Latest F/W I have is 64E5, here is LSB0210 says about the firmware corruption problem and how to prevent it.
SERVICE BULLETIN Land Mobile Division
Kenwood Service Corporation 2201 E. Dominguez St. Long Beach, Ca. 90810 (310) 761-8275 (310) 761-8290 FAX
SUBJECT: TK-250/350 Firmware Protection
DATE: 12/30/1997
(310) 605-5045 BBS (310) 761-4498 Direct FAX
Printed in U.S.A.
TK-250/350 Firmware Protection
The TK-250 and TK-350 microprocessor memorizes the user settings every time the radio power is turned off. If the battery being used cannot sustain radio operation, the user settings memory routine cannot be completed reliably. In earlier versions of the TK-250 andTK-350, it is possible to corrupt the microprocessor firmware by trying to operate the radio
with a weak or discharged battery.
Beginning at serial number 704xxxxx of the TK-250 and TK-350, C451 on the display section of the TX-RX unit (X57-44xx-xxC/3) was replaced with a 47k W resistor (Kenwood P/N RK73GB1J473J or equivalent). This resistor stabilizes the reset line when a low battery voltage condition exists - preventing memory errors which could otherwise occur as the radio powers down.
All versions of the TK-250, TK-350, TK-250G, TK-350G, and TK-353 with serial numbers higher than 704xxxxx include the C451/47k W resistor change. Any time that the firmware is upgraded in an earlier radio, or during routine maintenance, C451 should be checked and replaced to a 47k W if this has not already been done.