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  #31  
Old 01-09-2015, 04:44 PM
old_coot88 old_coot88 is offline
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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?
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  #32  
Old 01-09-2015, 09:19 PM
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Winky Dink Winky Dink is offline
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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.
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  #33  
Old 01-09-2015, 09:38 PM
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Kevin Kuehn Kevin Kuehn is offline
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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

Last edited by Kevin Kuehn; 01-09-2015 at 09:43 PM.
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  #34  
Old 01-09-2015, 09:43 PM
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Winky Dink Winky Dink is offline
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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.
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  #35  
Old 01-09-2015, 09:45 PM
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Not sure what you mean by the coupling tube?
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  #36  
Old 01-09-2015, 10:00 PM
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Is it possible you could have the polarity of one of the 1st RF transformer windings reversed?
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  #37  
Old 01-09-2015, 11:07 PM
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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.
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  #38  
Old 01-09-2015, 11:28 PM
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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.
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  #39  
Old 01-10-2015, 11:35 AM
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Kevin Kuehn Kevin Kuehn is offline
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Quote:
Originally Posted by Winky Dink View Post
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.
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.



Quote:
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.
I'm not clear on what you did, or did differently?
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  #40  
Old 01-10-2015, 01:08 PM
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Winky Dink Winky Dink is offline
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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.
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  #41  
Old 01-10-2015, 01:48 PM
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Ok, another goofy theory from jr... How about your indoor antenna is picking up the amplified signal from the TRF stages and the radio is actually oscillating at some frequency, obliterating the weaker signal from radio stations? Do you have an oscilloscope?... it might be worth checking the output of the last TRF stage for a strong CW carrier.

jr
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  #42  
Old 01-10-2015, 04:58 PM
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Winky Dink Winky Dink is offline
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jr: Not having an oscilloscope, I approached this theory differently. I took down the antenna. No change in symptoms.
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  #43  
Old 01-10-2015, 05:04 PM
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Winky Dink Winky Dink is offline
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Recap of the current status

Here's a recap of what the radio and I are doing:


Normal configuration on left. Configuration on right bypasses 1st RF stage.

Normal configuration, lid on, ANT post to indoor antenna:
No audible signal at max volume.

Normal configuration, lid on, ANT post to earth ground:
Receive 3 stations, some audio at zero volume, otherwise normal volume control.

Bypass 1st AF stage, lid on, ANT post to indoor antenna:
No audible signal at max volume.

Bypass configuration, lid on, ANT post to earth ground:
Receive 7 good stations and 3 weak stations with normal volume control. Only the strongest station gives some audio at zero volume.

(I pulled out the 1st RF transformer and it appears to be OK, with 2.2 to 2.4 ohms through each winding.)
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  #44  
Old 01-10-2015, 07:11 PM
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jr_tech jr_tech is offline
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A couple of questions;
1. Is there *another* path to ground perhaps?... such as a new 3 wire power cord grounded to the chassis for "safety" reasons, or perhaps "leakage" between the power transformer primary and the chassis?
2. Does the volume control pot work better if you connect the yellow and green wires together at the back of the pot?

This is a real headscratcher,
jr
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  #45  
Old 01-10-2015, 09:53 PM
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Winky Dink Winky Dink is offline
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I keep missing Kevin's posts. This is from yesterday. I will try antenna directly to grid RF2 and see what happens.

jr - No, I didn't ground the chassis to the house ground. I actually did that on the first-ever piece of electronics I worked on a few years ago. Couldn't figure out why the circuit breaker tripped every time I turned on the power. Then I read the sticker on the back panel, "DO NOT GROUND THIS CHASSIS!"

Potentiometer? I guess this is technically a pot, but I don't think they had started using that word in 1928. The wires exit the side of the Bakelite shell, and they're not accessible without pulling the chassis. The volume control apparently works properly when the signal goes through it, but the next time I pull the chassis I'll look at that.

Power transformer primary leaking to ground? Probably not, but easy enough to check.

So, this evening I'll (1) Try antenna wire to 2nd RF grid, (2) Check resistance between power transformer primary and ground, and maybe (3) Set the Model 42 up like the Model 37 using the antenna choke from the older model.

Oh, I almost forgot that I have a Heathkit signal tracer that I "restored" last year, but it's never been used. If a signal tracer would be helpful, I'll see if I can figure out how to work it.
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