[Amps] trouble with FT101E amplifier

[Eric Dobrowansky, KK2ED]

Hello gang,

I am attempting to repair a Yaesu FT101E I picked up on the cheap.     There
is a problem with the RF output PA. The pair of 6JS6Cs check out good. The
problem is that the PA Loading and Plate tuning is sporadic and the amp goes
into spurious oscillation very easily just by turning the plate or preselect
control, even when the unit is in RX mode. While looking at a spectrum
analyzer, I can see the PA fundamental output disappear and see wideband
noise being generated when it goes in oscillation.

So far, I have checked the parasitic suppressors. While the 51 ohm resistors
are no longer exactly 51 ohms, they are close, around 35-40 ohms, which I
would think should be fine. The neutralizing air-variable cap looks good, as
well as the 100pf cap which couples the adj neutralizing cap to the top of
the plate choke and the 1000pf cap from the Plate var. cap to the plate
choke.   If the drive level is lowered to less than 10w or so, the amp runs
clean and the plate and load tune smoothly on either side of a peak.
However, once the power is increased over 50w or so, the Plate tuning is
very erratic, and the Load control works as if it has a "dead spot" once it
is turned past 5 or so on the scale. However, both the plate and load caps
look fine at close inspection, and work smoothly at reduced power.

The bias seems to be working correctly, and is set for 60ma idle current.
My only suspicion at this time is maybe something changed in the value of
the plate choke. Physically it looks ok, but maybe a winding shorted?


Any suggestions? After about two hours with it I am stumped at this point.


Eric
KE2D