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.

https://lh5.googleusercontent.com/-7...42%2520200.jpg

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?

https://lh4.googleusercontent.com/-l...42%2520204.jpg

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.

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.

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.

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 ?

Good thought--I had to double check, but it's cloth.

Did you check it with an ohmmeter just to be sure?

Absolutely. And I scraped a clean spot on the core to be sure of contact.

Could there be a fairly fine wire broken off inside of the cloth braid?

jr

I just shredded some of the cloth braid. No wire.

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.

https://lh3.googleusercontent.com/-a...42%2520208.jpg

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.

https://lh6.googleusercontent.com/-R...42%2520221.jpg

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!

https://lh6.googleusercontent.com/-I...42%2520405.jpg

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.

Thanks. I'll give it a try.

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.

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.

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.

https://lh3.googleusercontent.com/-c...42%2520223.jpg

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

https://lh3.googleusercontent.com/-e...42%2520224.jpg

cancel, bad info. Ignore attachment.
Sorry, coffee hadn't kicked in yet.

Unravelling the Coupling Transformer
 

Actually I tried a couple of other configurations which didn't work, but they were based on my lack of understanding of the antenna coupling transformer. So I'll disassemble the set tonight to see if I can find faults.

Now, looking closely at the coupling transformer, which I viewed as a single coil, like a choke, tapped in the middle for some mysterious reason. I see that at the antenna connection (red), there are two wires--one for an inner winding which ends at the yellow terminal and the second for an outer winding which ends at the black terminal. So there are effectively two coils which mysteriously act upon each other when the signal current is present. Does this work like a step-up transformer?

https://lh6.googleusercontent.com/-U...42%2520225.JPG

The antenna transformer is what is known as an auto-transformer.
http://en.wikipedia.org/wiki/Autotransformer

If you know how a variac works then it makes the description simpler. Your antenna transformer is used as a voltage step-up transformer.

Of course I know how a variac works. You turn that knob on the top and watch the needle go up....
But seriously, I tried to research this earlier but I needed that word, "AUTOtransformer."
Thanks much.

Last Gasp Effort (for now)
 

I'm at the point where I'm repeating tests without changing anything and expecting different results. The only change I've made is to increase the plate voltages which were all lower than normal. This made a very slight change in the radio's performance.

These socket voltages look good to me:

https://lh4.googleusercontent.com/-h...42%2520228.jpg

Still, I can only pick up a broadcast signal if it's very strong and the antenna post is connected to my "earth ground." With the radio's lid on the output is medium-low audio volume at the rheostat's lowest, increasing as expected when the volume is turned up, like half the signal is going through the rheostat. But if I pull the coupling tube, there no appreciable change in the audio output.

On the other hand, if I use a signal generator (induction through two loop antennae) to the antenna post, then pulling the coupling tube stops the audio output.

I've also tried aligning the variable condensers but haven't changed anything. Notably, rotating the antenna coupling condenser 360 degrees has no effect on the output.

I'm hoping that these results will resonate with someone. Otherwise this set goes back in the box and I move on to the next project ("Going from failure to failure with undiminished enthusiasm").

https://lh5.googleusercontent.com/-C...42%2520901.JPG

I left that link at the end accidentally.

Could the fabric strip be a dipping thread? Electricians in the motor rewind shop used to dip the windings in Varnish, and used shoestrings or paracord - strong, light, and with just a slight flex they could break all of the dried varnish off it for reuse.

Do you have zero resistance between your antenna ground and your HV center tap (chassis ground)?

Possibly it's a dipping thread. The second transformer I looked at has one on top and one on the bottom, and they're long enough for that purpose. I don't think Atwater Kent would have needed to varnish the outside since they were entombed in sealer, but I don't know if Atwater Kent made the transformers or if they came from a different manufacturer.

The thick tar likely would not penetrate the windings as deeply as thin varnish (which aided in the dielectric strength of the windings.).

Kevin: Yes, zero ohms from the center tap to any other chassis ground. And (this is something I don't understand) zero ohms from antenna post to ground when the rheostat is at the lowest setting. When the rheostat is turned (440 ohm element), the antenna-ground resistance goes up to about 100 ohms, then back to zero at the highest volume setting.
Tom: Makes sense. I also think it might have facilitated placing the transformer into its chamber. It's a tight fit with very little room for fingers. I have to use a pair of pliers on the core to pull it out.

According to the schematic - with the volume control all the way up(volume control center wiper at bottom end of antenna coil), you should basically be reading 6 ohms across your antenna terminal to ground(DC resistance of the lower half of the antenna transformer in parallel with 400 ohm volume control). With the volume all the way down, the volume controls middle wiper should be shorting the antenna post directly to ground. So if you're measuring 100 ohms at some point, that would seem suspect. Have you taken resistance measurements of the two windings of the antenna transformer? It would be completely reasonable for that ant coil and/or volume control to have been zapped by lightning at some point, although that would normally leave something open circuit, it could also leave a carbon track. You can try hooking your antenna directly to the top half of the antenna coil(1st RF grid). At that point the volume should get much louder.

https://lh3.googleusercontent.com/-c...42%2520223.jpg

Thanks. I've covered this ground before, but I rechecked both issues. The antenna autotransformer shows 6.4 ohms and 18.8 ohms. I have a spare transformer that shows the same resistances within 1 ohm. The rheostat has been disassembled twice and totally rewired (but saving the shielded lead), carefully inspected and tested. I just tried bypassing the coil with the red lead (directly to grid) but it made no difference in any way. Putting the lid on the set reduces the volume by 80 percent.
The rheostat has no effect on the volume in this configuration.

I think I've tried every other configuration from rheostat to grid. I have a Model 37 that works well. It's nearly identical to the Model 42, but instead of the antenna transformer it simply has a choke from grid to ground. I was thinking of disconnecting the antenna transformer and using the choke from the older set just to see what happens. Your thoughts?

Hmm. I think something else is wrong. When you hooked the antenna directly to the 1st RF grid, it effectively took the volume control out of the equation. I need to think about this for a while. :scratch2:

What happens if you connect your antenna wire directly to the grid of the 2nd RF tube?

This is a dumb question, but have you visually verified that the filament of the 1st RF tube is lighting?
And can you verify that the tube pins for the grid and plate are contacting the socket lugs?

Just got back from dinner and connected antenna wire from rheostat directly to 2nd RF grid. Amazing. The three strong stations come in, each over a broad range, volume control works (but some signal is still present at lowest volume setting), and the rest of the stations I usually receive are present as well.

I can't relate the 0-100 scale to frequencies, but at the low end of the band a single station is present from 0 to 20. At the upper end the reception ranges become normal.

What do you think? I'm going to recheck continuities on the 1st RF transformer, then look at the 1st variable condenser more closely.

It's normal that you're loosing a bit of selectivity when only running two stages of tuned RF amplification. Now if you were to run your antenna wire to the grid of the third RF amp, it will tune even broader, and with less gain. But that's great news that you've narrowed things down to the first RF stage.

In case you don't already have it, here's more modern schematic that's much easier to navigate.

http://www.atwaterkent.info/TechData...AK09800sch.pdf

Whoops! I didn't notice that I had the coupling tube pulled. All of the above is without the coupling tube.

When I inserted the coupling tube it reverted to original--only three strong stations with no control from the volume rheostat.

I do have the modern schematic, but I learned to read schematics from Riders Vol 1-3, so the old ones are easier for me. They're more pictorial.

Not sure what you mean by the coupling tube?

Is it possible you could have the polarity of one of the 1st RF transformer windings reversed?

Sorry, I'm rushing too much.
The coupling tube is the 1st RF.
I just repeated trials with antenna direct to 2nd RF grid, both with and without the 1st RF tube. The result for both was the same--what followed "Amazing," lots of reception with good volume control of all but the strongest stations.

The 1st RF transformer connections have tested correctly according to the (old) schematic, and the connections have not been changed from original, and are the same as on my junk Model 42. But I'll recheck them.

The 1st RF transformer connection to the 1st RF plate lead was "iffy." I had to do some pushing on the test probe to make the connection. So I resoldered that one.

And somehow I missed Old Coot's response yesterday. The filament lights and I get good Gm and emissions with my testers. Plus, I tried swapping tubes anyway. I also cleaned all contact surfaces of pins and socket and verified that they were connecting.

I just put the coupling transformer back into the circuit--wired normally except the black lead goes to the 2nd RF grid. The result is the same: Three uncontrollable loud stations and the rest of the area stations come in normally.

Backing up a bit here. I measured the resistance between antenna and ground on my 42, and I get the same - close to 100 ohms at about mid volume. Looking at drawing again, it makes perfect sense - at mid setting the 400 ohms is basically split into two 200 ohm resistors in parallel, but the lower half is in series with the 6 ohm winding of the antenna transformer. So at mid setting you have 200 ohms in parallel with 206 between ground and antenna. A little hard for me to visualize looking at the drawing.

https://lh3.googleusercontent.com/-c...42%2520223.jpg

I'm not clear on what you did, or did differently?

Previous configuration was red (center tap) to 2nd RF grid.
I put red back to the center tap. Then disconnected black from 1st RF grid and connected it to 2nd RF grid.

Then I remembered to try it with the lid on the set. The lid blocks nearly all of the non-antenna signal. In that configuration the set receives normally, all stations volume-controlled--except for a negligible amount of the strongest signals getting through the cabinet.

So, it works well bypassing the 1st RF tube, 1st RF transformer, and 1st variable. condenser. I'm assuming there's a problem with one of those components or with the connections. Right now I'm removing the 1st RF transformer so I can inspect it thoroughly and check the continuities again.

The other part of the puzzle is that I cannot detect any signal if the set is connected to my usual 40-feet-around-the-ceiling antenna. I don't get any detectable signal unless I connect the antenna post to "earth ground." (I put that in quotes because it's not an REAL earth ground with iron rods 8 feet into the whatever)

And I would guess that the inability the get signal through my normal antenna is related to whatever keeps the signal from getting through the first RF stage.

I'll take a close look at the RF transformer, then try to simplify the results with only the essential variables.