HI, I have an S-38E that hums loudly for a few seconds after I shut it off, it sounds like the filter caps are discarging through the audio amplifier section, the filter and wax capacitors have all been replaced. I also have a National NC-60 that does the same thing, but my other S-38E does not have the same problem and the radios work fine otherwise. Any ideas? Thanks!

Does this change when you reverse the line cord?

When the line cord is plugged in the other way the radio hums like the filters are bad, but this problem does not happen.

:Does this change when you reverse the line cord?

Well, I'm looking into my crystal ball here. I know the problem, but I'm having difficulty discribing it. Are you grounding the chassis? The schematics shown on this web site show this set as an AC-DC set. AC-DC sets do this when the chassis is grounded in some way (a chassis that is connected to the line through a condenser). The explanation for this is long and complicated.

Thomas

It is not a wise idea to ground AC-DC sets. Sets that have the chassis isolated from the line through a condenser (such as yours), however, technically can be grounded (the chassis, that is). The hum problem you experience is this: Omit the idea in your head for one moment, that ground is some sort of magical cure-all shield. Remember that one side of the AC line in your home is tied to ground. Now if you plug in your radio receiver so that the chassis is grounded (through the condenser) through the line cord, and you also ground the chassis directly with a wire to a water pipe, all audio circuit control grids will be at this same potential. No hum will be heard. When you turn off the radio, the switch as it is wired disconnects all of the components that were at ground potential from ground. These components are now connected to the "hot" side of the line through the tube filaments. The chassis, however, since it is directly connected, is still grounded. Once again, omit the idea that ground is some sort of a shield. THink of it as only the other side of the AC line. All of the grid circuits in the audio section are now at the "hot" potential with the switch off. The chassis is at the "cold" potential, which is opposite in phase to the hot side. The capacitive effect of the chassis radiates this opposite phase into the grid circuits of the audio tubes. Since the electrolytics retain a charge for a moment, the amplifier will amplify this radiation and you will hear a hum similar to that when you touch the control grid of one of the audio tubes with your finger.

Now, if you reverse the line cord so that the switch feeds from the "hot" side, your radio will hum when it is on. THe reason is the same as before, only all the grid circuits that are at the same potential as the switch will be directly fed from the hot side. The chassis will be at the ground or "cold" side. The opposite phase of the chassis will radiate a signal into the grids of the audio tubes and the radio will hum while it is playing. If you turn the radio off, it will now cease to hum. With the switch open, the hot side is disconnected from the radio. All circuits are fed from the cold side through the tube filaments. They will then be at the same 60 cycle alternating current potential as the chassis, and no signal will be imposed upon the grids.

Certain circumstances can also bring on this condition with AC sets, but it is not as problematic. Placing a condenser from the chassis to the line cord and then having this side of the line plugged in so that it is hot will cause this hum. The simple remedy is to reverse the line cord.

The problem you have with your AC-DC set is not really a problem, but perhaps an annoyance. As a rule it is not a good idea to ground an AC-DC set even though the chassis is isolated from the line cord by a condenser. Still, in many radios a condenser is placed across the line cord to eliminate radio interference. There is no more chance of the condenser to your chassis shorting out than there is of this condenser shorting out. Under the right circumstances, both would cause the same result--a shorted line cord (blown house fuse). It is not necessary, however, to ground by wire an AC-DC chassis. The condenser to the line cord affords enough grounding.

The problem I described above can perhaps be described with much more technical detail than I have used, which may paint a clearer image in your mind.

Thomas

Thanks for the info. I did a little investigating and found out that down the line of the power outlet I was using is a GFCI outlet, when I use another outlet in the house this problem goes away. Do you think that the GFCI can be at fault here?

Could be, but I have never had that trouble. Are you setting the radio on any metal surface or something that would ground the chassis (if the cabinet is metal)? If someone miswired the outlet, this could have caused the problem. If someone rewired the radio with a 3 wire plug (grounded), this could cause trouble, but you say that you can reverse the plug, which is not possible with a grounded 3 prong plug unless you use an adapter.

Thomas

You may also try shielding the 1st audio tube. Does it do this with the volume control all the way down or only with it turned up? Are the volume control and the switch one unit (making it impossible for the volume to be up when the receiver is turned off)? Also, if it does this with the volume control turned all the way down, is it possible that someone miswired the volume control? The lead that goes to the condenser that goes to the grid of the first audio tube should go to the center terminal of the volume control. Connecting it to an end terminal will leave the grid wide open to interference even with the control turned down.

It is somewhat normal for amplifiers of any make to hum when turned off by the power switch if the volume control is able to be left in the medium to high position.

Thomas