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

jr: Not having an oscilloscope, I approached this theory differently. I took down the antenna. No change in symptoms.

Recap of the current status
 

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

https://lh5.googleusercontent.com/-Q...42%2520226.jpg
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.)

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

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.

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?

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.

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.

https://lh5.googleusercontent.com/-T...42%2520231.jpg

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

https://lh4.googleusercontent.com/-x...42%2520232.jpg

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?

https://lh3.googleusercontent.com/-j...42%2520238.jpg

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.

:yes:

The little signal that is getting through is probably feedback through the common filament bias network.

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?

Missed Kevin's last post. What little signal? I only understand 85 percent of the thousands of meanings of "signal."

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.

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.
https://lh3.googleusercontent.com/-j...42%2520238.jpg

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.

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. :scratch2:

I mean any little signal that's getting past an apparently defective(shorted plates) tuning capacitor.

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.

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.

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."

BINGO! Good find!

jr

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.

https://lh4.googleusercontent.com/--...42%2520985.jpg

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

Absolutely on steel wool being banished from the electronics work area. Ditto keep it away from speakers: their magnets want to "wool gather."

I'd like to know what you used to get those capacitor components so sparkling clean? Have you by chance documented this complete AK 42 restoration process some place?

If those plates are aluminum like on the model 40 sets, then they tend to stay fairly shiny and clean on their own...I've worked on/with 3 model 40's, and opened the lids of a few more..

Interesting. The ones I find seem to have a dull gray patina on them, but I admit I haven't done a thing to this one besides vacuuming the dust off. Was just thinking...in another 10 years a lot of these late 20's sets will be 100 years old, true antiques. :)

http://i452.photobucket.com/albums/q...g?t=1421170164

Gray Patina?
 

That looks like it spent its life in a coal-fired factory. Here's a photo showing the Model 42 tuner before it was cleaned.

https://lh4.googleusercontent.com/-6...20Interior.jpg

I have an untouched AK Model 30 chassis sitting behind me, and the variable condensers look just as shiny as these.

I just posted an album of photos of the Model 42 restoration. You don't want to miss the definitive guide to restoration of Model 42 ball feet!

https://picasaweb.google.com/coldrb/...eat=directlink

Excellent slide show of the restoration! Thanks for the info.

Winky, You have some amazing restoration skills there! :thmbsp: