[radio1]

Anyone know what the brass(copper?) ground strap is used for on the air cap 1400 KC H.F. osc. tap? It grounds to the cradle and connects to the screw point, and I have to insulate from this to get reception when connecting to it. Strange.

[rfeenstra]

It is essential that all of the movable portions of the tuning capacitor have a secure ground. Hence you will see multiple "ground straps" on the rotating shaft. On these Philco's they used these spring wire type ground straps. On later models they used flat spring straps that ride in groves that were machined into the shaft. If the rotating portion of the cap is not well grounded, you will have reception problems and the oscillator will not track correctly. Judging from the wire that is on the ground strap and on the stationary portion (non cloth covered wire), this radio has been worked on previously. Check all connections to see that they are correct and secure.

[radio1]

Thanks for that info, but that's not what I am referring to, it's a flat strap that that is riveted to the cradle of the air cap (you can see the top of it under the 'left terminal lug'  - above and to the right of the oscillator trimmer hole). When I said grounded, I meant grounded to the cradle, not chassis. I don't recall how it was originally hooked up at this point as it had several metal and insulating washers IIRC, and wonder what purpose it serves if any. I thought it might disable that section resulting in a two stator, but just guessing. I have used one of the original insulating washers there currently to get reception, the metal wiring attachment lug does not touch the metal strap. None of the other attachment points have this strap. This is the oscillator gang section, maybe it does have some significance, I question my hookup here.

[Radioroslyn]

I would suspect that the strap should be fitted between the stator plate (where all of the plates are jointed and the two screws hold it the frame thru the insulator). The other end the strap is fitted under the solder lug which it held by the mounting screw. This provides a good low impedance connection to the cap, coil and grid.
If you haven't rewound the osc coil your wasting your time trying to make it work properly. It needs to have the right ratio between capacitance and inductance. I would break out the 38G wire and rewind. If you don't know how many turns use the holes in the form as a rough guide.
GL

[radio1]

Yes I know, I should have a spool of 36awg delivered tomorrow. The best the my current 30awg windings will do is up to around 160khz if - which is what more or less what I calculated. I used the 30awg on the first rf coil also, so gonna rewind both and looking for the right inductance on the oscillator. I finally tried a real alignment with a 324 I just purchased. Radio plays, and I can dial it in by ear better to get my main listening station based on what it is at the moment. Still in process of building my lab back up. 

I am not sure I follow you on the strap, will take another pic of it to better show where/what it is. Thanks.

[radio1]

        Here's another pic of the strap, see the lower connection to the cradle (I think - I ohmed it, would have to take it apart again to double check).

Also, I was getting intermittence on the field coil substitute  wire rounds I was using, a 2.7K and 3 100 ohms, so I found some old 1 (maybe 2)  watt Allen Bradley 1K carbons in my box and put these in. My plate voltages fell to 250 on the nose, it was at 269 - go figure. The carbons have drifted and the sum is approaching 3100 ohms. Don't know how long they will hold up. I can imagine what people thought when they paid 49.99 for this radio, equivalent to around 1,000 bucks today, and the radio went to the wild side at night. Well, throughout the last the last 20-25 years, we went through the same thing with computers (remember bad caps and such, software glitches - and those computers were not cheap at that point), history/technology repeats itself.

[rfeenstra]

So I pulled my 70 out and, with a flashlight, tried to see what you are looking at. Mine has the strap as well only the bottom side is riveted to the the chassis. that rivet is what is holding the insulator in place. I can't tell on your photo if the rivet is missing or if it is discolored and just looks like it is missing. that end of the strap should be connected to chassis ground through the rivet. The end under the screw should be isolated from the screw that connects to the stationary portion of the tuning cap. On mine, the wire is connected to the other screw - probably makes no difference as long as the wire has connection to the stationary part of the cap. The strap is then acting as a low value capacitor between chassis ground and the stationary part of the tuning cap.

[radio1]

Yep, that's it on the strap, so some sort of capacitance for whatever reason, mine is attached the same way. Received the 36G wire and immediately saw it was really thin, so remeasured the original osc. wire against my 30G and looking at AWG tables. I was pinching too hard on the caliper , it is 32g. I used 30g which is around the 75% one vs. the other of what I thought - .008 vs. .0100 inch. If I use 36G, will have to compensate to get the correct inductance, might be difficult. If I go against the original form holes, will end up with many more wraps, so would have to back off and scrape and measure to know when to quit. Still thinking what to do, use it or not.

[Vic Gibson]

So I thought I’d throw in my 2 cents on that brass strap cap on the tuning cap.  And after 3 years of not restoring anything, I got ambitious and pulled out my junk model 70 – an early build ser. no: A72252.  Sure enough, it has the strap. That strap is permanently adding maybe about 30 pf to the cradle plates of that section.  Which means that section will track about 30 pf above the other 2 sections.
   

But then I see that the copper adjustment cap. plate for that section is shorter, smaller than the copper plates for the other 2 sections.  So the short copper plate is adding less capacitance to that section than the other 2 copper plates add theirs.
   

I’m just guessing that the strap is making up for the difference between that shorter copper plate and the other 2 longer ones.  In other words, the strap just adds back what the shorter plate takes away.  Seems like adding that strap is an odd thing. Speculating even further, maybe during manufacturing they had some shorter copper plates for some reason.  Error?  Maybe from another model?  I note that later 70s don’t have that strap and also have all the copper adjustment plates the same size.  Again, just my 2 cents.
 
Vic

[radio1]

Vic, good guessing there. I imagined errors in the day also. Don't really see how it adds that much capacitance through that thick insulator. Did you measure it? Also, was your unit connected to that terminal or the other side? If you have the original oscillator coil in there and have an inductance meter, what inductance do you get on the two tapped oscillator windings as installed in the can? Mystery for sure. Thanks.

[Vic Gibson]

Yup, guesses are me.  So I did measure each section of the condenser, without the copper adjusters installed, with my somewhat flaky cap meter.  That section did track about 25 to 30 pf above the other two, depending upon where I set the rotator section.  When I installed the copper adjusters all three sections pretty much tracked the same plus or minus maybe 5 pf. 
 
Also, the adjusting cap for that section is in fact connected to the grid of the 27 and to the top of the osc. coil, just like it shows on my schematic.
 
Now unfortunately I’ve never acquired an inductance meter, so I cant help there on that osc coil.  The coil is original.  I did ohm it out a while ago and it is 6.8 ohms total (5.2 ohms and 1.6 ohms where its tapped with a 50K.)  So I’m assuming the coil is OK.  
 
I’m basically just getting started on this unit – plan to recap and replace any resisters out of tolerance etc etc
 
Best, 
Vic

[Vic Gibson]

BTW I just noticed that the goofy brass strap is on my schematic as part number 13 and they call it, "Compensating Condenser (Part of Gang Assembly)" 

[Radioroslyn]

Bro,
As for your coil you could backwards engineer it by using an inductance/cap calculator. Use a capacitance meter to confirm the value of the osc section of the tuning cap. I would think it would be around 350-400mmfd. Use the calculator to figure the inductance to tune the frequency of the osc (810kc to 1760kc). Also check the operation of the trimmer. It should vary the capacitance by 10 or 20mmfd

Then use an inductance calculator to figure how many turn of what size wire on a given size form will give you the proper inductance. This is for the top half of the tapped coil. The bottom half is the feedback section. Using a triode osc typically that winding would be about 15% of the total of the top winding.

If you just want to wing it 85t of 38G and 14t of 38g for the bottom winding. This would be a starting point. It may or not work.

GL

[radio1]

You guys hit with the capacitance on the strap.
I've been through several windings on the osc.
and det. coils to experiment, and could struggle
to get a 'decent' sound, it was unstable, esp.
volume wise. I went with 32g on the oscillator
last night, still something not right. This morning
I removed the air cap and switched the osc. connection
to the other side with the brass washer, and then screwed
the strap back with the insulated washer.
A real pain. All back together and what a difference,
everything changed, sound much smoother, can hear exactly
what the trimmers are doing and can push up the
volume with stability. Still don't know what hooking up
the other way was doing, but the chassis hole lines up
with the right side screw, should have been a clue.
Probably all my windings would have been acceptable, a forgiving radio,
just find the IF you end up with. I used plain old
Pen Gear Matte Finish invisible (scotch) tape wrapped
to the thickness of the original celluloid with 32G wrap
on top for the RF osc. coupling. Is it heterodyning the original way, no, but acceptable.
It does a strange thing, after around 5 minutes of playing is suddenly dies, and have to tune several KHZ down
to get it back, and the sound is not quite as loud and good, maybe a tube, the 27 is getting weak. It's a fun twiddle machine.

[radio1]

Radioroslyn, I have been hitting the calculators. I still have not figured out which side of the am band for the osc. to create the spec'd 260KHZ if. Almost seems like it is a mistake.
From what I currently see I need .26 mh to het to 260, right now I am at .23 which I plugged in the calcs and was at around 230 KHZ or so, thus I IF'd it at that with the signal generator, and it dialed right in. I am comparing the sound of this unit to many other old vintage radios I have had, and at times it amazes me, but it is sensitive to every adjustment. The original OSC. wire measures most like 32AWG by my mic, .02 off. The way it is coupled back to the det coil may have something to with it, series. It tracks well, 1100 am on the spot, 600 hits high around 620.

'Now unfortunately I’ve never acquired an inductance meter, so I cant help there on that osc coil. The coil is original. I did ohm it out a while ago and it is 6.8 ohms total (5.2 ohms and 1.6 ohms where its tapped with a 50K.) So I’m assuming the coil is OK.' - Instailled I get around 7-8 ohms top primary .23 mh and 1.8 feedback .03 mh (milli henry) on the osc. coil. Seems like if my RF coil winding is off, it still would be passing the correct frequencies to hit the osc, I can zero beat correctly to a station. I can say this though, hearing the original with the bad coil, I am close but not there yet. Could the the 'power detector' causing distortions, might look into some tweaks there, maybe a diode or cap combination to solve it. Grid leak to try. 1500 into the grid, 1 meg to ground see what happens.