Saturday, September 25, 2010

FT-920 - High SWR on 50 Mhz

Main Unit

Q1084 - burned

After replace

The FT-920 experienced with the following problem recently:

1) high SWR on 6m
2) no control / adjustment of VHF power on the internal VR pot
3) TX on SSB , no mic input and getting full pwr output - at this point pwr cannot tune down
4) the above not appear on HF
5) turn on the ATT to 12 / 18, the above problem goes away.

I sent out help in the yahoo FT-920 group, but no solid hints on this. Two weeks ago, I saw Kevin, KI4NBO post that he had the same problem. He finally found out the problem realted to Q1084 - 2SC2714.

Q1084 is part of the T/R switching at the front end. It is turned on in transmit to keep the TX signal out of the RX antenna circuits. It is in the direct line of fire if the radio gets a lightning hit, so it goes along with those other JFETs being burned. (very lucky that the JFET is working at my radio) The attenuator circuits are also connected in at this point.

I have followed his hints to repalce the same, the radio back to normal now. Many thanks to Kevin, KI4NBO for his great work.