[Eric Adams]

This a strange one. All caps replaced resistors checked, replaced as nec. Tube test ok. Radio aligned and sounds great. Just one thing. This sound when I shut it off. Any ideas?
Check it out: http://youtu.be/GE-8t0KIIJM



Schematic: http://www.nostalgiaair.org/Resources/991/M0014991.htm

Thanks,
Eric

[morzh]

I think it reminds me of a microphone effect.

Look for anything that can resonate at this frequency.
A tuning cap, tube.....basically it looks like the power switch introduces a quick jerk annd then something resonates at the frequency you hear.

[Eric Adams]

Thanks Morzh, I'll check those things. Have you ever had this problem? Do you think it could be caused by the switch?

Eric

[morzh]

yes it potentially can be the switch itself if it bounces a lot.

here's the test (somewhat dangerous...that is requiring observing some safety, so be careful. Use an isolation transformer and never hold two wires or a wire and the chassis with both hands at the same time....you know the spiel);

Desolder one wire to the switch, turn the switch on and keep it on, so you can have the volume where it works, and use that desoldered wire to make/break connection. If the sound persists, it is not the switch, if it goes away - it is the switch.

[Eric Adams]

Well Morzh, it looks like it's the switch. My deoxit order should be here on Thurs or Fri. I'll get back to you after I treat it.

Thanks,
Eric

[morzh]

You're alive, that's a relief.

Deoxit is a good stuff but it may not necessarily cure the problem: if the switch has an excessive bounce Deoxit won't do a dang thing about it. It is mechanical. Then again, if this is the extent of your problem, it's nothing really bad.

[Eric Adams]

Yup! Gotta keep one hand in the pocket!

I'll try the Deoxit before I take the switch apart.

Thanks,
Eric

[Paul Philco322]

I have one of those, your set is prettier, mine does not make that sound. Actually for a small set they perform quite well. I DX on it sometimes.

Paul

[Eric Adams]

Thanks Paul, it took a lot of cleaning and polishing. I haven't got my Deoxit yet and from the testing I'm sure the problem is in the switch. If I put pressure on it and shut it off there is no noise.

Eric

[codefox1]

Yup, just about always that. Causes a transient that briefly saturates output transformer above its limit to suppress such stuff. Best fix it lest you looking for new iron.

[Eric Adams]

Hey Morzh,

That did it. Just a little spray into the control and...no noise.

Thanks,
Eric

[codefox1]

Yup! Probably has hsppend to most of us.

[morzh]

Great! Well, dirt also causes bounce. Obviously.