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My Brand Z G725 - Eric Adams - 12-25-2014

Hi Guys,

Here's my latest project Zenith Model G725 . When I got it all it did was have a loud hum at any volume. I replaced all the electrolytics and wax caps. Replaced any resistor out of specs. I tested all the tubes and they check ok.
So, now, with the volume turned all the way down I get a loud hum. Once I move the control any amount, the hum goes away and the radio sounds great-AM and FM.
Any thoughts on what might be causing this?

Thanks,

RE: My Brand Z G725 - OldRestorer - 12-25-2014

I am far from a chassis guy so I wont give any advice but I like to respond to posts I read.

Kirk

RE: My Brand Z G725 - Ron Ramirez - 12-25-2014

Heater-cathode short in a tube? That's the first thing that comes to my mind...

Also make sure all leads are properly soldered to the volume control; if the low end of the control comes unsoldered from ground or B-, it could also produce this effect.

RE: My Brand Z G725 - Eric Adams - 12-25-2014

Thanks Ron,

I swapped each tube with known good ones, one at a time. I resoldered each terminal of the vol control. Could it be the vol control itself?

RE: My Brand Z G725 - Ron Ramirez - 12-25-2014

Could be...measure across the two outer terminals and see if you get anything close to the rated resistance. If not, then you have a problem.

Preferably using an analog VOM, measure between the center terminal and one of the outer terminals. Holding the test leads in place (alligator clips come in handy here), slowly turn the control from one end to the other. If the resistance does not change smoothly but jumps open here and there...then you have a problem.

RE: My Brand Z G725 - Jayce - 12-25-2014

This might sound silly, but did you clean the controls? Sometimes a dirty control pot or even tube socket can cause hum as well.

RE: My Brand Z G725 - Eric Adams - 12-25-2014

No problem with any resistance readings, Ron. Jayce, I wish it were that simple. That was done while replacing all of the components.

RE: My Brand Z G725 - morzh - 12-26-2014

The box of the pot is groundedvi a a cap.....have you cheked it?
Also, the quality of the speaker voice coil grounding, look at it.

RE: My Brand Z G725 - Eric Adams - 12-26-2014

The cap checks good, Mike. Voice coil ground is ok.
Now, for the good news. There's a 1k resistor that goes from the low terminal of the volume control to ground. The 1k resistor was right on. I jumped it with another resistor and the hum went away. After some experimenting I settled on a 470 ohm resistor to replace the 1k. I'm sure someone can tell me why it works, but it sounds great now!
Thanks everyone for your input. I hope you all had a good Christmas and looking forward to a healthy and happy new year.

RE: My Brand Z G725 - morzh - 12-26-2014

Could be an inductive wirewound resistor you had before that picked up noise.....which then could mean that if you re-oriented it the noise could've been attenuated. A wild guess.....

RE: My Brand Z G725 - Eric Adams - 12-27-2014

Nope. Carbon resistor.