Yaesu FT-840 power output issue. - UK Vintage Radio Repair and Restoration Discussion Forum

Scimitar · 15th Mar 2020, 11:54 pm

Receiver works as expected. In transmit, the output power is unstable, even into a dummy load. Varying the power control clockwise gives an increase in output which then often dies as the control is advanced. It can then come and go at random.
The problem affects all modes. When running correctly it seems to be producing nearly the expected 100 Watts. There is rather a lot of RF in the shack too, even on the dummy load which makes me think maybe PA dry joints or interconnects tarnished.

Has anyone come across this before and can suggest what to look for please?

crackle · 16th Mar 2020, 9:50 am

Dirty potentiometer??

Mike

Steve G4WCS · 16th Mar 2020, 4:07 pm

parasitic oscillation ? may benefit from checking all the electrolytic caps around the PA area.

mickm3for · 16th Mar 2020, 7:51 pm

I have had this very same fault on this radio, was dirty pot, took longer to dismantle than fix
Mick

Radio Wrangler · 16th Mar 2020, 7:57 pm

Can also be dry joints on some radios.

This makes output power uncertain and heating from use at higher powers can make it go bad just when you thought it was having a good patch. Hysteresis in the effect is common.

David

15th Mar 2020, 11:54 pm	#1
Scimitar Heptode Join Date: Oct 2015 Location: Gloucestershire, UK. Posts: 720	Yaesu FT-840 power output issue. Receiver works as expected. In transmit, the output power is unstable, even into a dummy load. Varying the power control clockwise gives an increase in output which then often dies as the control is advanced. It can then come and go at random. The problem affects all modes. When running correctly it seems to be producing nearly the expected 100 Watts. There is rather a lot of RF in the shack too, even on the dummy load which makes me think maybe PA dry joints or interconnects tarnished. Has anyone come across this before and can suggest what to look for please?

16th Mar 2020, 9:50 am	#2
crackle Rest in Peace Join Date: Mar 2011 Location: Basildon, Essex, UK. Posts: 4,100	Re: Yaesu FT-840 power output issue. Dirty potentiometer?? Mike

16th Mar 2020, 4:07 pm	#3
Steve G4WCS Heptode Join Date: Sep 2004 Location: Preston, Lancashire, UK. Posts: 632	Re: Yaesu FT-840 power output issue. parasitic oscillation ? may benefit from checking all the electrolytic caps around the PA area.

16th Mar 2020, 7:51 pm	#4
mickm3for Heptode Join Date: May 2015 Location: Chatham, Kent, UK. Posts: 964	Re: Yaesu FT-840 power output issue. I have had this very same fault on this radio, was dirty pot, took longer to dismantle than fix Mick

16th Mar 2020, 7:57 pm	#5
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,902	Re: Yaesu FT-840 power output issue. Can also be dry joints on some radios. This makes output power uncertain and heating from use at higher powers can make it go bad just when you thought it was having a good patch. Hysteresis in the effect is common. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done