Yaesu FT-840 no transmit PLEASE HELP!!!

[Lkaskel]

Hi Everyone,
I have on the bench a Yaesu FT-840 that quit transmitting. I am assuming that the owner did not open it up and change anything. I had found the voltage regulator section baked because of the 8.2 ohm resistor and I replaced/upgraded all resistors and replaced all of the caps in that section. I also replaced the 2 regulators and I now have good voltage in that section. This is a very well documented issue and can be seen on Mikes Radio YT channel. After the repair the radio still does not transmit. I then re-flowed the boards (also seen on Mikes YT channel), No transmit. This is a modular radio as many ham radios are. The service manual I downloaded does not show voltages on the schematics so I am unsure what the best process would be. I was thinking that I could test the transmit at the coax transmit jumper to the amp section. What should I expect to see? When I hook the scope up I get nothing. Any help would be appreciated!! I really want to get this back to the owner if possible.

Thanks as always!!

 

[sp5it]

Take a RF probe and go backwards from antenna socket to find where power disappear.
Mike

 

[Lkaskel]

Hi Everyone,
It's been a while. I have been blessed to have been very busty at work and therefor have had no bench time. It time to get back at it. I sat in front of this Yaesu again tonight and I tried something different. I injected a tone at the antenna to see if I would get any reception. I got nothing. So the updated diagnoses at this point is the radio does not transmit or receive. I see the suggestion from sp5it to use an RF probe. I do not have one but don't mind buying or building one. Do you have any suggestions on what to buy or build?

Thanks as always!!

 

[nomadradio]

Ahhh.. No receive?

Points to some function shared by transmit and receive, both. I'd worry more about getting the receiver to work before tackling transmit troubles.

Getting it to receive again might also fix the transmit problem, if it's a shared fault.

73

 

[Shadetree Mechanic]

Did it get hit by lightning, or a power line spike? I would look at the components that get voltage from the regulators you replaced. The components might have seen high voltage from when the regulators went out.