Pictures of diagram and board layout. (click to enlarge). Please note that capacitors C626 and C628 are shown reversed, but that they are correctly mounted on the board !
After several years of operation, the TS850 sometimes is presenting an instability in the transmitter output power. Power varies and drops after a few minutes, comes up again and stays low...
There are several possible origin for this problem, we are treating here only the instability due to an ALC problem, which is the most common. Other reasons are treated elsewhere on this site.
The RF-UNIT unit is divided in several boards, one of these is smaller than the RF UNIT main board and supports the ALC function. This board is mounted verticaly and has the part number X59-1100-00. This board delivers an ALC voltage of -6.1V derived from the main +12.5V. This voltage converter is built with a astable multivibrator using 3 transistors, and a full-wave rectifier with the help of 2 diodes. Two capacitors C626 and C628 are filtering the ALC voltage. The Zener diode D605 limits the output voltage to -6.1V
If you have a minimum of technical knowledge, you can repair this ALC board.
Start in measuring the ALC voltage which should be close -6.1V on pin 1. This voltage should stay stable in time and when using the TX. If it's not the case, it's time to heat the soldering iron !
Start in replacing the resistors R2 and R3 from 22kOhm to 18 kOhm. Very often this modification is enough, in particular when the transceiver is operated with a low supply (12V).If this does not help, all other components can be incriminated, but experience has shown that mostly the transistors, diodes and capacitors are faulty. These capacitors are not located on the ALC board but on the main board ! Even if you do not detect any problem with them, it is wise to replace them beacause they mostly finish to leak or dry...