Tektronix 2445A backup battery

[c4757p]

The memory backup battery on my Tektronix 2445A is starting to go flat. Normally, after replacing it, the whole calibration routine has to be run, because the damn thing doesn't even have factory defaults in ROM. I'm a bit short the necessary equipment (and time) to do a full, proper calibration, and it's currently spot-on so I'd like not to mess with it. Sadly there is no way to read out the constants and then write them back in. Would I have any problems doing something like this?

Connect a power supply to the circuit in parallel through a resistor (2-5k or so). This should limit current into the dying battery - I don't care if I damage the battery, since it's going right in the trash, I just don't want it to blow up. I'd imagine the backup current is low enough that the voltage drop in the resistor shouldn't matter. (I'd use the current limit on my power supply instead if it weren't so pathetically coarse at low currents.) Desolder the battery. Connect a new one with a series low-drop diode (BAT85S, V_F around 0.25V for I_F around 100µA) to protect it from the power supply. Disconnect the supply, short the diode for full voltage.

Any potential to damage anything here, besides the original battery? (Or any reason why it just plain won't work and I'll lose my cal constants anyway?)

[kolbep]

If the Tek  has a bbattery that is float charged (check terminal voltage what it is plugged in and turned on - should be higher than when the unit is unplugged), then if you can get to the battery, change it while the unit is on. That way the memory  will be powered by the charger.
But check that the battery terminals are not at A  high voltage to the chassis, or you would get a boxing day tickle.

[c4757p]

It's not even rechargeable! It's a standard lithium, long life battery (LTC-7P, 750 mAh, supposedly 15 years shelf to 80% capacity). In fact, it's even available with a built-in diode to prevent accidental charging, though this one lacks that. I have no idea if the memory would be retained if I removed it with the circuit powered on and I'd not like to find out if I don't have to.

Seems kind of ridiculous to me to use a non-rechargeable battery for memory backup and not even bother to socket it or anything - should you really design something that needs a soldering iron taken to it every ten years or so? And they really should have included a routine in the self-test menu to play back calibration constants so you can write them down before changing the battery...

[SeanB]

Power it on then prepare a new coin cell in holder with preprepped wire leads to go in it's place. Cut old one out then solder the wires to the cut leads with an isolated soldering iron ( a soldering gun is good here for the isolated tip or use a gas powered soldering iron) to keep the memory alive. Place new coin cell holder in a convenient location with double sided foam tape ( 2 layers ) so you can pop in a new one when needed.

[kolbep]

if you want it powered off,then connect a new battery with flyleads as per Sean's suggestion, but solder the leads on the bottom of the pcb or some nearby pads (so the old and new batteries are in parallel. Then snip off the old battery.

[alm]

An unplugged soldering iron can also serve as isolated soldering iron. You can rely on thermal mass to keep up the tip temperature. Of course this won't work if the battery is soldered to a large ground plane or something else with a large thermal inertia.

[amyk]

According to the 2445 service manual (without the A), it uses an ER1400 EAROM (early form of flash), so it doesn't have a battery.

The 2465A uses an LTC-7P battery to power two CMOS SRAMs, I'm guessing this is also true of the 2445A. It's a 3.5V lithium thionyl so a standard 3V coin cell might make it warn about low voltage (although the SRAMs will hold down to 2V). The 2465A does have a way of reading out the calibration data.

[peter.mitchell]

I'd suggest putting 2x battery holder in, so when you need to replace the battery in future, you put new battery in holder, then quickly take out old battery.

[c4757p]

AAARGH    Just realized why I have been having trouble - the service manual I downloaded is labelled 2445A - even has the 2445A cover, but is the 2445 manual! It's just similar enough to have proven useful a few times and I never noticed. Off to find the right manual...

Thank you, everyone, for the advice.

[c4757p]

I have been sitting here for God knows how long wondering why I can't find the god-!$@#@-damn battery in the !@#$ schematic...

[c4757p]

I like the "isolated soldering iron while the scope is running" idea - I see from the schematic I finally found that it will most definitely retain memory while running if the battery is removed, nothing to worry about there. Might require a bit of cable-wrangling to get to the back of the board with all the power connected, but much better than screwing around with a diode and a power supply.

[wkb]

you could use one of these lighter-gas fired soldering irons if you are worried about an electrically heated soldering iron.  Weller make them, just to name one vendor.

[aghp]

...putting 2x battery holder in, so when you need to replace the battery in future...

These batteries do not need change very often....

I have here couple of LTC-7P "Keeper"'s from Tektronix oscilloscopes.
I have changed these before battery is dead.

They are removed inside about 1 year ago now. Time when they are removed they all keep memory without any problem and assembled on the board voltages show still near around nominal.
They are stamped year 87 and 88.  These celss are amazing good quality. Around 25 year and battery have still life. (one example, stamped 02/87  25 year inside Tek.  15 second load  test with 330ohm resistor still give 2.75V. (This was wrong, it was other type of battery) I read more carefully my notes and:  I use for LTC-7P: After 15 second 200 ohm load and measured loaded: Result =  2.3V  (with this value battery do not really need change perhaps still long time but... if you sell oscilloscopes it is better to change  - it is too ecxpensive if it fails just after sold.)


If now change new same (or better) quality Lithium cell next time it need change  before year 2035.
Who think it still need change agen... but who knows... it may be 50year old and it is still alive but I'm little sceptic even for Tek 2400 series best models.


Common:

If want good.  One good is Tadiran - example SL-850/P  (1/2 AA, 1.2Ah 3.6V)

http://www.tadiranbatteries.de/pdf/EPCdeu.pdf

And then littlebit more "theory and practice"

http://www.tadiran-batterie.de/download/eng/LBR06Eng.pdf

[c4757p]

...putting 2x battery holder in, so when you need to replace the battery in future...

These batteries do not need change very often....

Sure, but it doesn't hurt anything. They made the scope in 1988 and the battery lasted 24 years - great! - but now it's 2012 and some poor guy is stuck figuring out how to gracefully change it because they figured it was long enough not to bother with easy change hardware. I've learned my lesson about "good enough" many times over - with my luck I'll buy a dud of a battery and it will have to be replaced again in a week. The battery holder is cheap and well worth the expense.

[BravoV]

I have been sitting here for God knows how long wondering why I can't find the god-!$@#@-damn battery in the !@#$ schematic...

Any success ?

Although I have not read the service manual for this model, I guess maybe all 24xx series have similar circuit, here the shot of my 2465B control board, is it similar ?

Watch the large black rectangle box labeled Dallas DS1225Y in the middle near the blue connector, that is the SRAM with battery embedded inside that big IC body, so in the circuit you will not find the battery symbol. 

One of the photo from other thread -> 2565B Tear Down

[c4757p]

I actually haven't done it yet. I have to replace another part as well and I am waiting for it to arrive.

The 2445A definitely has a discrete battery instead of that. My board looks fairly similar but with older-style design, all through-hole and with a lithium backup battery in the corner. I'd take a picture but I am currently lying in bed...

[amyk]

Seems kind of ridiculous to me to use a non-rechargeable battery for memory backup and not even bother to socket it or anything - should you really design something that needs a soldering iron taken to it every ten years or so?

The reason is that it uses a thionyl battery, and that chemistry was not designed to be user-replaceable. From http://en.wikipedia.org/wiki/Lithium_battery#Chemistries :

High cost and safety concerns limit use in civilian applications. Can explode when shorted. Underwriters Laboratories require trained technician for replacement of these batteries. Hazardous waste, Class 9 Hazmat shipment

[kawal]

I also have a 2445A with a discrete battery. Just in case someone needs calibration values after loosing a battery I posted a Youtube video: