Tektronix 2455B ram battery replacement advice needed

[11mrjones]

Hello all -

My first post here, and need advice on my first scope which I got a few days ago, tektronix 24xx series, a 2455B. Currently it is fully functional but of course the Keeper battery is dated 1988. First question is (1) should I replace the battery now? It is going to fail soon so I might as well get it over with? On the assumption that I should replace it, here is where I am a little paranoid about losing the cal data. So after researching this here on eevblog I just want to double check to make sure I don't screw up.

Plan is to remove the A5 board, leave the board floating, attach a temporary battery (a 3.6V lithium battery) across P(B) and P(C) (see attached photo), de-solder original Keeper battery (soldering iron tip is grounded through the solder station mains plug) and replace with new Keeper battery. So question (2) - Is this correct approach or should I also connect a diode and 10k current limiting resistor to the temporary battery between P(B) and P(C). Hoping I can just connect the temporary battery straight across P(B) and P(C).

I will have board on ESD mat, mat grounded to power strip gnd, ESD wrist strap grounded to power strip gnd, solder station grounded to power strip gnd through the 3-prong power cord.

I am really paranoid about this because I have read here on the forum where a member basically did this as outlined but still lost his cal data somhow. (I have already recorded my working cal data off the scope through the diagnostics mode). I don't want to lose cal as I don't have any equipment for a re-cal.

Any advice/confirmation is appreciated.  Thanks

[Edison]

Hi, definitely replace the battery, these batteries basically do not have a discharge curve, they are around 70-80% capacity for many years and then suddenly fail. I have the battery-less version of the 2465 oscilloscope, but with other devices I've done it by connecting a spare power supply in parallel with the backup battery and then removing that battery and replacing it with a new one - always without complications. In more than thirty years of servicing industrial technologies, I have solved hundreds of such circuits.
If someone didn't succeed, they must have made a mistake somewhere - for example, they short-circuited the backup power supply circuit through careless handling.

[Kwakerman]

When I've done it in the past I used a PSU and accurately measure the voltages of the old and new batteries.  Sent the PSU to the voltage of the old battery before connecting it to the A5 then remove the old keeper battery.  Turn the PSU up to the voltage of the new battery and then solder the new one in.  By matching the PSU to the old and new voltages you will prevent current flow so the new battery won't try and back feeding the PSU and deplete some of its capacity.  Obviously if you feed the A5 from the PSU via a 10K resistor then that would also limit current flow in itself.

Don't rely on croc clips to make the PSU connection, a good multi strand cable should be soldered to the PCB to prevent any accidental disconnections when turning the PCB over

[factory]

For the 2465B I have, I temporarily soldered wires to a similar type of battery and taped it to the ribbon cable, working on a non conductive piece of hardboard while changing the battery.

Be careful to avoid shorting anything with the solder.

David

[11mrjones]

My first pcb work and need some ideas. Soldered in my temp battery and secured it onto ribbon cable as suggested , all good there. Problem is that I can't get the damn original keeper battery out. You can see where I used wick with some flux to clear out the solder on the under side of the PCB. While heating the joints I'm giving the keeper a firm tug from the other side of the pcb but it doesn't budge. I'm thinking there is a good amount solder on the top side of the board holding the pins still. I don't want to pull too hard or heat too long so as to cause damage. The keeper just will not budge it seems. It can't possibly be glued down from the topside. Any ideas, hints, tips are appreciated.

Thanks

[11mrjones]

Are those 2 solder bumps that are inside the rectangle outlined by the 4 corner joints of the keeper also attached to the under side of the keeper and need to be de-solder?

[11mrjones]

There are not any pins sticking through them, just look like bumps.

[factory]

No those two are vias, they are not connected to the battery.

Two pins of the battery are connected to the metal case of the battery (which is hidden by the plastic casing), the other two are the + - connections.

The attached picture shows the underneath of the Keeper battery, which has been kept & often gets used as a quick check source, for any vintage digital voltmeters I acquire.

If my desoldering iron didn't free up the pins, I suspect I used the technique of heating pins on each side and pulling on the component to lift slightly each time, this would be repeated for each pin until it was free.

David

[11mrjones]

Thanks gentlemen for hints and tips. Who knew de-soldering could be such a pain in the.... well such an art form.

Finally managed to replace the battery and retained the cal data successfully. I think my problem was not enough heat, was using an Aixun T320 station, t245 handle with a c245-k200 tip. I was nervous the whole time, especially at first power up upon completion.

Anyway, thanks again, phew, baptism by fire! 

Victor