The ALC Meter suddenly be out of scale, then output power goes down low. Shortly after that,
the power comes up again .. and down .. and ...
This RF power oscillation is ordinary symptom with old TS-850.
The problem is caused by DC-DC(X59-1100-00) daughter-board on the RF Unit(X44-3120-00).
The DC-DC board, supplying negative voltage (-6.2V), has an astable-multivibrator in it.
When the symptom occurs, the astable-multivibrator stops running due to a little bit of voltage fall down from the power supply.
Looking into WWW, I can see many people tried to fix this symptom. Some one changed the daughter board. Someone changed Tr's on the board. Someone reduced the value of resistors(R2,R3) at Tr's Base.
The power supply voltage and the Hfe of Tr's are very important conditions to keep running astable-multivibrator.
I suppose that the astable-multivibrator which switches nearly 13V is very hard job for Tr's having only 5V Vebo, and then Tr's would get Hfe degradation in time.
Replacing Tr's to active one would be OK, and putting Diodes between Tr Emitter and ground is recomendable to avoid relapsing.
By the way, nowadays, we can obtain good devices for this kind of circuit.
This time, I tried a Voltage Converter IC to fix this problem, and it seems to work fine.
I chose the LTC1144 of Linear Technology.
In this way, LTC1144 runs in 10kHz oscillation, meanwhile original board oscillates approx 15kHz.
I don't find any problem about that for now.
But You (maybe a HiFi SSBer) may dislike that DC-DC runs audible frequency.
Then you can connect Pin No.1 (Boost) to the V+ and you'll get 10 times higher frequency.
And to tune (lower) the frequency, add Capacitor between Pin No.7 and ground.