Yaesu FT-450 hums on transmit

It was recently brought to my attention that my Yaesu FT-450AT hums when I transmit. A friend of mine told me that the hum can be caused by the display dimmer, and can be cured by setting the dimmer to it's highest or lowest setting, and is caused by bad grounding between the faceplate and the chassis.

See this (closed) thread for reference: http://forums.qrz.com/showthread.php?152913-new-ft-450-hums-on-TX

The dimmer can be set between 0-4.

I hooked up a monitoring oscilloscope and set the dimmer to it's lowest setting (0):


Then I set the dimmer to the medium setting (2), which is where I always have it. Yuck:


I took the top and bottom covers off the radio and removed the faceplate. On the inside of the faceplate are three bendable metal tabs that make contact with a flat plate that sits between the faceplate and the chassis:




The recommended fix from Yaesu is to bend those tabs so that they make better contact with the metal plate. This just seemed wrong to me, so I came up with a completely over-engineered solution that fixes the problem permanently.

1) I identified a good and convenient spot on the bottom board to run a ground wire from the ground point to the faceplate. I used an ohm-meter to make sure that the flat plate and the spot I picked are at the same ground potential.


2) I bought some snap connectors from Radio Shack to make a connection that can be disconnected if I want to remove the faceplate. I used 14AWG stranded wire and soldered one end to a ring connector, and screwed that into the spot on the board that I picked in step 1. I soldered the other end to one half of the snap connector.

3) I cut three lengths of 20AWG stranded wire and soldered one end of each wire to each of the three bendable tabs on the faceplate. I then soldered the other end of each wire into the other half of the snap connector. That way, each of the three ground-points on the faceplate will be connected to a common ground point, which should prevent a ground-loop.


4) I used heat-shrink tubing to cover the metal tabs so that the tabs can no longer make electrical contact with that flat metal plate. Also used heat-shrink on both ends of the snap connector for neatness.




5) Here's how the finished product looks. The 14AWG wire is run through a slot that's underneath the rubber piece, and then through one of the existing holes in the flat metal plate. I made the wire a little longer than it needs to be so I can easily remove the faceplate without disconnecting the snap connector.


6) Put the faceplate and chassis covers back on and retested with all 5 dimmer settings. No trace of hum on my oscilloscope.

 

You would have thought the engineers that designed and manufactured the FT-450 would have caught that problem and resolved it before putting it out. But that's just me.
Good information and well presented thanks for the post.
73
Gary

 

One thing I forgot to mention is that the hum was only there when the MIC audio was connected. I tried putting a PTT switch into the MIC jack and used that to transmit and there was no hum. The hum only appeared when I connected the MIC audio. I tried with two different MICs just to rule out a problem with the MIC itself.