Re: How to change battery of Tek 2446 Scope


Greg Muir
 

Although not really applicable in this instance, changing batteries in equipment where you must retain the saved data (cal factors, etc.) can be a bit tricky. There are a few methods and precautions that need to be followed lest you end up with the undesired results.

You can go in and replace the battery while the unit is under power but I prefer not to do that mainly because of any slips that might occur with the soldering iron causing damage to surrounding circuitry. Of great concern is going in with a soldering iron that is grounded to the third wire AC. Since the majority of equipment has its power supplies reference to the frame of the instrument and, hence the AC third wire ground, you will simply short out the supply voltage providing power to the memory that you are trying to save. Even if you float the soldering iron by use of a third wire ground “cheater” (those adapters that go between the AC plug and the outlet to which it is connected) depending upon the type of iron that you use (mostly the step-down transformer types) there may be enough stray capacitance within the iron circuitry to affect the battery supply circuit via transients and such when the tip makes contact. ESD may also have an effect since you have now removed the iron from AC ground.

My method is to first disconnect the instrument from AC power then connect a bench supply with floating (not reference to 3rd wire AC ground) output adjusted to the battery voltage using around a 1k ohm resistor in series with the positive lead of the power supply. You could also use an external battery supply provided it is close to the range of the battery voltage that you are replacing.

The leads of the power supply(/battery)/resistor are connected to the circuit of the target battery somewhere remote from the area where you will be removing it. The resistor will serve as a “soft” current supply to the battery circuitry so as not to inject high current into the non-rechargeable battery until it is removed yet allow a supply of voltage to the RAM while the battery is being replaced.

Following the above procedure will allow you to keep your iron connected to the third wire ground and lower any chance of any ESD damage. But you must remember to first disconnect the instrument under repair from the AC supply to float it while you work.

I use this method on all of my battery backed memory instruments and have excellent success. Of particular note is a model line of HP spectrum analyzers that actually have the battery backed RAM and associated battery contained on a small plug-in printed circuit board that can be removed for battery replacement. Otherwise it is usually a chore trying to get to the underside of a printed circuit board and having to do considerable disassembly to reach the trace pads for unsoldering/resoldering.

The bottom line is to take your time and be very careful as to not short out the supply to the RAM while doing all of this or suffer the obvious consequences.

Greg

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