Tektronix 2440 Scope Backup Lithium Replacement
Most of Jestine’s Repair Blog Readers probably already read about the completely Short circuiting Tektronix 2440 500MS/s Memory Scope that was successfully repaired not long ago.
You can read that article post below:
Before this I just recently had bought a Tektronix 2465A 350MHz Scope in Texas USA. And from the 2465A Tek I already replaced the Backup Lithium by the same LTC-7P version to ensure not losing its Calibration Data in the Internal SRAM chips. Because an empty Lithium Battery easily could mean a useless Oscilloscope afterwards or having to pay about $600 US Dollar to a Firm that specializes in Calibrating these now about thirty Years old but great Instruments!
My Tektronix 2440 was brought to life after I replaced the Bad components but also was calibrated by yours truly with the Help of the Great Tektronix build in computer software and its terrific Diagnostic Hardware! And after repair I finally also bought it from my friend and old colleague Frans who in fact got it for free after the 2440 only blew up the internal Scope’s 5Amp Power fuse and the 230VAC Mains fuse. (As can be read in the already published 4 Part article about the 2440). And because even the badly working 2440 Scopes that fail at the Calibration test go on Ebay (for about $349 dollar!) over to another owner it was a good deal after I got it fixed.
But also this great piece of Tektronix Device had the (probably) still original old Lithium Backup Battery inside. Which means I also have to prevent losing the Calibration in time if the Backup Battery runs low.
So this article is about my plan to replace the old Lithium inside of the Tektronix 2440.
Of course a plan is nothing without the Schematics of the Tektronix 2440. And I already had printed out in a very Big Book (516 Pages!) the previously downloaded Service Manual revised version July 1993. (first Printing version was July 1988). Scope Serial Number B014064 & Below 070-6603-00.
Above part of the System A12 Processor Board shows were the Backup Lithium with name BT800 can be found. This is page 428 from the 2440 Tektronix Oscilloscope Service Manual. To give an indication size of how big the BT800 Lithium really is: the pin to pin (+ and – Lithium wires) raster size is exact 39 mm. The Eagle Picher Battery is a 3.5V KeEper II 1.6AH LTC-16P-CO-F-S2 Lithium type.
Sadly these original Batteries from Manufacture Eagle Picher can’t be obtained anymore. And I was advised to look for another equivalent Lithium Type. So I bought 2 type SAFT LS 14500 3.6V 2.6AH Lithiums hoping they could be used to replace the old type. But sadly their pin size was with 51mm a lot longer. If I would remove the original Lithium, the SAFT AA Lithiums easily would fit in place of the old BT800 when of course also adjusting the Battery wire length.
Previous 2 photos showed the BT800 on the A12 Processor Board Top and Bottom side.
Next picture (page 429 of the Service Manual) is a view of the Battery circuit around the BT800 Backup Lithium. The circuit shows why it is not a good idea to attach a second new lithium to the original BT800. BT800 is connected to CR802 (a type Schottky 1N6263 – see page 8-50 = page370 / 60V, 2.25PF), and also to R900 10KOhm and R802 10MOhm (to the Opamp to detect the Battery Voltage). And if we would attach a new Lithium to an old almost empty Lithium in Parallel, we would only create big trouble. Not only the new Lithium could be drained by the empty Lithium but it even could cause Fire! And if we would place another New Lithium Battery via another 1N6263 Diode to our old circuit, the U940A Opamp (TL072CP) wouldn’t be able to detect the new Battery either!
So the best thing here is just to replace our old Lithium by a new one.
Parallel connecting of Batteries is probably only allowed when they are rechargeable and of the same type Li-Ion Cells. With some safety measures. As is the case in Laptop Battery Packs to prevent fire and high short currents.
Next Photo of the A12 System Processor Board Component side shows another view on our BT800 Battery. This Panel is the first Board placed inside the 2440 right above the Power Supply Board and the CRT Scope Tube.
The Photo following shows how the A12 Board is connected on the down faced side of a black ‘plastic’ mounting plate. On the other side of that ‘plastic’ mounting plate there is the A11 Time Base/Display Board. Connector J148 with cable from Board A11 is Horizontal and Vertical Display Output (page 466). And to be able to open both the A12 and A11 Panels, that are mounted on the Black Plate, we first must disconnect all connectors going to both Boards. Beginning with all connectors to Board A12.
When we forget to disconnect these connectors and their cables we could damage them when we turn open the Mounting Plate with both Boards by force. The Black mounting Plate with A12 and A11 attached, swivels/tilts open on Hinges at the one side were most if not all Flat cables go into both Boards. On pages 429 and 446 of the Service Manual this is also visualised.
Because my original Lithium BT800 Battery still measured 3.7V which probably is a good sign, I decided not to replace it by my new longer SAFT LS4500 3.6V 2.6AH Lithium cell. (Maybe the BT800 already was replaced before because of the Marking ‘MLP´ carved into the A12 Board next to the BT800). And also because I already successfully managed to calibrate the Tektronix 2440, as I had written in my 2440 repair article.
I do hope that the Internal Tektronix 2440 Diagnostic Startup Menu will warn me in time when the Battery Voltage Level is going just below the still safe value. Which gives me some time if the old Lithium indeed needs to be replaced. Lithium batteries are also used in modern electronic Watches because they drop the Voltage instantly when they go Bad. And thus immediately prevents the Watch from displaying any wrong Time and Date. Which would indicate there in fact would be no time left to safe the Calibrating SRAM Data in time if this resembles the BT800 Lithium.
But because I couldn’t get the original BT800 3.5V KeEper II 1.6AH LTC-16P-CO-F-S2 Lithium Battery, I do not replace it by the SAFT Lithiums yet. And maybe in future I’ll be able to find a better matching replacement Battery with the exact same size. And when replacement no longer can be postponed, this article will come in handy.
Next 2 photos show the first cables that connect onto System Processor Board A12 and have to be disconnected to be able to open Boards A12 and A11 without damaging those cables.
And the photo following shows how the Boards are secured onto the Black ‘plastic’ Plate. On Black Mounting Bolts that can be tightened by screwing the top so that it fastens its grip on the hole in the Board (or vice versa).
If the Cables are all removed from the A12 and A11 Boards, including all Flat cables at the Hinges side, the complete A12 Board can easily be taken out of the 2440 Scope and be removed from the black mounting Bolt(s) and Black bottom Holders. Which is a good thing when the Lithium Battery needs to be replaced in the (hopefully not nearby) future.
And to protect the Calibration Data in both SRAMs luckily I now already have 2 new SAFT LS14500 Batteries of which at least one can be used to connect with its PLUS + side attached to the kathode of diode CR802, and with its – MINUS wire to the top connection of R900 (10 KOhm).
Or just use 2 new AA (Duracell) Batteries instead of another Backup Lithium like explained in the Tektronix 2465A Backup Battery Replacement article (28 March 2015) written by Robert Calk from Texas USA.
But of course only do this when the 5V Always Power line to the Kathode of CR802 is not active anymore! (While keeping the >2V Lithium Backup Voltage on a safe level!)
And keep all your electronic devices safe and in good working order.
Life is hard enough without them. Just as living without good Friends and Family! Until next time.
Albert van Bemmelen, Weert, The Netherlands.
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Note: You can read his previous repair article in the below link: