There is a known problem with one of the components within this series of Yaesu transceivers. The problem involves the FT101, 901 and 902 models. It may involve other models as well, but these are the ones I know for sure. The interstage coupling capacitor between the driver stage and the PA stage can become leaky over time. When that happens, the DC voltage present on the plate of the 12BY7 tube driver can cause an increase in plate current on the 6146B PA tubes. It can also cause the output power to appear to "fall off" giving the impression the tubes are soft. In other words, in the Tune Mode, power will register maximum initially but will decrease to a lower level within a few seconds.

If the leakage is severe or the capacitor actually shorts (which it often does) damage can occur to the PA tubes. I have read that in some cases, if the circuitry is configured such that high voltage is present even during receive, damage can occur to the radio even if you do not put it in transmit mode. The capacitor in question is a 100 pf 1000 VDC mica. The solution is to replace it.

The capacitor is labeled C01 on the schematic for the 902DM. It goes by other names in the other models. I think it may even be a different value in some of the other models. I believe the value and label are the same in the 901DM. Either way, it's easy enough to find on the schematic. It is between the plate of the driver tube and the grid circuitry of the PA tubes.

My 902DM had exhibited the hint of the symptoms indicating either softness of the tubes, or perhaps that this capacitor was at least slightly leaky. Not wanting to risk damaging my radio, I left it powered off until I obtained and installed a replacement.

The capacitor is not particularly easy to get at, but some of the descriptions of the process I have read made it sound much worse than it really is. It is located on a circuit board which is just below the two PA tubes. In fact, the PA tube sockets are mounted on this board. The description given here applies to the FT-902DM, and I believe the FT-901DM. I do not know how it may differ from the earlier models such as the 101, 102, etc.

To get to the circuit board you have to (after removing the two half outer covers) remove the screen top off the PA cage, the side cover and the bottom screen cover of the cage. Two wire leads will have to be unsoldered in order to lift the board out to gain access to the capacitor. One lead is from the plate circuit of the driver. It is red in my radio and I presume that is its common color at least in this model. It passes through a feed through seen in the lower right in the first photo. You must also unsolder the bare lead from the board to the phono plug on the rear panel.

Some correspondence I've seen stated that the capacitor number is stamped on the board. It is not on my radio.

Board in position, screws removed in preparation to lift board	Board exposed showing the original C01 above the right hand tube socket
Topside view PA tube sockets partly in position	New silver mica capacitor installed
Board back in place Leads resoldered	FT-902DM PA stage closeup

I really noticed the dust when I started editing the photos. Don't worry, I did some cleaning before I put things back together.

Though I tested the original C01 capacitor out of circuit both with a high impedance ohmmeter and with a capacity checker and it indicated that it was ok, I would not trust it considering the reputation of this component. After putting the radio back together and going through the tune up procedure, the radio tunes fine and it produces a consistent 100 watts output on bands 17 and 15 meters. It produces 75 watts on 10 meters. I didn't try the lower bands but I suspect it will produce 100 or more watts on those as well. It did the last time I tried those.

Whether this capacitor in my radio was actually defective, I don't know. Yes or no, it was worth the effort to change it as doing so, removes at least this one element of risk to the continued good performance of this fine radio.

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