Boredom got the best of me: AA5 hum and loop antenna
 

So I revisited this old Emerson I recapped a few years ago but never used. It's a 50A-1 standard AA5, and after recap, had a significant hum (vol up or down) and distorted sound.

It had the dual section filter changed before I got it so I just assumed it was wired wrong and I put it away for another time.

Time being now, I took another look at it and confirmed the filter was wired correctly.

To cut to the chase it was a 12AT6 with heater to cathode leakage. I mean, this replicated the sound of a bad filter perfectly.

With too much time on my hands, I tried measuring the leakage with an ohmmeter (something I never tried) and indeed it read 2.5M ohms from the cathode to the heater. I put it on my leakage tester and it was reading about 20ua at 100V. I then put the leakage tester on 600V (what the heck) and after a few minutes, the leakage dropped like a reforming capacitor until it read zero leakage.

Back in the radio, the hum was gone. I let it run a few hours, and still quiet. I tried it again the next day and still no hum.

I pulled it back out, marked it as test only with int hum and put the new one back in.

BUT, I have a question specifically regarding the loop antenna which has *three* wires on the antenna board. The wires were broken off, but I wired the loop section between the osc and first IF tubes as normal, and indeed it works fine. The third wire on the antenna board is not electrically connected to the loop, but is a wire about 4" long that runs along side the loop coupled inductively.

The schematic shows it, but shows it not actually connected to anything - just hovering over a ground symbol.

Here is the schematic: http://www.nostalgiaair.org/PagesByM...1/M0004651.pdf

Any ideas where the wire goes?

John

Usually this kind of second loop was used to add an external antenna to radios which are line powered. The inductive coupling provides isolation from the AC line. Is there any evidence of an antenna terminal on the back of the radio?

If I read the schematic correctly, there should be both an antenna terminal and an earth terminal on the back.

None. I'll take a pic and post it tomorrow.

Thanks.

John

There are no terminals, but then, the schematic I linked may not be the exact one. I searched on line for it the exact model and someone on a different public forum said this the one I linked was the same chassis, but perhaps not.

The tube lineup is the same, the hard parts layout is the same, the tuner and dial cord stringing is the same, and all the resistors and caps are the same, so I assumed it was the same chassis with a different cabinet. The schematic even shows the loop and the third unattached segment.

The antenna loop board makes up the backboard, and there are no external connections to the radio of any kind.

I'll take a pic and post it tomorrow. Thanks.

John

Here's a link to the exact radio. No model number:

https://a2zmodern.com/products/other-88

One of the pictures shows the rear antenna board. No external connections. This picture shows evidence that someone soldered a wire to one of the connection rivets, but my radio shows no such external wire was ever soldered to the rivet.

John

According to alignment instructions 2-3 there would have been antenna and ground wire lead connections hanging out the back. Was a common practice on low end sets.

John,

Sorry for making three posts to answer one question. This thread with pictures sheds more light on the situation.

https://antiqueradios.com/forums/vie...307249&start=0

Are you kidding? Dad always told me that beggars can't be choosers!!! I'll take advice any way it's packaged. :yes:

I think I understand now. The point of the unattached segment on the loop board is to be able to add an external antenna while providing isolation from the chassis, yes?

Thanks again.

John

That must be the answer, although there is not a terminal on the back. Depending on which way the plug is put in the socket, the loop section of the antenna can read 115V above ground, so we definitely don't want to connect anything external to that.

The isolated section of loop of course has no voltage on it. Kevin provided a picture of a similar Emerson that ran a wire outside the back, so it must have been shipped with a coil of wire wrapped up around back. The wire on mine was long enough only to reach some point on the chassis, so I thought it was connected internally somehow. Must have been cut at some time in the past.

Thank you sir.

John

Yes that's my understanding as well. And if you notice on the schematic they show an earth ground symbol for the external antenna, which is not chassis ground. In theory when it's wired correctly you would have your antenna on one end of the isolated loop and earth ground(water pipe) on the other. Doing that should isolate anyone touching those two connections from shock hazard. Unless of course there's a capacitor shown on the schematic connecting those two grounds, which I didn't notice with my quick scan over. Make sense?

I'm not sure what you mean.

The antenna fiberboard has three wires exiting the board: two form the continuous race track loop, one end of each ending up at the grids of the osc tube and first IF tube like most AA5s. Typical stuff at this point.

The third wire on the antenna fiberboard is a single trace that lays alongside the edge of the racetrack loop. One end of that wire is terminated and literally buried into the fiberboard where no electrical connection can be made to it. Looks like it was punched in with a tool. The other end is connected to a wire which presumably is the wire that would serve as the external antenna lead in.

Do you mean that single lead would go to an antenna, or to an earth ground? It wouldn't be possible to connect that isolated track on the antenna board to both an antenna and an earth ground.

Thanks again.

John

I assumed that your back looked like this one from the ARF thread. It appears to have 4 connections, which would match what's shown on the schematic.
Basically one loop with another around the perimeter. But maybe I'm seeing things. If that's not what's going on there then I'm a little lost myself, because that's normally how an antenna matching transformer is designed. Let me go study that schematic again. :scratch2:

https://c5.staticflickr.com/9/8298/2...525501d0_c.jpg

From Riders 17-26 in the general notes, it says But then in the alignment directions 2 and 3 it specifies to connect the signal generator Those alignment directions are what through me, as there's apparently no external ground lead. Possibly it's typo and they meant chassis ground? In any event I'm sorry I confused the matter.

I think these are the intended connections for the antenna as pictured below. The outside antenna wire and ground form a single turn around the perimeter of the internal spiral loop. Internal loop connects between AVC line and the grid of the 12BE6. The outside antenna ground should be left disconnected or returned to an earth ground, but not the AC chassis ground, unless you don't mind having a shock hazard.

Yeah, that's unlike what i have:

https://i.imgur.com/ENhIo2o.jpg

In this radio, there's only three connections on the antenna board, the two for the loop and the third (blue wire attached) which is not connected to the loop in any way. You can also see that the third runs inside the loop, and is pinned into the fiberboard.

It can be connected to an antenna, to an earth ground, but not both.

John

Looks like pure capacitive coupling from the external antenna is what's intended.

Thanks for posting the picture. Obviously they made variations over a period. I'm now done beating the subject to death. Until next time :thmbsp: :D

What do you suppose is the significance of the little right angle bend at the end?

[edit]I thought there was a bend on the left end, but after enlarging the photo it seems it's only a mark on the fiberboard. Now I'm really curious how those were manufactured.

I should have posted the picture when I first asked the question. But this one must be out of the ordinary. In any case, I do appreciate the help from you, OC88, Max, etc.


Yep. It looks like the end was stamped/pinned/guillotined into the fiberboard to keep it from wandering loose. The mark is just the divot.

John