Yaesu FT-902DE ALC/outputpower problem

[PA3BBV]

Hi guys, I'm new to the forum here, thanks for letting me in. I am trying to repair a classic tranceiver type Yaesu FT-902DE and I would like some advice...

The radio does not give full power. maximum approx. 60 watts on all bands. The strange thing is that I can get full power in ssb if I switch the s-meter to the ALC position. The ALC meter always is full scale when transmitting. I'm afraid there's something wrong with the ALC. However, I can't figure it out, because the overview diagram is printed on A4 copies, it is very difficult to follow the wiring. Could somebod explain how te ALC works? Anyone has a suggestion?  Thank you all ver much.


73es Leo

[m3vuv]

heres a quote from the fox tango site,dont know if it helps?:The meter problem nagged me: The meter works fine in
receive mode, but in transmit I get no deflection in either
the IC, ALC or PO position.
After studying the (rather small copy) schematic, I saw
that the meter is switched via relay RL1, and, because I
had read that the relays could be a problem, I cleaned the
contacts, and measured the contact resistance.
They were all good, but the problem remained. So it had to
be the rocker switch on the front who switches the meter
to the IC, Power output and the ALC.
Strange, because all other switches are working normally.
Well, it was disassembling time again. After taking out
some modules, the counter, VFO and the frontplate, I can
take the switchboard out.
Fortunately I have the Service Manual, with the foil pattern
and a detailed schematic of the board, and after some
searching I find a connection point (15) that is not attached
to anything, but should connect to the relay. After prying
the cable harnass apart, I find the loose wire. It is so short,
that it must have been broken off, the first time this switchboard was worked on.
There were signs that the switchboard was taken off earlier, by one of the previous owners, maybe to try to clean
the switch contacts, because some of the small screws of
the switches are missing.
I think they have been looking for the same fault.
I made the connection with a short piece of wire and some
shrinking tube, to get more stretch in the wire, to prevent
future problems.
While I was at it I sprayed the contacts of the switches
with tuner spray, it can be applied via the upper mounting
screw holes. I was able to test this by disassembling a
similar switch, that was in my junkbox. The switches have
sliding silverplated contacts, and should normally clean
themselves.
The switches are ALPS, and are also used in some audio
equipment of the time.
After re-assembling, the meter works fine, and I am able
to measure the IC current, ALC and the output power.
link:http://foxtango.org/FT901/FT-901-902%20Web%20Page.htm

[m3vuv]

this maybe better :http://foxtango.org/Files/FT901-902%20SG5.pdf