Kenwood TS-850S Battery Replacement
For informational purposes only. Use at your
I found the memory backup battery BA1 in my TS850 was leaking, enough so that there was evidence of the leak on the bottom cover of the radio. There are several stories around the 'net of batteries that have leaked enough to short across circuit traces or the connectors on the bottom of the Digital-Unit PCB. Not a nice thought.
Photo 1: The original 3V Sanyo Battery, the leak was on the bottom half of the battery
The original battery in my TS850S had no part numbers, just "Sanyo MnO2 Li 3V". After measuring the leg-spacing on the battery a short investigation revealed the Sanyo CR2430-FT1 3V 270mah battery was a suitable (if not direct) replacement. Mouser Electronics stocks the battery as mouser stock number 639-CR2430-FT1 for a whopping $2.30.
Many webpages mention adding battery-holders wired to the radio, but since the battery only needs to be replaced every 10-12 years I decided I'd worry about adding a holder later if it became an issue. Total time for the project was about an hour.
Replacing the battery
The battery is mounted on the front of the Digital Unit PCB, located immediately behind the front panel of the radio. To access the board take the top and bottom covers off, remove the remaining two screws holding the front panel on, then carefully unplug the two ribbon cables on the lower edge of the digital unit PCB (see photo 2).
Photo 2: Bottom view of the front panel and
digital unit PCB (on edge).
After unplugging the two ribbon cables carefully turn the radio over and unplug the cables around the perimeter of the digital unit PCB. The remaining ribbon cables along the bottom edge of the PCB do not need to be removed! If you chose to remove all the cables you may want to label the cables first. Be very careful with the ribbon cables. Some older radios may have brittle ribbon cables, flex them with caution.
The battery should be obvious, mounted on the upper-left half of the Digital Unit PCB (see photo 3).
Photo 3: Front/upright view of the Digital Unit
PCB with the four cables unplugged around the
The front panel should be free enough to swing forward to allow access to the Digital Unit PCB. It may help to block the main chassis up an inch or so. There are two ribbon cables (each split in two) connecting the radio to the RIT/NOTCH/NB controls - they can be unplugged to allow the front panel to swing out even further if needed.
At this point you should be able to remove the gold screws holding the Digital Unit PCB in place. Carefully remove the screws and carefully swing the Digital Unit PCB away from the main chassis to allow access to the solder points on the back side of the PCB (photo 4).
Photo 4: Rear solder points for BA1 on the back side of the Digital Unit PCB
The location of the solder points for the battery should be obvious from the rear of the PCB. I marked their location with a red sharpie before I started working on the board. Carefully desolder the rear solder points, using care near the SMT components seen in Photo 4. I used a 15W iron and #1 desoldering braid (if you use this method be sure to work just a little at a time to avoid overheating the board and surrounding components).
The battery legs are soldered on both sides of the board, so the front side may need to be desoldered. It is possible to desolder the top (-) leg from the top/front of the board. Once the top leg is free the battery can be bent away from the PCB to allow access to the lower two (+) legs of the battery (photo 5). The battery should also have a piece of foam tape holding it in position - remove the tape for better access. The difficult part of the project is done once all three legs of the battery have been desoldered.
Photo 5: BA1 bent over, revealing the two lower legs.
Carefully tin all three legs of the new battery with solder, then slip the new battery in position. Reuse the original foam tape (if possible) to hold the new battery in position on the board. Carefully solder the new battery from the back of the PCB, check each leg to make sure the solder joints are solid on both sides of the PCB.
Photo 6: The new battery is installed and ready to use
With the new battery installed (photo 6) all that remains is to put everything back together and test the radio. My battery was only off the PCB a short time - I didn't even lose any memories. The new battery should be good for another 10 or so years.
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