TRC 457 AC Hum on RX

[Blackpaw]

I've got a TRC 457 that's got some AC hum while RX'ing. Is this a symptom of failing caps in the AC/DC stage of the transformer? What I've noticed is that when I am bench testing a mobile radio in my shack (12VDC on a regulated PS) on my base antenna and no antenna on the 457 and RF gain turned down, I get a nasty AC hum on receive in the 457 while testing the other radio. I suppose I could attach a 12VDC power supply to the 457 and see if the problem goes away, but am hesitant to do so, I don't want to cause more problems by changing up how my 34+ year old 457 gets power. It's running just fine on it's own 120VAC to 12VDC supply. I had another mobile set up as a "monitor" radio, so I could try that. Looking for any input!

Thanks, 73's, happy holidays to all

Greg

 

[Robb]

A) Reverse the AC power plug.
B) If it has a +12v plug on the back for mobile use with a switch, plug in a 12v power supply and see if it sounds OK then.
C) If it does; then the filter caps of the power supply inside the radio are probably showing their age and need replacement.

 

[unit_399]

When you remove the AC power cord fro the 457, it will make the12 volt plug active. Connect an external supply or a battery to the rig and see if the hum goes away. if it does, check C304 in the power supply. If it's bad, replace it with a 10,000uf computer grade electrolytic.

- 399