 1,7 = 13,7 V, in this case plus the voltage drop caused by the rectifier diodes, leading to about 14,5 V altogether. 12 V AC (or pulsating DC) will result in about 1,4*(12 - 0,7) = 15,8 V which is fine.

Addendum: I just measured my two own 82066Bs. Connected to 230 VAC and with no load, both of them give the same output voltage of 11,5 Volts AC.


Edited: 8 Jan 2011, 1:41 p.m.


You say no output from the charger when it is connected to the battery pack, so I am guessing that you have checked and found that there is AC present at the end of the cord when it is not connected to the battery pack? If so looking in the schematic for the battery pack posted in the other thread one of my first suspects would be 100uF capacitor on the input side of the 78L12. Electrolytic capacitors are a common point of failure and can fail short, or partially short which may pull down the input voltage to the 78L12. According to one of my data books the drop out voltage for a 78L12 is 1.7V this means that the input voltage must be at least 1.7V greater than the output.


Thanks for all the very helpful answers!

What I meant was that I checked the AC output at the cord when the adapter was pluged into a 230VAC receptacle. I did not measure any voltage there so I'd say it is dead.

I wouldn't consider myself someone with a very good background in electronics. I do know a bit more than basics though (at least I can recognize the hot end of the soldering iron when I see one, so to speak ;)).

So summing up your answers it seems, that building a "self-made" AC adapter for the battery pack sounds quite managable to me. I checked the datasheet of the 78L12 here and the recommended operating conditions are an input voltage of 8-20V. So a simple AC source with a voltage of approx. 15VAC as you recommend should work indeed. Maybe a fuse to limit the output voltage of this adapter in order to protect the battery pack circuit would come in handy as well...

However, I do have some colleagues at work which are really knowledgable when it comes to electronics, so I will try and show them the battery pack circuit diagram. Maybe they have some additonal inputs as well.

I'll give you some feedback once (and if) we've found a workable solution. In the meantime thank you very much again for your replies and best regards!



Well, the 78L12 may handle input voltages as low as 8 Volts, but in this case it won't provide the desired 12 V output of course... ;-)

Building a home-brew AC adapter may be fun, but on the other hand working solutions are cheap and readily available. In your case your local Conrad shop may be worth a visit: take a look at this 15 V AC adapter for instance - for less than 10 Euros you get stabilized 15 V DC. The maximum load (800 mA) even is way higher than required. For a few Euros more you'll get an adjustable solution for voltages between 9 and 24 V, again stabilized and with enough power to handle even higher loads, making it an universal solution for various other applications.

Simply connect one of these with the 82066B's cable, either directly or with a self-made adapter that connects the AC adapter's plug with the HP-cable.



A few more alternatives:

Do you have any old PC power supplies lying around (if you have as many old PCs as I do, the price is right: zero!). They are wonderful sources of +/-12 VDC, +/-5 VDC, and maybe others (all nicely regulated, and in the case of +12 VDC, at currents up to 10s of amps). You usually have to short out the right pair of pins somewhere on the motherboard power supply plug to fool the power supply into thinking that its PC is turned ON. Check the web for pin outs (i.e. search "PC power supply pin connections" or something similar).

Are there stores in Europe similar to Goodwill in the US? These are places, usually connected with some sort of charity, that resell items that people bring in and give to the store just to get the stuff out of their house - and get a tax deduction. (Sort of like ebay - except you GIVE your stuff to them and they keep ALL the sales price.) The stores here have many of the little power supply "wall warts" that plug into the AC line and supply power for all of our modern gizmos (calculators, modems, telephones, etc.). They might sell for as much as $1 and they come in all varieties of AC or DC output voltage (read the label!) but are easily hacked by cutting off the power tip and replacing it with what you need or have.


My golly, that penny took a while to drop!

I was so pinned on an AC power supply that it wasn't before reading Dieters message with the Conrad-Links that it dawned on me. Of course the diode bridge in the rectifier doesn't care if you supply DC voltage to it, it's simply that one of the branches of the bridge isn't used at all... Oh dear!

Dieter, Dave thanks for your tips. I have indeed an old PC as well as an additional old PC power supply. However, I think I also have some old AC adapters lying around here that deliver the required 15VDC output. I thinks that's the solution I will go for. Nice and easy, all I have to do is replace the cord of the new AC adapter with that of the old HP-adapter. And above all, it's by far the safest way of dealing with the 230VAC connection that is required...

Many thanks for your continued support, I really appreciate your input!


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