Yaesu FT-902 boat anchor losing TX power after long powered on periods in RX?

[Chris Wilson]

I was falsely blaming my laptop for reduced audio drive to my FT-902DM
when running USB digital, but having now seen it do the same from a
proper PC, with a different interface, and seeing as it's getting
worse, I need to check it out. I am thinking it's time or temperature
related as it's fine in a m morning after being turned off all night.
But after a few hours of being left on, TX output power has dropped to
about 5 to 10 watts even with plenty of audio drive in digi modes.
Pressing the "Tune" knob gives more output, but nothing like enough
even with everything plate and loaded, and trimmed up properly, with
full carrier. It seems fine when cool, and gives its 100 watts OK.
Without more testing I can't say if this happens if I leave the tube
heaters off until such time as I want to transmit, and I am not sure
if I should be running it at all for prolonged periods. Only whoopsie
is I blew the main fuse (which has always been fast blow, 3
Amps),three times when my ATU had an internal issue and I couldn't
load the set up properly. It ran fine for some long period ( a couple
of weeks or so) after these dramas, so not sure if it's relevant. Can
anyone suggest some tests I can perform to see what's happening, and
is leaving it on, in receive, to see if the heaters being on or not is
relevant, a good idea? All PS caps were changed a good while ago (4
months or so) except the two very big ones in the middle of the case,
I didn't have to time to ferret them out. I have new ones for it
though. I have changed the notorious very high voltage ceramic for the
power tubes though, I forget its number designation.

I have a decent scope, a Wattmeter, signal gen and frequency counter,
as well as decent bench meters, but as yet am at a very low level in
the learning curve. I am aware of the high voltages present in these
things, and residual in the caps, and how to preserve myself for a
little longer Thanks.

[vk6zgo]

Chris,do you have a dummy load?
If so, the best thing I can think of is to leave the Radio running continuously keyed in CW mode,& see if the power drops after a while.
You probably won't be able to do this for long,as most dummy loads aren't up to really long periods of full power.
If you don't have the problem with continuous carrier,the fault is probably in your SSB generator,or some other circuit which is used for SSB,but not CW.
If it is in low power stages,you may be able to monitor it with your Oscilloscope.
I don't really know the FT-902DM,so can't help much more.

Have you tried asking on QRZ.com.or eHam?
There are some very knowledgeable people on this forum,but few of them are Hams,so you may do better on those forums.

73,VK6ZGO