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  #46  
Old 01-10-2015, 11:23 PM
old_coot88 old_coot88 is online now
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Quote:
Originally Posted by Winky Dink View Post
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.
The symptoms you've delineated here could occur if the grid of the 1st RF tube were shorted to (chassis) ground.
With the tube removed, and the coupling xfmr disconnected from the grid lug of the socket, there should be infinite resistance (open circuit) from the lug to ground. Is that what you're getting?
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  #47  
Old 01-11-2015, 01:03 AM
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Quote:
Originally Posted by Winky Dink View Post
I keep missing Kevin's posts. This is from yesterday. I will try antenna directly to grid RF2 and see what happens.
You did that, only you included the volume control and antenna transformer, which is fine. I'm fairly confident there's nothing wrong with your volume control, because it seems to control the volume going into the 2nd RF stage just fine.
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  #48  
Old 01-11-2015, 05:35 PM
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New Antenna, Dump the Earth Ground

Old Coot: There is infinite resistance between 1stRF grid lug and chassis ground (With no connection of grid lug to anything).

I replaced the old (magnet wire) indoor antenna with 50 feet of 14 gauge, 7-strand copper "antenna wire." It works better, and I am no longer connecting the antenna to earth ground for reception. So this is a bit of a restart which will simplify things for me.

1. In "normal" configuration, I get very weak audio at maximum volume on two or three strong stations.

2. With antenna to 1st RF grid, as below, I get slightly better audio output from the strongest stations.



3. With antenna to 2nd RF grid, as below, I get loud audio from many stations.



I've also pulled the grid resistor and the RF transformer to examine them out of the circuit, and I can't find anything wrong with them. I played with the signal tracer, but all I could do was pick up audio.

Question: With the diagram below, I still read "short" between stator and rotor. I see no physical connection of the stator with chassis ground. Is this short to be expected--is enough current conducted between rotor & stator plates to show resistance of only 4 ohms?

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  #49  
Old 01-11-2015, 06:31 PM
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No tuning capacitor should ever show DC resistance (the only kind a DMM will measure)!

If that capacitor is shorted then it would completely explain the symptoms you've been discussing. Find that short and fix it.

Also if the grid lug of the first RF tube shows resistance to ground with the tube removed and nothing connected to it, then the socket is bad...Which means find and remove the conductive path or replace the socket.

Both of those problems will kill a whole lot of signal if left be.
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  #50  
Old 01-11-2015, 07:16 PM
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Quote:
Originally Posted by Electronic M View Post
No tuning capacitor should ever show DC resistance (the only kind a DMM will measure)!

If that capacitor is shorted then it would completely explain the symptoms you've been discussing. Find that short and fix it.


The little signal that is getting through is probably feedback through the common filament bias network.
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  #51  
Old 01-11-2015, 07:24 PM
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Confused. The set is not in operation. 1st RF tube grid lug has been disconnected from the coupling transformer. Tube is pulled. Those are the only electrical connections to the lug. It's mounted on Bakelite and has no electrical connection. The DMM show infinite resistance between the lug and ground. There is no conductive path. What am I missing?
The statement about tuning capacitor is also confusing to me. It shouldn't show DC resistance--but if it's shorted it's bad? I'm missing something here as well.
Help?
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  #52  
Old 01-11-2015, 07:27 PM
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Missed Kevin's last post. What little signal? I only understand 85 percent of the thousands of meanings of "signal."
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  #53  
Old 01-11-2015, 08:29 PM
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Quote:
Originally Posted by Winky Dink View Post
Confused. The set is not in operation. 1st RF tube grid lug has been disconnected from the coupling transformer. Tube is pulled. Those are the only electrical connections to the lug. It's mounted on Bakelite and has no electrical connection. The DMM show infinite resistance between the lug and ground. There is no conductive path. What am I missing?
NOTHING! That is how it should be. One of YOUR earlier posts indicated that the test you just described gave you a resistance reading that was less than infinity...So I was telling you readings less than infinity in that situation are bad.

Quote:
Originally Posted by Winky Dink View Post
The statement about tuning capacitor is also confusing to me. It shouldn't show DC resistance--but if it's shorted it's bad? I'm missing something here as well.
Help?
By should not show resistance, I mean there should be INFINITE resistance (so high you can't measure it) between the side of that capacitor that is connected to the RF transformer and ground in the circuit shown below.
TO BE DOUBLE CLEAR IF THERE IS A SHORT OR A RESISTANCE LOWER THAN INFINITY THEN SOMETHING IS WRONG.


I usually use the term 'no resistance' to mean an open circuit....I guess I should use 'no measurable resistance' next time to make it more clear. If you don't know yet that a capacitor is supposed to have infinite resistance between it's terminals (unless something is wired in parallel with the capacitor), then you REALLY need to brush up on the basics.
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  #54  
Old 01-11-2015, 09:30 PM
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Hi Tom, There's a lot of folks enjoying the radio restoration hobby that don't necessarily have a formal background in electronics and basic electrical principles. And everyone learns at a different pace, so we need to cut them some slack. Now I'm curious to find out what is causing that tuning capacitor to be so fully shorted.

Last edited by Kevin Kuehn; 01-11-2015 at 09:35 PM.
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  #55  
Old 01-11-2015, 09:34 PM
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Quote:
Originally Posted by Winky Dink View Post
Missed Kevin's last post. What little signal? I only understand 85 percent of the thousands of meanings of "signal."
I mean any little signal that's getting past an apparently defective(shorted plates) tuning capacitor.
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  #56  
Old 01-11-2015, 11:28 PM
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Kevin: I've been looking for a leak to ground, and that's why I was asking about my finding of a short through the first tuning capacitor. The stator is grounded through the RF transformer secondary. The rotor is grounded directly. I recognized that the rotor and stator would short through their ground connections, so disconnected the RF transformer ground before testing stator-to-rotor connectivity.

I can't see or feel any contact between stator and rotor plates yet I still found a stator-rotor short, measuring 4 ohms. The RF transformer coil also measured 4 ohms, which seemed like a peculiar coincidence.

So, I was trying to find out if there might be a another explanation for a stator-rotor short other than plate contact before I proceeded.

Thanks.
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  #57  
Old 01-12-2015, 12:20 AM
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Well you can turn the rotor so it's completely out of mesh from the stator - that eliminates any possibility of plates rubbing. You can disconnect the wire wound(grid stop) resistor and the RF transformer wire from the stator by removing the top and bottom nuts on the rear of the stator - that eliminates any possible leakage through those two points. Other than that you have the stator supported to chassis by the top and bottom phenolic insulators. I suppose there's a remote chance that something metallic is bridging across the phenolic pieces to chassis ground. I can't see any other possibly path to ground.

Last edited by Kevin Kuehn; 01-12-2015 at 12:31 AM.
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  #58  
Old 01-12-2015, 12:37 AM
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Yeah. I figured out the un-meshing thing. The plates were in a position that couldn't be unmeshed without first pulling the chassis, loosening the pulley, and repositioning the plates. Still had 4 ohms. Having done that, I pulled the capacitor from the chassis and still had 4 ohms. Then I saw the problem. I'll give you a hint, "NEVER USE STEEL WOOL AROUND ELECTRONIC EQUIPMENT."
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  #59  
Old 01-12-2015, 12:53 AM
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BINGO! Good find!

jr
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  #60  
Old 01-12-2015, 05:56 PM
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The Culprit

Thanks to all who had the patience to help me track down the culprit. I'm posting this photo so others with limited experience might come across it and benefit from it. The lesson is, "Don't use steel wool near electronic equipment." This problem resulted from my use of steel wool two feet away from the set.



I think this set's going to work beautifully when I reassemble it. I'll do one last post when it's complete.
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