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  #1  
Old 11-20-2014, 02:08 PM
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AK Model 42 Transformer Ground?

I have an Atwater Kent Model 42 in about a dozen pieces, ready to reassemble and rewire. While I'm fitting the power transformer into the power unit case, I realize that I have no ground lead as shown in the diagram.



I have the other 12 leads properly identified and voltage-tested, but I think there should a separate lead to ground--I can't see any other way that the transformer would be grounded. I've been staring at the transformer for several weeks, but the solution has not appeared.
So, perhaps I broke off a copper-wire lead and can't find it?



Another question: What for is that reddish fabric strip (upper left)? I saw a photo somewhere that explained why it was there, but I forgot the answer and can't find the photo again.
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Old 11-20-2014, 02:20 PM
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Check the resistance across the leads of the rectifier plate winding, then check to see if you get about half that value between either end and the iron core...If you do then the core is your ground.

Good luck.
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Old 11-20-2014, 02:41 PM
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Thanks. I did that already--300 ohms across the HV secondary, looked for 150 ohms from that winding to every possible contact--but no continuity to the core or any other place. I'll recheck though.
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Old 11-20-2014, 04:23 PM
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Winky are ya sure that IS a fabric strip , and not a strip of woven fine copper wire that could be your missing HV winding center tap ?
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Old 11-20-2014, 04:26 PM
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Good thought--I had to double check, but it's cloth.
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Old 11-20-2014, 04:46 PM
old_coot88 old_coot88 is offline
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Did you check it with an ohmmeter just to be sure?
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Old 11-20-2014, 04:59 PM
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Absolutely. And I scraped a clean spot on the core to be sure of contact.
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Old 11-20-2014, 05:35 PM
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Could there be a fairly fine wire broken off inside of the cloth braid?

jr
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Old 11-20-2014, 06:47 PM
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I just shredded some of the cloth braid. No wire.
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Old 12-31-2014, 12:05 AM
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My "Aha!" Moments

I got a good replacement transformer. What I lost on the original wasn't the ground connection; it was one end of the primary. The missing connection was on the inside of the primary coil, about two inches up where the stick is.



Now that I have power, I got all the components cobbled together on the bench; chassis, power supply, speaker, variac, and isolation transformer. All components meet original specs except the three filter condensers which were replaced with .5-, 10-, and 10-uF caps as recommended in an article posted on atwaterkent.info by Ken Owens.



With 110V input, I got good reception and very good audio with NO HUM. BUT...the medium-strength volume was practically unaffected by the volume rheostat and I couldn't receive the signal through my indoor antenna--I had to connect the antenna post to an earth-ground. After quadruple checks on my wiring and disassembling the rheostat I ran the voltage tests, and everything was good except the power tube +B voltage was only157V versus the specified 180V.

So, I reassemble everything in the cabinet, turn it on, and get no sound at all. Quick check shows (Aha!) no filament voltage at the power tube socket (but the voltage is present at the power supply, so that's an easy one, even for me). BUT...now the +B voltage to the power tube is 200V!

So, I disassemble the power supply until I find (Aha!) a broken contact on the speaker choke (also referred to as the plate choke).

So, I repair that and reassemble the power supply and think the problem is solved. BUT...now I have 240V!! +B to the 171A. Further investigation discloses (Aha!) that I have a broken connection to the RF plate circuit resistor.

So, I pull the chassis, repair the broken connections to the plate resistor and the power tube filament voltage and try again. BUT...now the power tube plate voltage is 300V!!! (and the RF plate voltages are high as well). Only the detector seems normal.

I retested all the tubes to make sure that I hadn't killed any of them. I need one more "Aha!" to proceed with the project. Would I be risking damage to the power transformer if ran the set without tubes in? Does anyone know where I might find another "Aha!"?

On the bright side, the refurbished vernier dial works perfectly!

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Old 12-31-2014, 12:30 AM
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It's probably okay to run the set without (any) tubes, but it might be a good idea to put a dummy load on the B+ winding just in case its voltage is able to increase beyond the insulation rating of the transformer (which is a bad thing). Can calculate that load based on the current drawn by the B+ and the voltage using ohm's law.
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Old 12-31-2014, 01:23 AM
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Thanks. I'll give it a try.
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Old 12-31-2014, 10:29 AM
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As far as volume control goes, if the 42 is like the 40 and 44, don't
expect much control until it is in its in its case with the top on.
You're probably picking up local stations on the input to the 2md
RF.
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Old 12-31-2014, 12:59 PM
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And any sets that use a volume control in the antenna circuit need a good external ground connection in order to properly adjust the volume.
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Old 01-03-2015, 02:17 AM
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No More "Aha!" Moments

I understand the issues with ectopic signal reception in these sets. I have that problem with a Model 37 (but only if the lid isn't on the set), and I'm pretty sure that the situation with my Model 42 is different.

I'm not sure how I accomplished this, but I got all of the voltages OK at the power supply and at the tube sockets. After I was sure that problem was fixed, I spent a lot of hours checking continuity, resistances, and capacitances throughout the set. But I still have the same issues--reception only when the antenna is connected to my earth ground and barely perceptible volume control. Good quality audio, though.

(My "earth ground" is 10 feet of bare braided copper wire running down the side of the house, buried 2 feet deep [where the soil is always wet] with the strands splayed out over an area of about 2 square feet, which I hope is enough to ground a radio signal.)

Also, I get the strongest reception when I hold the ground wire (attached to the antenna post) and put a finger directly over the 2nd RF tube. And putting the metal lid on the cabinet changes nothing. Except, of course if I forget to take my hand away from the 2nd RF tube--the lid is pretty heavy, so I get bruising on my wrist.

So, the signal is either bypassing the volume control, coupling transformer, and 1st RF tube (?); or the volume control is somehow screwed up (?). I'm posting this now to see if someone has a suggestion other than to pull the chassis again to disassemble the volume control.

I don't understand how this is supposed to work. Here are two diagrams of the same thing except the right diagram doesn't show the shielding on the antenna lead.
With the volume control set up like this, I get resistance between the antenna post and the 1st RF grid from 16 ohms to 230 ohms and then back to sixteen.



I have a Model 37, which is set up like this. This works pretty well with no external ground.

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