[Jason63]

This past Summer, I rescued a 42-380 from a neighbor's garage where it had been stored for several decades.  The elements were not kind to the radio, but I thought it would be a fun project.  Not being much for woodwork, I paid a pretty sum to have the woodwork restored and took the radio to a local repairman to restore the radio.  Though I have a decent electronics lab in my basement, I've been busy with work and figured someone with experience repairing these radios would be able to do a better job than I could.

Several months later, the radio was finished.  A quick check sounded fine at the repair store, so I paid the man and took it home.  After setting it up in my living room, I started playing with it.  Sure enough, there was an annoying 120 Hz hum present throughout the AM broadcast band, especially bad on the lower end.  The hum is present with or without a station being tuned, although it is more noticeable when some stations are tuned than others.  I contacted the repairman and he was clueless about the cause.  Given the hassle of transporting the radio back to his repair shop again, I decided I would attempt the repair myself.

The first thing I did was go through the full alignment procedure specified in the Philco 1942 RMS Year Book.  It was clear my repairman had not performed this alignment given how far off some of the controls were.  When I finished, the dial was considerably more accurate and the sensitivity was also noticeably improved, but unfortunately, the 120 Hz sum was still present.

Looking through the schematic, I note the only place where 120 Hz is generated is the rectifier circuit consisting of a 6X5G tube connected as a full-wave bridge rectifier.  The output is low-pass filtered by a PI-filter consisting of C61, L59, and C20A.  C61 was a 475V 18uF electrolytic capacitor.  L59 is the field coil in the speaker.  C20A was a 475V 8uF electrolytic capacitor.  A look inside the radio revealed that the electrolytic capacitors have been replaced as follows:

C61:  22uF at 450V
C20A:  22uF at 450V

I also measured the inductance of the field coil at 10.8 H at 120 Hz with a series resistance of 6.5 kOhms.  The schematic says it should be closer to 1.1 kOhms which means I need to have a closer look at the speaker to see what could explain this large discrepancy.

Closer inspection of C20 and C20A revealed that one of the capacitors was dented due to the overzealous tightening of a retainer clip on the part of my repairman.  I decided to remove all electrolytic capacitors and test them further:

C20:  21.2 uF, ESR of 2.48 Ohms, leakage current of 0.45 mA at 400 Volts.
C20A:  22.1 uF, ESR of 1.96 Ohms, leakage current of 0.55 mA at 400 Volts (this one is dented).
C61:  21.6 uF, ESR of 2.3 Ohms, leakage current of 0.32 mA at 400 Volts.

According to the Nichicon specification, I should have seen leakage currents of under about 0.9 mA, so I guess those values are within specification.  The ESR values seem high to me, though I'm accustomed to lower voltage applications so perhaps they are reasonable given the ~200 Volts they have across them during normal operation.

Next, I did some searching on the forum to see if I could find any other ideas to try.  I found a note about R60 -- a 15-30-146 Ohm compound resistor used for biasing.  Sure enough it had already been replaced by my repair guy and was reading correctly when I checked it.  I spot checked a few other original resistors for correctness and found them to be within 20% of their rated value, drifting high.

The wiring inside the chassis looks very questionable from a modern perspective where we work with fast-changing signals, but I'm assuming outside of avoiding routing the power lines close to any RF input stages, it should be fine.  Some of the original wiring was replaced by my repair guy and I can only assume he routed it in the same manner as he found it.  I'm a bit worried about the placement and wiring of C20 given that it appears to provide additional power supply filtering to the IF stages and is placed next to the power supply filter capacitors.

I'm getting ready to hook up an oscilloscope to the radio and start probing around to see if I can find anything.  I'll use two probes in A-B mode in order to avoid the floating ground issue that would otherwise be a problem.  Meanwhile, I'd certainly appreciate any advice or suggestions anyone here can provide as I've not worked on any electronics of this vintage before and have probably neglected to try many things which would be obvious to those who have.

--
Jason

[morzh]

Do you have any switching converters / ccgl lamps etc at home?

Often times the hum is a result of them working. You could try switching things one by one to see if it disappears.
Also if you have a transistor radio, set itnearby and see if you have the same hum.

[Jason63]

I do have quite a few switching converters at home.  As a SWL enthusiast, I have tested most of them and replaced the small handful that have noticeable EMI issues.  However, your point is well taken as they are constantly being added whenever a new appliance is purchased so it would be easy for me to miss one.  I just tuned my Icom to the AM broadcast band and I do in fact hear a fair bit of what I would describe as shot/ignition noise there, which diminishes at higher frequencies.  The main difference in behavior between it and the 42-380 is that when I tune in a reasonably strong AM station on the Icom the noise disappears.  Not so on the 42-380 where the 120 Hz hum remains quite audible on most, but not all stations.  As far as CCFLs go, I can't be certain but I think most of the displays in the house (TVs, computer displays) are LED backlit.  It would be an interesting experiment to selectively power off each circuit in the house while listening to the 42-380 or Icom to see if that might locate the source of the problem.  I will add that to my list of things to try.

--
Jason

[Radioroslyn]

So if you turn the volume all the way down do you hear the hum??? Does the volume control have any affect on the hum??

[klondike98]

welcome to the phorum!

[Jason63]

Terry:

The 120 Hz hum rises and falls in volume with the volume control.  When turned all the way down, it is inaudible.

If you were suspecting the field coil, I measured it with an Ohmmeter yesterday at it came in around 1040 Ohms  -- close enough to the 1100 Ohm value specified in the schematic.  The 6.5 kOhm ESR value at 120 Hz is much higher due to the field coil's high inductance at that frequency.

Bob:

Thank you.  I am very happy to find and join such an active community of Philco radio enthusiasts!

--
Jason

[morzh]

You are overcomplicating stuff. You don't need to know field coil ESR as it is irrelevant to anything so much so Philco never shows it anywhere, showing only DC resistance, nor do you really need lytics' ESR. As long as you use proper lytics (most could be any type except the one right after the rectifier - high ripple rating is preferred there).

If your hum appears stronger in certain places of the dial, this is likely an outside interference source(s).
Could even be a power transmission line nearby.

Again, a good way to tell is using another radio.

As for the chassis, you could try to ground detector output (or top of the volume regulator), see if hum persists.

[Jason63]

Thank you for the suggestions.

I tend to measure everything when I take electronics apart, because it's easy to do so when it's apart and not so much when it's all assembled. You're probably right that I will never need most of the measurements, but they are occasionally useful.

As for the capacitors, I thought it would be a good idea to at least measure the leakage current near their rated voltage. I've seen plenty of capacitors which measure fine in terms of capacitance and ESR fail when a higher voltage within their rated ranges is placed on them. Just wanted to be certain this wasn't going the case. The other measurements were gratis since I already had the caps out of circuit at that point.

There is a strong AM radio station here at 1410 kHz. When I tune it up on another radio, there is no noticeable hum on it. However, when I tune it on the Philco, there is a distinct 120 Hz hum. This is distinct from typical EMI. In particular, looking at a real-time power spectrum of the audio output shows a significant peak at 120 Hz on the Philco -- something that is not present on another radios tuned to the same frequency in the same room.

I suppose many would just live with this annoyance, but I can't believe this is normal behavior. If it is, then I suspect there are modifications that can be made to reduce it to a more acceptable level. In any event, it's a learning opportunity for me to familiarize myself with this old tech and maybe expand my collection if it continues to be an enjoyable experience.

If I understand the schematic correctly, the plate of the 7C6 is the detector output, and is biased to 100 Volts through a 220k resistor. Given that hum increases and decreases in proportion to the volume, and disappears completely when the volume control is at minimum, doesn't that rule out the detector and audio amplifiers as the source of the problem?

I should probably go over all of the work my repairman did before pulling any more hair out trying to track down this problem -- it would be unfortunate to discover the problem is caused by using the wrong-valued component somewhere or incorrect wiring.

--
Jason

[morzh]

Audio amp - yes, detector - no. Then again, detector is rarely the problem, if ever.

Does gounding he chassis make any difference?

At any event, if you could by tracing (say by pulling one by one RF tubes before detector) see if the hum stops at wone point, then it is likely an interference. I saw several cases where a radio sounded fine before having been relocated, usually those were cases of electrical noise, in Mains or radiated by home dvices / Power lines.

Moving the radio inside the house could show difference, switching off lighting sources one by one. In my old house it was the fixture in the lobby. Sometimes the opposite is true: turning on some loads attenuates hum. Or maybe even binging it
back to the repair guy's house to see if the hum is there.

The other radio you have - is it a transistor? Or does t have a frame/loop/ferrite antenna? If yes, does rotating the radio around gets the hum to appear?

[Radioroslyn]

OK so if there was an issue w/the p/s filtering the ripple would be throughout the set and would not be effected by the volume control. +1 Mike I thinking that it's rf interference from somewhere. OBTW your p/s doesn't use a full wave bridge circuit, that would have four diodes. Just a full wave.

GL

[Jason63]

Indeed I meant to say "full wave rectifier" and not "full wave bridge rectifier" -- I corrected that. Either way the output has a 120 Hz frequency component which led me to suspect it. In fact I'm still convinced this is the source of the hum, though thanks to Morzh's suggestion, I believe I've found the source of the problem.

Moving the radio to a different position had no effect on the 120 Hz hum. However, rotating the frame antenna inside the chassis did have a significant effect on it. In particular, whenever the wires from the frame are near the speaker, the hum is significant. And when they are as far apart as possible it is essentially gone. Unfortunately, it wants to rest in a position with the wires near the speaker, but I think it will be easy to use a wire tie to hold it in the correct position to eliminate the hum.

I'm guessing that the 120 Hz ripple current going through the speaker's field coil is being radiated by the metal body of the speaker and this is finding its way into the front end when the metal frame antenna passes near the speaker. I can certainly try a few more things to confirm this, but it's nice to at least be getting closer to solving the problem.

I do notice a tremendous amount of noise on the lower end of the AM radio dial, but it is not a 120 Hz hum and is very different in character. I think this is the EMI from all of the wall-warts, CCFLs, and other devices that have been mentioned here and are common around the home these days.

Thank you everyone for your ideas and suggestions. If anyone has any other ideas for me to try, I'm happy to give it a try and report back here. Otherwise, I'm inclined to call this problem resolved.