[Yaesu] FT901 power out problem

[Martin Sole]

Dear list,

I have an FT901 on my workbench at the moment. It is in reasonable shape 
and for the most part seems to be working. I am however having great 
difficulty with getting much power out. Several modifications appear to 
have been done and I am trying to undo them but may have missed 
something. Currently having removed what I think are modifications I 
have about 25 to 30 watts out on 20 metres. PA tubes and driver check 
good in another radio.

Initially I found the PA HT was only about 450 volts. I traced this to a 
rebuilding of the power supply as a full wave bridge rather than a 
voltage doubler circuit. This seems to have been deliberate on account 
of the cut pcb tracks. Circuit rebuilt correctly and tested.

Underneath the PA compartment I found what I believe should be R06 to 
have been replaced with a choke, about 15 turns of 28 guage wire wound 
on a 3/16" form by the look of things. The schematic shows no value for 
this resistor. I believe it may be 1 ohm.

At the connection to the output socket from the change over relay I 
found an additional PI network, not sure of the cap values but the coil 
is 9 turns of 26 guage wound on a 1/2" form or thereabouts. The manual 
schematic suggests this should just be a piece of wire from relay to socket.

I would be interested to see some good photographs of the PA section, 
particularly around the toroid core output network inductor as I suspect 
there may have been modifications in this area also. Would also like to 
hear any other thoughts as to why the reduced output power.

Regards
Martin, HS0ZED