The PHILCO Phorum

I have a 48-230.  I got this from my Father when I was very young, it used to work but hasn't been used in approximately 45 years and it was lost for awhile, looks like something heavy fell on it.

The Cabinet is long gone, I had to replace the speaker and output transformer.  The IF Transformer behind the speaker had a big dent and I repaired the wires torn loose inside and got out much of the dent.   Totally recapped.   Cleaned up the chassis rust.  Replaced a few resistors as needed.   

Now, since the cabinet is gone so the antenna loop no longer exists.  According to the schematic this part appears to also be a functional part of the main tuning LC circuit.   I need to know the approximately dimensions of the antenna loop and the number of turns. 
The Photofact shows that the coil is DC resistance of 2.5 ohms.  Is t possible for me to make a coil based on that resistance?  Or do I need specific xx uH microhenries of inductance for this to work.

I can inject audio into the grid of the 14B6.  Currently I get a lot of heterodyning at various frequencies but I think that is because of the lack of the antenna coil.   I believe I have the IF roughly aligned by viewing it on the scope for maximum amplitude on the modulated 455kHz.

I have a tube tester, oscilloscope, Jackson Electric Frequency Generator, VOM and a low voltage Asian component tester - and a few other tools.

Once I get this running again I will need to determine what kind of cabinet to give.   I made a 3D printer and I've thought about designing a new front panel similar to the original Flying Wedge that it had.  I'll post that in the other section when that time arrives.

Hi Daniel and welcome,

Yes you are looking for a specific inductance which the resistance won't tell you. I think it's roughly 225 uh but sometime I get lost in the zeros. Won't hurt any thing if it's not right just won't peak any where. If it's more sensitive @ the low end There's too much L so need to remove some turns. Same goes for the high end poor sensitivity @ the low end it needs more L. You maybe able to get a rough idea from pics in the Sam's or on line as to the dimensions & # of turns.

To adjust the IF I would use a rf choke in place of the loop (value isn't all that critical) and feed the 455kc signal at the ant connection. Set the generator for 455kc and use some sort of method to check that the generator is spot on. Turn the volume up and generator output down so you can just hear it. Peak and repeak the IF adjustment for max speaker output. Most of the time it's no more than a turn or so on any one of the adjustments.
> Jackson Electric Frequency Generator
Hmm could have voted for FDR in his 1st go around...

GL

A little more research and I found that the dual variable capacitor on this set is 18-479pF for the Main Tuning and 19-165pF for the Local Oscillator.
I have some 28ga enameled magnet wire and so I understand that the AM band is 535 to 1705kHz.
So, maximum pF at 1705kHz would be 18.191uH.

Now, if I went to a website like the one below and calculated a 18.191uH coil, does the DC resistance mentioned in the Photofact for this Philco 48-230 Radio of 2.5 Ohms have to be observed? Can other resistances be used? What is to be gained or lost by other resistances?
http://tesla-institute.com/%21app/sim/acic.php

Looks like I may have found something else. It appears that Maximum 479pF is actually on the low freq. end of the dial when all the plates are meshed together. That is actually 424.38uF not 18.191.

The problem is that the resistance varies with the gauge of the wire. The smaller the ga the higher the resistance per foot so the ohms measurement doesn't mean much unless you know the ga. Then there is the Q of the coil/loop. The higher the Q the better in this case. The higher the Q the better the rf selectivity which saves the IF from filtering out other garbage.

Thank you

So I found a Q calculator on the web and used numbers for a 10" x 1/4" wide 30 turns Antenna Loop which calculates a Q of 285. Then I ran my first calculation of 0.5 inches x 1" with 272 turns and it calculated a Q of 586. Both of these with the same 424.38 Microhenries.
So, based on what you said the tradeoff is that the smaller coil (in this case) has a better Q which nearly doubles selectivity, however the smaller more selective coil looses much of it's ability to be a loop antenna due to size. I would have to rely more on an external antenna in that case.
34 years ago I recall some on these things on the Amateur radio exam but that was some time ago and I didn't use it much.

But you got your license!!

The resistance matters very little and the gauge of your wire will mostly determine the robustness of the soldering spots and the wire itself rather than the Q. The reason being, at this frequencies the resistance of the wire is determined by the skin effect, that is the thickness of the layer where the current actually flows. Unlike in case of DC current, AC current creates a magnetic field that "pushes out" the current from the center of the wire to its outer layers. This way the Ohmic resistance is very different from what you would measure with your DMM.
DO not confuse that with the inductive (reactance) impedance: this is still the active resistance, but dependent on the frequency of the current.

Therefore, if you want to use a bit thicker or a bit thinner wire, it is OK and the criteria are: 1) ability to fit the wire in the frame (too thick), 2) flexibility (too thick), 3) breakability during handling (too thin).

Been a Ham here for about 30yrs.

de N3GTE

I checked my work a second time and it was a good thing that I did. I had forgotten to select something in the online calculator and came up with the wrong inductance. So, it's 184.76 Microhenries at 0.535mHz with a 479pF Variable Capacitor.
Using a shipping tube I can make a 3.125"ID coil with 32 turns of 28AWG wire. These are things that I have on hand to make it.

While you're at it you can add a second few turns of wire as a primary for an external antenna.

Thank you, that's a great idea

I finally got it to pickup a number of radio stations (not just WLW), a full NOS tube swap out (except rectifier) and a slightly longer antenna coil.  The audio was weak and distorted but it was there.
The same audio distortion increases in amplitude when the screwdriver contacts the primary of the 2nd IF transformer.  That makes me wonder if this is a problem area.

To date, the Philco 48-230 has:
New speaker
New output transformer
New NOS tubes, except rectifier
All electrolytic and paper caps replaced.
Antenna coil fabricated
Cleaned up tuner
Took dents out of 1st IF can and resoldered.  Rust present on one trimmer cap screw
2nd IF can inspected and resoldered

It's definitely challenging to practice on.

I have some 455kHz crystals on order for making a fixed frequency oscillator so I can align this thing more accurately.

I took IF cans apart and cleaned rust off of the screws.  The mica washers looked fine.  Put them back together. 
I'm getting WLW which is only 3.5 miles from here.  I believe I got the coil right, the station is in the right place on the dial.
The audio is really weak, even with an external antenna connect to wires wrapped around the antenna coil.  I have replaced all the tubes except the rectifier.
Any suggestions?

A photo below, the antenna coil runs off the photo on the left.