[Milsurplus] German PRC-6/6 question

[Yong Surk Lee]

I am adjusting a few German 6 channel PRC-6/6 to operate on 51.0MHz. I install a crystal for 51MHz(51-11.38MHz) and I adjust coils(square knob on turrett) for maximum reception and I also adust TX oscillator frequency(11.38MHz) for 51MHz. And I adjust for maximum output power. 
One problem I encountered was that the TX frequency drifts when I transmit with the radio. It seemed the AFC is not stable. 
Any opinion what needs be done? Any component or tube needs be replaced?
I worked many times with US PRC-6 with no problem.
de HL1FB/WW6N
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