The HP 3468A multimeter repaired in the last post uses something called “closed-case” or software calibration. Instead of making physical adjustments to controls inside, calibration involves applying known reference levels and having the meter store calibration constants. These are essentially offset values that will allow the meter to show a correct value even though internally there are small errors due to normal component tolerances. It is obviously important to store this data permanently.
This model (and it’s cousin the 3478A) use a 3.0V long-life lithium cell to maintain the power supply for a static RAM device (U509) when the power is off. Shelf life is generally considered to be 10 to 15 years for this type of cell, although they can last much longer. The problem is that the discharge curve for this type of battery is very flat right up to the point at which it is almost completely discharged. Just the fact that it reads 3.0V or better does not mean that it has plenty of life left. If it’s old or its age is unknown, it’s probably best to replace this battery to avoid an expensive calibration/adjustment procedure from a professional metrology service.
The procedure described here involves working on a mains-powered instrument with the power on. If you choose to duplicate these steps, then please exercise due caution.
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This particular unit has a SAFT LX1634 cell in it. There are no signs that it has ever been changed, even though the date codes in the unit indicate it was manufactured in early 1987. The voltage reads 3.021V. I measured it’s dimensions at 0.64 inches wide by 1.37 inches long. (16.2mm X 34.9mm)
The replacement chosen is a Panasonic BR-2/3A
with PCB pins (Digi-Key P/N P226-ND). It is physically the same size as the original and should drop right in.
In order to replace the cell without disrupting the power supply to the SRAM, the battery will be replaced with the power on. To avoid having a grounded-tip soldering iron cause problems, a butane gas-powered soldering iron will be used. And just in case of a mains power interruption, I will also add a 3V primary battery pack for insurance.
The 3V battery pack (2 fresh 1.5V cells) will be connected via a diode (to prevent back-feeding the battery) and will be soldered in for security. The diode used here is a 1N5817 Schottky from my junk drawer. Just about any small diode will do though. I put heat-shrink over it to avoid accidental shorts.
The ribbon cable connector for the LCD display is in the way, so it has to be disconnected and moved. It fits into the plated-through holes on the LCD board, and can be gently pried off.
The SRAM that must remain powered (U509) is in the corner. It is a 22-pin NEC μPD5101L 1024-bit (256×4) RAM. Pin 22 is Vcc. Minimum supply voltage for data retention is 2.0V. Standby current is 10μA.
The 3.0V lithium battery and the on-board +5V supply rail is diode-ORed to supply Vcc for the SRAM. The +5V rail goes through CR550, and the battery goes through CR551. The junction of the cathodes is Vcc for the SRAM. This is where the diode-buffered external battery pack will be attached.
The points of interest for this operation are just above the battery. The junction of the cathodes of CR550 and CR551 leads to the Vcc pin of the SRAM. Care should be taken not to short this node to anything else during the procedure.
The external 3V battery pack is temporarily soldered to the PCB. As long as the device is left on and the mains power is steady, this pack should not actually be used. I just added it to safeguard against mains power interruptions (it happens.)
With the power ON, the old battery leads are snipped off. This makes it very easy to remove the stubs from the PCB and clear the holes of solder, all while working from the top side of the PCB. Watch for solder splashes and be very careful about shorting adjacent leads with the soldering iron.
The new battery fits the PCB holes perfectly and is soldered in from the top side. The holes are fairly large and require quite a bit of solder to fill up.
The battery terminal pins also serve as the only physical mounting for the battery, so the connections need to be very secure.
The power is still ON at this point. The external battery pack has been removed. Reconnecting the LCD ribbon cable results in garbage on the display, but it doesn’t seem to hurt anything.
It’s not a bad idea to verify voltage measurements before turning the power off and trusting the new battery. Here, the SRAM Vcc node is a diode-drop below the +5V rail at 4.40V. CR551 is therefore reverse-biased, so the battery is not being used at this point.
The new battery voltage is 3.39V.
The battery voltage also appears at the anode of CR551, so the new battery must be connected OK. It should be safe to rely on the new battery.
The mains power is turned OFF. Checking the SRAM Vcc at the diode junction shows 3.19V. The battery is now supplying Vcc. The stand-by current is so low that the forward voltage across CR551 is much lower than the usual 0.6V.
Upon turning the power back ON, the self-test passes. There is no indication of “Error 1”, which would indicate the loss of calibration RAM data.
A quick check with a 5.000V reference shows that the meter is still calibrated and operating correctly.