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Author Topic: YAESU FT-ONE 40m TX Mystery-Drivin' me crazy!.  (Read 1438 times)
ZS5WC
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« on: November 04, 2013, 01:29:07 AM »

 ???I have a mint / restored Yaesu FT-one--it SHOULD be FB on all bands-but for some mysterious , unexplained reason the RF output drops on 40m, and ONLY 40m.(all tests into 50ohm dummy load)
TX Any Carrier above 40w, and the RF wattage drops on the meter, whilst the PA / Finals current starts climbing.
At around max 25w the PA current draw /  output is normal.
Also, 50w out on any other band except 40m will draw around 11A at the PA, whilst on 40m the PA draws around 15A for same o/p and the current  increases with time.
Initially I thought it could be the directional coupler causing excessive alc due to self resonance-which proved not to be the case.
The coupler gives the same reading for the same RF output on all bands.
I suspected internal supply problems-but the rig does the same on the external DC supply.
Although the output drops , the ALC also increases, but forward power is down, and reflected power reads zero.
ALC is developed from forward and reflected power readings from the coupler.
I swopped the LPF filter board, still same.
I checked the signals on a scope for PA instability-but none seems present, also into an 1:1 swr antenna , the SWR does not rise as would be the case if the finals did oscillate .
The power meter too is fine.
Heck, I am stumped!. Angry
Any ideas?.

73 de William
ZS4L
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ZS5WC
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Posts: 411


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« Reply #1 on: November 09, 2013, 11:25:25 AM »

 :)Problem Solved!--turned out to be a NON-original pre-driver transistor not even thermally connected to the PA case.

Transistor gets HOT--HFE goes up, pa ascillates on voice peaks.

All OK now--just to get the reference oscillators to stay put--maybe new ceramic trimmers will do the trick..

73!
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