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40-190 hums intermittently - Printable Version

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40-190 hums intermittently - fossilexpress - 07-23-2013

Hi all - new to this great group and pretty inexpereinced with radios. Have two Philcos, a 37-620 and 40-190. The former was professionally repaired by a retired repairman (found it thrown out in the street!).

The 40-190 is in pretty good shape, except for some dings on cabinet, including a tiny bit of missing veneer. It is also missing the cardboard tube around the built in antenna. The unit does play both AM and "Police" bands, but there is a noticeable hum. The odd thing is that this comes and goes if I turn and/or tap the volume button. The shortwave stations similarly drop in and out, with pretty poor reception overall.

I have tested out all the tubes and they were ok when I last checked - I do own a tube tester. So: what is my next step(s)? Should I replace the capacitors? Does the volume pot need to be cleaned and or replaced? Anything else? Any advice appreciated. Thanks!

RE: 40-190 hums intermittently - fossilexpress - 07-24-2013

A bit more detail - the hum comes and goes with any tapping at all.

RE: 40-190 hums intermittently - BrendaAnnD - 07-24-2013

Sounds like a poor ground. As for the poor SW reception, you probably need to attach an antenna. The internal one doesn't do much for it.

RE: 40-190 hums intermittently - fossilexpress - 07-24-2013

Thanks. I actually don't have it grounded yet. Will do so immediately. I have a short wave antenna on the other radio, will switch it over and give a try.

RE: 40-190 hums intermittently - BrendaAnnD - 07-24-2013

I was referring to a chassis ground inside the set. Is the hum 60 Hz or 120 Hz?

RE: 40-190 hums intermittently - Mondial - 07-24-2013

You might check if the nut mounting the volume control bushing to the chassis is loose. Sometimes if the case of the volume control is floating with respect to ground, a hum problem can occur. This might be the case as you mentioned that just touching the volume knob or shaking it can make the hum occur. It certainly seems that you have an intermittent loose ground connection somewhere within the audio circuitry.

RE: 40-190 hums intermittently - fossilexpress - 07-24-2013

Ok. It is a 120 HZ hum. I have an app on my MacBook that does sound frequency analysis. BTW, does the unit need to be physically connected to an external ground anyway?
Sorry for all the newbie questions!

RE: 40-190 hums intermittently - fossilexpress - 07-24-2013

Curiouser and curiouser! I took the chassis out and brought it into my shop. When I flipped it up to look at the bottom, it was pretty clear (I think) what the problem is. There are three capacitors, which look like they are replacements (they say "Mallory") that were not soldered into place, but were simply twisted on the existing wire. They are quite loose! So - before I bring out the soldering iron - any idea why this was done? Anything I should watch for?

RE: 40-190 hums intermittently - vinzer - 07-24-2013

You should replace them if there are very old and make sure the new ones are not hooked up to old original ones.

RE: 40-190 hums intermittently - fossilexpress - 07-24-2013

They look relatively new, but there is an old one in clearly bad shape. I will get a replacement. Does Radio Shack still carry such things?

RE: 40-190 hums intermittently - BrendaAnnD - 07-24-2013

Shack hasn't had anything applicable to tube stuff in decades. They quit selling component parts some time ago. There are lots of places you can order from, though, such as Mouser and Digi-Key. Also AES and others.

Sounds like you found the problem all right. If any of those twisted wires are loose, they could be oxidized and breaking contact, which would cause your hum.

RE: 40-190 hums intermittently - klondike98 - 07-24-2013

It might be useful to do a little reading before you go too much farther if this is all new to you. A good start is here: http://www.antiqueradio.org/begin.htm and then some info on capacitors here: http://www.antiqueradio.org/recap.htm . Repairmen would sometimes tacked on capacitors when the original would fail in the radio but they did not always disconnect the old ones. In your case it sounds even worse in that the tacked in caps were only twisted on. As vinzer suggested they should be replaced with new ones. You can find a schematic and some info on your model here: http://www.audiophool.com/Philco.html .

RE: 40-190 hums intermittently - fossilexpress - 07-24-2013

The most incompetent repair job I have ever seen (on anything; I usually do toy trains). Looks like the old caps and a resistor were cut out and the replacements twisted on. I have now properly soldered most of the connections (one or two more to go.) I will give it quick try when done, and then order new replacements for the lot, as well as the remaining original caps. Will check back in when done. Again, my thanks for all your advice!

RE: 40-190 hums intermittently - Phlogiston - 07-25-2013

I have seen that done when people were hunting for bad caps. They clip one end and then twist it back together.

RE: 40-190 hums intermittently - fossilexpress - 07-25-2013

Hum is gone! Still some static, but much improved! Now to order a full set of new caps....