Grundig 2320 hum, capacitive coupling between chasis and ac line input?

[g***@gmail.com]

Hi,

I have a Grundig 2320 AM/FM/shortwave radio. I recently re-capped it and it has been working perfectly for a week. The sound was amazing. This morning I removed the volume control to spray it with contact cleaner (it was scratchy). When I put it back in I have awful hum.

Here is the schematic:
https://www.vintageshifi.com/repertoire-pdf/pdf/telecharge.php?pdf=Grundig-2320-Schematic.pdf

Here is what I have figured out so far:

- I checked the resistance between the heaters of the EABC80 and EL84 and the cathodes and they are fine, So i don't think there is a short there.

- Hum increases in loudness when I turn up the volume control.

- When I short the grid of the EABC80 to ground, the hum almost completely goes away.

- Disconnecting the wiper on the volume control from the grid circuit of the EABC80 does not get rid of the hum.

- Hum happens on all bands and the phono input.

- When I remove the EABC80 from the tube socket, hum completely stops, so I don't think the problem is in the audio power amplifier (EL84).

- When I ground the chassis to the ground of the wall outlet, hum gets much softer.

- When I plug the the radio into the wall outlet one way, it hums more than the other.

- When I connect a voltmeter between the chassis and the prong of of the wall outlet, it measures about 110 volts AC. Between chassis and the other prong, about 30 volts AC.

- Ohmmeter (goes up to 20 M Ohms) shows infinite resistance between chassis and both prongs of line cord (when disconnected from outlet).

- There is no connection between the AC line input of the radio and the rest of the circuit or chassis. No capacitors. It is only connected to the fuse, power switch, and transformer primary.

Does anybody have any clues? Again, it was working perfectly this morning, not a hunt of hum.

Thank you!

Gabe

[Peter Wieck]

Gabe:

In no particular order:

a) You miswired the re-installation and/or you pinched a wire in the process. Check the control very, very carefully.
b) You knocked something loose inside the control, and/or you rinsed some contaminants across the controls or wipers such that a current-path was created. This is more common than you might think.
c) You changed the location of the feed wires and/or displaced shielding or similar.

Note that Euro radios are exceedingly complex as compared to similar-vintage US-origin radios as they never use one part where four-or-more might do. Further, they integrate the chassis with the cabinet making even routine servicing an adventure. Were I to guess from here, I would suspect b), then a). But, it really could be either.

Peter Wieck
Melrose Park, PA

[Peter Wieck]

!!One more thing!!

That the hum reduced when the chassis was grounded strongly suggests that you shorted something to the chassis when you were messing with the volume control. Obtain a good VOM. With yourself not grounded, and the VOM on ACV, then DCV, check to see if there is _ANY_ current from the chassis to a known-good ground both when the radio is off and when it is on.

If so, UNPLUG that radio immediately, and do not plug back in - except through a proven isolation transformer - until the problem is identified and fixed. The life you save may be your own.

Peter Wieck
Melrose Park, PA

[g***@gmail.com]

Peter, Thank you for your quick reply!

a) I checked the wires very carefully, but I will check again.

b) I don't think it's a current path in the volume control because I tried isolating the control from the chassis by both suspending it in mid air and resting it on a piece of cardboard not touching the chassis. No change in hum.

c) I didn't move the wires around at all, and there was no shielding. Also, Before I played with the volume control, I tried the radio out in the position that I was working on it: upside down on bench, no cabinet, wires running from audio transformer to cabinet speaker a few feet away (so I could hear the sound), and the radio sounded fine.

d) About the chassis being shorted. Why would messing with the volume control short the chassis to the AC line? The AC line comes on the other side of the radio and neither neither AC wire touches chassis or any other part of the circuit except the fuse, transformer primary, and power switch. I will do what you say though by checking current from chassis to ground with VOM.

One more thing. The volume control is a 1.3M pot, with a tone control tap about 400k from the bottom end. I have a 1M pot. If I try replacing it with that, what should I do with the tone control wire?

Gabe

[Peter Wieck]

Gabe:

Please note the interpolations.

Post by g***@gmail.com
Peter, Thank you for your quick reply!
a) I checked the wires very carefully, but I will check again.

Yes. The pinch/short can be anything from a stray strand to a bit of solder bridging. A magnifying glass would no go amiss.

Post by g***@gmail.com
b) I don't think it's a current path in the volume control because I tried isolating the control from the chassis by both suspending it in mid air and resting it on a piece of cardboard not touching the chassis. No change in hum.

I mean inside. A bit of carbon or some such rinsed up against the VC cover providing a current path.

Post by g***@gmail.com
c) I didn't move the wires around at all, and there was no shielding. Also, Before I played with the volume control, I tried the radio out in the position that I was working on it: upside down on bench, no cabinet, wires running from audio transformer to cabinet speaker a few feet away (so I could hear the sound), and the radio sounded fine.
d) About the chassis being shorted. Why would messing with the volume control short the chassis to the AC line? The AC line comes on the other side of the radio and neither neither AC wire touches chassis or any other part of the circuit except the fuse, transformer primary, and power switch. I will do what you say though by checking current from chassis to ground with VOM.

Note that the entire B+ goes through the audio output transformer primary. Such that any "misplaced" current will jump directly into the audio path.

Post by g***@gmail.com
One more thing. The volume control is a 1.3M pot, with a tone control tap about 400k from the bottom end. I have a 1M pot. If I try replacing it with that, what should I do with the tone control wire?

I would suggest you find a tapped pot. Not easy, but they are certainly out there:

https://www.ebay.com/p/1963-CTS-2-Meg-Ohm-d-Shaft-Dual-Stereo-Potentiometer-Volume-Pot-Loudness-Tap/2208509098

***@comcast.net And Mark can make them to order.

You could create an RC network around the non-tapped pot, and the tolerances are not excessive. But if you can find a reasonable tapped pot off-the-shelf, go that way. You could also try rinsing the pot again, that might dislodge the first cause.

Good luck With It!

Post by g***@gmail.com
Gabe

[g***@gmail.com]

Post by Peter Wieck
Please note the interpolations.

Post by g***@gmail.com
Peter, Thank you for your quick reply!
a) I checked the wires very carefully, but I will check again.

Yes. The pinch/short can be anything from a stray strand to a bit of solder bridging. A magnifying glass would no go amiss.

Post by g***@gmail.com
b) I don't think it's a current path in the volume control because I tried isolating the control from the chassis by both suspending it in mid air and resting it on a piece of cardboard not touching the chassis. No change in hum.

I mean inside. A bit of carbon or some such rinsed up against the VC cover providing a current path.

Post by g***@gmail.com
c) I didn't move the wires around at all, and there was no shielding. Also, Before I played with the volume control, I tried the radio out in the position that I was working on it: upside down on bench, no cabinet, wires running from audio transformer to cabinet speaker a few feet away (so I could hear the sound), and the radio sounded fine.
d) About the chassis being shorted. Why would messing with the volume control short the chassis to the AC line? The AC line comes on the other side of the radio and neither neither AC wire touches chassis or any other part of the circuit except the fuse, transformer primary, and power switch. I will do what you say though by checking current from chassis to ground with VOM.

Note that the entire B+ goes through the audio output transformer primary. Such that any "misplaced" current will jump directly into the audio path.

Post by g***@gmail.com
One more thing. The volume control is a 1.3M pot, with a tone control tap about 400k from the bottom end. I have a 1M pot. If I try replacing it with that, what should I do with the tone control wire?

https://www.ebay.com/p/1963-CTS-2-Meg-Ohm-d-Shaft-Dual-Stereo-Potentiometer-Volume-Pot-Loudness-Tap/2208509098
You could create an RC network around the non-tapped pot, and the tolerances are not excessive. But if you can find a reasonable tapped pot off-the-shelf, go that way. You could also try rinsing the pot again, that might dislodge the first cause.
Good luck With It!

Post by g***@gmail.com
Gabe

I removed the fuse and switch from the circuit and connected the line cord directly to the transformer primary wires. When plugged into outlet, the meter reads 5 uA AC between chassis and ground prong of same outlet. When I reverse the plug, it reads 25 uA AC. Could there be some leakage between the primary and secondary winding in the transformer? Or perhaps this could be capacitive coupling between the windings, or between the wires in the primary and the metal case of the transformer, which is screwed to chassis. What would be a normal ammount of leakage current?

I removed all the wires from the volume control back to the terminal strips, and soldered brand new wires to a 1 meg pot. I've checked the wiring probably 5 times now against the schematic. There was no change in hum compared to the original volume control.

When I connect the grid of the EABC80 to the ground, the hum diminishes. When I connect the grid of the EL84 to ground, the hum almost goes away completly, but there is still a slight noticble hum in the speaker.

Also, when I used to turn on the radio, there would be no sound at all until it warmed up. Now when I turn it on, there is immiedtly a faint hum in the speaker, which then becomes loud after about 10 seconds.

[Peter Wieck]

OK - More speculation:

a) Did, by any chance, you reverse an electrolytic cap? If so, it will hold for a while, then fail. More-or-less gone are the days when caps treated this way would explode spectacularly, but it is something to check.
b) Have you checked the volume pot for any leaks or shorts?
c) Have you checked for any bad rectifiers? Is the OEM rectifier getting warm in any way?
d) Have you measured for AC on the rectifier output and/or in the B+?

And, to your best guess, is the hum 120 Hz or 60 Hz?

Peter Wieck
Melrose Park, PA

[Peter Wieck]

Gabe:

Any updates?

Peter Wieck
Melrose Park, PA

[MarkS]

Post by g***@gmail.com
Hi,
I have a Grundig 2320 AM/FM/shortwave radio. I recently re-capped it and it has been working perfectly for a week. The sound was amazing. This morning I removed the volume control to spray it with contact cleaner (it was scratchy). When I put it back in I have awful hum.
https://www.vintageshifi.com/repertoire-pdf/pdf/telecharge.php?pdf=Grundig-2320-Schematic.pdf

My neighbor just asked me to see if I could fix his Rekord Senior 60. This is also German radio from about '59. The radio's problem? Hum! The schematics between the Grundig and this Senior 60 are very similar, has the same power supply arrangement 50/50/8 lytic, output transformer arrangement etc. Assume Rekord is some offshoot of Grundig. I plan to dig into this thing over the next few weeks so will see what's what.