[RFtech53]

All,  I'm restoring this radio and found something I've never seen before.  It has a round metal case that is grounded,  has one wire going to the A3 Bandswitch contact... and that same switch contact also has a wire going to the square ceramic Compensator (see my yellow drawn line).

Can anyone tell me what it is ... and what symbol it is the schematic?

               

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[Arran]

This is a good question, to me it looks a bit like a doorknob capacitor, but with terminals for soldering. However it has a metal case, that you say is grounded? An RF choke of some sort? That cloth covered wire winding around the outside of one of the coils has me curious too. It has the hallmarks of being "Hammed". By the way the 1936 and newer Philco models had brown rubber power cords, even though the cloth one looks cooler.
Regards
Arran

[Ed Locker]

I think that is a thermal capacitor that is undocumented. Notice how close it is to resistor #12 , 10K 3 watt? As the resistor heats, the capacitance changes to prevent the oscillator from drifting.
Hope this helps.

[RFtech53]

Ed, that would make sense.  I'm thinking you are correct.  I should have included the schematic ... which I'll do here. Do you see it in the schematic? 

I'm thinking it's the dotted line ground symbol that says "used in Model 38-7 only".  Though I still don't understand what it really does.  When it gets warm enough,  it grounds that line??

Aaron, I just replaced the 'ratty' rubber power cord with the new cloth covered one.  
   

[DaleHCook]

It is documented in the Philco service manual (38-7, codes 121 and 124, is on pp. 113-116).

Part 7A on the schematic is shown as a "Compensator Model 7 (1500 KC)," and is Philco part 31-6196. What you show in the photo, though, does not look like a compensator, which is a trimmer capacitor.

[RodB]

I think it's a compensator of sorts being that environment heat is slightly changing the capacitance, similar to a ceramic cap in an FM front end. Probably an experiment on Philco's part to see if the buying public understood what they did. On a hot summer Saturday afternoon you don't want the radio drifting when the Babe is at bat.

[Ed Locker]

It is further explained in "Changes in Models" #280 .

[DaleHCook]

It is further explained in "Changes in Models" #280.

Ed Locker hit the nail on the head. What he found is the 38-7 121, 124 Run 3 and Run 4 sections on page 2 of Philco 1938 Changes in Models (Changes No. 6). They describe the thermal compensators. Although that section applies to Service Bulletin No. 280, the thermal compensators are not described in the bulletin, just in the changes.

[morzh]

So it was your post on FB yesterday? See? Here it is that much better.

Someone even suggested a bias battery.
Transformer radios do not need them - they can generate their own negative bias. It is mostly used in AC/DC sets.

[morzh]

OK, this is the description of it.

https://antiqueradios.com/forums/viewtopic.php?t=244627


Look at the post #10.

They replaced an existing compensator 7A they used up 'till Run 3, with P/N 31-6206 (a comp. cap).
Then the took a thermal compensator and soldered it IN PARALLEL to 7A compensator.

This is what you should have, yours is likely the Run #3.

Then in the next Rub #4 the took out the thermal compensator and replaced it with two caps P/N 30-1097.

[RFtech53]

Thanks to all for helping teach this old dog a new trick !

And Morzh, I'm not as savy to forums as you are. I felt that posting to two different threads would be a good idea.  I've deleted my latest post on this subject,  on this site,  in another Philco 38-7 thread. 

As for posting here and on Facebook, if I correctly understand your reply about my same post on FB, I see no issue with posting in two places. Just because someone is in a FB 'Radio group',  that doesn't mean they're also on this site.  So I'm going for the most exposure to all radio restorers to find answers.

If I misunderstood that part of your post,  I apologize.

[Arran]

The square ceramic object is most likely a padder condenser, which is for the tracking on the lower end of the dial, in some sets they call it a "600 padder". This temperature compensating device is certainly an oddity, I have run into zero temperature co-efficient caps in some Rogers built sets, but not one that intentionally varies with temperature, not in an 89 year old radio. I would guess that Philco must have have produced them in house as I can't recall seeing them in anything else, on first glance I thought maybe someone was playing around in there, I've seen chokes and other things added before to try and quash some interference, or sub another defective part. Someone was doing that with the antenna coil, either the primary is open or someone was experimenting with it in some way.
Regards
Arran

[RFtech53]

The antenna coil ... could a tube have shorted and pulled enough current through the coil to fry it a few turns from the end? 

I recently restored a 1938 Motorola 9A Ch=9-3 Chairside and it had the first IF coil partially re-wound. I was told by someone who restores a lot of 30's radios that it's a fairly common issue when a tube shorts in the first or second stage.

[Arran]

Lightning could have been the culprit, if it looks burned, or some form of mechanical damage to the winding. The tubes themselves seldom short, and the shorts are usually between one grid, and another, or between the heater and cathode. What can cause an IF winding to fail might be the IF amplifier tube being biased improperly drawing excessive current through the primary, possibly due to a leaky capacitor, which is usually connected between the B+ and the plate of the IF amplifier tube. Sometimes they fail for mechanical reasons, like the wire physically breaks, or corrodes.
Regards
Arran

[morzh]

RFtech,

No, no issue with posting on Facebook; I post there too. It's just the experts on Philco are here. And if you need good expertise on just any radios, Antiqueradios.com. even though it is kinda competitor place, are also very good, plus the sheer number of people there is helpful.
The post I pasted here is from there.

But as for posting in several threads, it would be better if you gave a link to it, or even copypasted the post from it here, but sticking to one thread per one radio makes it more readable and clear to those who will open it in the future. I myself am not sure I'd understand why all of a sudden a post about the thermal compensator appeared in that other thread.

Mike.