AK Model 42 Transformer Ground?

[old_coot88]

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

[Winky Dink]

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?

[Electronic M]

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.

[Winky Dink]

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.

[Winky Dink]

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:



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

[Winky Dink]

I left that link at the end accidentally.

[Findm-Keepm]

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.

[Kevin Kuehn]

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

[Winky Dink]

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.

[Electronic M]

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

[Winky Dink]

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.

[Kevin Kuehn]

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.

[Winky Dink]

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?

[Kevin Kuehn]

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.

[Kevin Kuehn]

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