I was organizing all my Rider packets I've printed from Nostalgia Air and checking radios piled up in the closet awaiting attention. It looks like the older 56X2, 56X3 series from 1946 has the plate while the more recent 66X11 (1947) and 9X641 (1950) do not have the plate. They all have the same chassis layout except the 9X641 has no 12J5 but uses a 12SK7 RF amp. followed by 12SA7 converter. The main difference though is that the 56X series is from 1946 so I'm thinking maybe the existence of a beacon prior to 1947 may be the correct answer.

That won't stop IF signal blowby. The loop connects directly to the mixer grid. That is how any unwanted signal would most likely enter the IF stages--stray pickup by the loop, then straight through the mixer stage to the IF.

You'd need a series tuned trap to eliminate an unwanted beacon signal at the mixer grid. Or a balanced mixer could be used, at greater expense and complexity. Of course, you could wrap the loop in foil and ground it to the chassis and that would stop the IF blowby, and all desired signals as well.

The low-frequency radio range (LFR), also known as the four-course radio range, LF/MF four-course radio range, A-N radio range, Adcock radio range, or commonly "the range", was the main navigation system used by aircraft for instrument flying in the 1930s and 1940s, until the advent of the VHF omnidirectional range (VOR), beginning in the late 1940s. It was used for en route navigation as well as instrument approaches and holds.

Based on a network of radio towers which transmitted directional radio signals, the LFR defined specific airways in the sky. Pilots navigated the LFR by listening to a stream of automated "A" and "N" Morse codes. For example, they would turn the aircraft to the right when hearing an "N" stream ("dah-dit, dah-dit, ..."), to the left when hearing an "A" stream ("di-dah, di-dah, ..."), and fly straight ahead while hearing a steady tone.

As the VOR system was phased in around the world, the LFR was gradually phased out, mostly disappearing by the 1970s. There are no remaining operational LFR facilities today. At its maximum deployment, there were nearly 400 LFR stations in the U.S. alone.

http://en.wikipedia.org/wiki/Low-frequency_radio_range

[Had a nice photo to add but it exceeds restrictive size limit .. Sorry]

The Four Course Radio Range
http://www.youtube.com/watch?v=p-VqtNY8vpw

I know what a beacon is. Cite one that was assigned to 455kHz.

Then explain how that plate is in anyway involved with the path of least resistance for unwanted signals entering the input of the IF amplifier,
which is likely due by direct pickup on the loop, and straight through
the first mixer stage Leakage paths around the loop to the IF stages
are probably down -80dB compared to what gets directly into the mixer
grid. The hot end of the loop connects directly to the mixer grid and
that shield has no effect.

When you align the IF in a AA5 the normal injection point for the signal
generator is at the mixer grid.

My post was merely to demonstrate an antique navigational beacon system as a possibility and to provoke thought. You seem to be very adept at telling us what it's not. In your estimation what is the purpose of our celebrated metal plate?

You were asking for opinions.

I did my best to explain why the plate would not provide any rejection
for an unwanted signal on 455kHz bleeding into the IF stages,
and I backed up my position. If you don't agree, that is fine. But
maybe someone else following the thread may take the time
to understand what I was trying to convey. If so, cool.

Based on known issues, I mentioned in the last thread that the plate
could shield the loop from unwanted second and third harmonic radiation
of the 455kHz IF signal that is generated in the detector. I also provided
some links to earlier threads on the that subject. Again, if you
don't agree, fine. Perhaps there are other readers who be might be interested
in learning about what causes 910kHz problems in AA5 sets, or maybe they
don't. If not, I'm sure they've already killfiled me.

Pete

Remove it and see what happens.

RRM

I agree, but it may be a case of knowing what to look for to see if there
is a difference. Otherwise, I suspect one would think the plate did nothing.

Pete

I have had a problem in a transistor radio where tuning to 910kHz or thereabouts the thing would burst into oscillation. Twice the IF getting into the ferrite antenna. The can of the last IF transformer was not very well secured to the chassis by twisted tags so I soldered a wire to the tag and grounded it to the nearest solder tag. Fixed. Your screen may be a fix for the same problem.

Money can't buy happiness, but you can be miserable in comfort.

You can listen for birdies. If you get them you'll know what it does. If not you still won't since this kind of thing might only be a problem with a certain brand of tube, some other production variation, signal strength or ...........

You can also set things up to see if there is some effect on the antenna that makes a significant difference.

I wouldn't be surprised to find that unless something breaks into oscillation you won't be able to tell if it's there or not.

RRM

Yes, you could listen for birdies, but that assumes the OP would have a test
setup that provides a signal source on 910 and 1365 kHz.

It could be a weak or strong hetrodyne on weak signal on those frequencies, or
full blown oscillation if the feedback path is strong enough to substain
oscillation as in Marty's example. A GT detector tube would radiate
stronger harmonics.

The test procedures should be validated to be meaningful.

Pete

Hello and thanks for joining the discussion. We've already established in an earlier thread that the plate does noting. What we have is the RCA 56X/X2/X3 series with the metal plate and the 66X series without. Both have the same tube layout and schematic, were re-caped about 3-months ago, and work equally well. The focus has kind of drifted away with a lot of conjecture but what we're trying to find out is what was RCA's original intent when including the plate on the older 1946 models. In other words, hoping that someone might be aware of some design reason from the distant past for the plate's inclusion. Of course now in the year 2012, it's a moot point -- only an unsolved curiosity.

I might add that I thought I had broken my buying addiction but unfortunately, just bought a couple more pieces of junk on eBay. I'm accumulating quite a collection of useless metal plates of late. Somebody please help me!

I was looking for that thread, lost track of it. Was confused to see this new one.
Is there a wire loop antenna on the rear of that set when it's in the cabinet? Saw a picture online of a supposed 56X3 set
with the rear panel on, couldn't see the metal plate in the view I was looking at. Wasn't the best pic though.
Just curious what it's supposed to look like with the chassis installed in the cabinet.
Anyone have good pictures? Didn't see any in the other thread either. Just the chassis.

We've already established in an earlier thread that the plate does noting.

For the sake of us following this thread, who is the "we" who established that the plate provides no purpose?

Can you elaborate on what tests, procedures and what equipment was used to prove the shield does nothing, and exactly what were the tests
looking for?

As I said before the shield might appear to do nothing unless one knows
what to look for. The change could be subtle. It could be subtle enough
for the bean counters to deem the shields an unneeded expense in
later models.

Casual observations are meaningless.

Pete

Well, I have the 56X schematic, and the 66X1 shcematic open in my browsers, and they look different to me!

Sunday I had a post all prepared and then submitted it only to find the thread had been locked. Lost the post and got irritated.

Well Since I like a mystery I did some research. The only pictures I could find of the 56 series with the metal plate were the plain 56x and the 56 x2. The x3 and above don't have it apparently.
As I said in a previous post the 66 series is a completely different set RF wise. The 66xx has a 455 kHz Trap like Peter mentioned, the 56xx doesn't.

Here's a 56x2 from Ebay with the plate:
Here's a link to Radio Attic of a 56X: http://radioattic.com/item_sold.htm?radio=0550217

There was a production change involving the 1st IF listed in Ryders here in the red box.:

My hunch is that the early 56 series was prone to oscillation due to the 1st IFT and the ant coil and the metal plate eliminated the problem. Regardless of the purpose of the plate in question, if you bring a conductor close to a coil, it will cause the inductance to decrease and the AC resistance to increase, it's just physics.

In a nut shell I believe it was a simple way to get a set out the door without screwing up the rest of the production line.

Good points Chuck.

But, looking at the photo a major section of the lower horizontal
portion of the loop winding is already close to the chassis apron.
The plate isn't as close to the windings as the chassis itself.
My gut feeling also is that it is related to some stability/feedback issue.
If someone has a Q Meter and one of those sets it would be fairly
simple to measure whether the ferrous metal plate affects the Q,
and to what degree by measuring the coil Q with the plate present
and removed.

Pete

Pete,
Yep I agree 100%. Too bad we can't ask D.D. Cole,

Maybe the plate is one of those Matched Elements that Cole is prattling about

It would be a hoot if it was a Q spoiler to improve the receiver fidelity at the low end of the dial.
Those loops are sharp enough at 540kHz (arithmetic selectivity) to limit the receiver bandwidth.

Pete

Maybe the radios with the plate were found to have oscillation and rather than redesign already produced chassis they found the plate was a solution that worked. Then in the next model they added the IF trap.