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I recently received an HP-21 which does not work. After opening it I saw a black solder spot on the component board. The same spot is black on the other side of the board too.

Part of component board, top left: o--R--o o--R--o

C C C x--D--o o--R--o

x marks the black solder spot. The resistor next to the diode has some brown stuff on it. What happened and why? Is it sufficient to replace the diode, by what type, or is it likely that more components are damaged?

It is very difficult to give a safe advice, but I would like to suggest some things that may give you some orientation. Please, be careful. NO WARRANTIES.

You may check if the components belongs to the AC adapter/recharging circuit. To check for this, you should find a printed circuit trace (gold plated on those days...), that starts at one of the AC adapter connector pins. Then the trace will go to a diode and a big resistor, and after that it will connect to one of the batteries contacts.

The diode is a 1N4001 type, Silicon, 1 A max current, 50 V max inverse voltage. It is a standard type, you can replace with ease.

The resistor is (almost sure) 82 ohm, 1 Watt type, carbon, 5% tolerance. Its color bands should be Gray, Red, Black, Gold. If possible, read the color rings on the old resistor and let me know. It is a easy part to replace, too.

If the components belong to other parts of the calculator, then it will be harder to know what to do, but it seems that the black spot is consistent with the part of the calculator where more heat and power are present and disipated. It also is one of the few zones of such calculator that you may understand without a circuit schematic and maintenance manuals. So let us hope the problem is there.

The black spot may be produced by an excessive heat disipation, probably caused by an overcurrent. That may happen if excessive voltage was applied at the charger pins; or, more likely, if a short circuit happened at the batteries. In this later case, if the diode is now open, it is just possible that the rest of the calculator was not damaged. Again, let us hope so.

If the diode failed and is now shortcircuited, then AC voltage may have reached internal parts of the calculator and the outcome is not favorable. But it deserves a test anyway.

You may test the calculator WITHOUT THE AC ADAPTER CONNECTED, using two standard AA alkaline batteries. BE SURE TO CONNECT THEM WITH THE SAME POLARITY THAN THE RECHARGEABLE BATTERIES.

If it works, then I suggest that you can replace both components. IT IS CRITICAL TO OBSERVE THE DIODE POLARITY !! Rebuild the battery pack with new NiCd, AA cells; and verify the AC adapter with a voltmeter before using it again. It should deliver some 8 V AC with no load.

Clean the batteries contacts to avoid any corrosion or dirt, it is very important to have a good connection to the rechargeable batteries when using the AC adapter.

AND NEVER USE THE AC ADAPTER WITHOUT GOOD RECHARGEABLE BATTERIES INSIDE.

I apologize for any error, I am telling this from memory, and a 20 years old "memory register"!