Kenwood TS-850 Memory Backup Battery Replacement

Kenwood TS-850 Memory Loss

Because of the age of the design and components used, the circuit uses a RAM chip. Using RAM has a drawback in that it requires power to retain information. This power is supplied from a small memory backup battery. Modern designs use NVRAM (Non-Volatile RAM) which retain the information without requiring power.

As a now aging radio, the chances are the memory backup battery is failing, or indeed has failed. A symptom of a failing battery is that the memories and operating parameters are lost when the radio is turned off for any length of time. This results in the radio performing a “reset” at power on.

It must be noted that when the battery fails it can leak resulting in the possible destruction of the surrounding foil tracks on the PCB. Therefore if yours is showing signs of failure, if for no other reason, it is a wise idea to replace it.

Locating the memory backup battery

This section assumes that you have already removed the radio covers. If you need help at this stage, then I would advise you not to proceed further UNLESS you’re happy. Remember, if you are unsure what to do, take the radio to someone who is competent – don’t risk ruining your radio.

With the covers removed, remove the screws that secure the front panel to the main chassis, this will allow you to gain access to the digital board which is located behind the front panel.

Replacing the battery

You will need to carefully remove the connecting cables. If you’re careful, the ribbon cables along the bottom of the board need not be removed as it is possible to replace the battery (shown by the arrow in this picture) without completely removing the PCB.

Kenwood TS850 Digital Unit PCBRemove the screws holding the board in place and gently lower the top of the PCB forward. On the reverse of the PCB, carefully de-solder the three pins on the battery tags. There is one for the negative tag, and two for the positive tag. You can then replace with a new Sanyo CR2430-FT1 battery, or if you wish, you can fit a holder for a CR2032 type battery. Should you decide to fit a holder, you may have to tease the pins through the holes in the PCB as the holder pin spacing is slightly different to the tags on the replacement battery.

Battery loses charge frequently

A common cause of premature battery discharge is using cheap batteries. Always use good quality, branded batteries. Batteries to avoid are those found in multi-packs, although they may have a brand name on, they can be rubbish!

There are other possible faults that may be apparent in the radio. Firstly, IC23 (MB3780A) may have a small internal leakage current. Secondly, either, or both, C194 and C195 may be leaking current to ground. It is most probable that C195, being an electrolytic type, has dried out and leaking current. Simply replace C195 to verify if this cures the problem.

Using a rechargeable battery

My TS-850 exhibits signs of frequent battery discharge. Replacing C194 and C195 didn’t solve the problem. My solution to replacing the battery on a regular basis was to use a rechargeable battery.
Firstly, I used the method above to install a battery holder. To provide a supply voltage to keep the battery charged I used a feed from the +5 volt rail on the PCB. There are plenty of places on the PCB where the 5v supply is available, the closest being pin 1 of connector CN3, just above the battery location. To prevent the battery discharging through the radio common 5v rail, I used a blocking diode. An 1N4001 diode was inserted in series between the 5v supply and the positive tag on the battery holder. This can be seen on the modified schematic. The 5v supplies just enough voltage to trickle charge the 3.3v battery.

TS-850 Memory Backup Modified Schematic

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