[Arran]

Philco liked using those Bakelite block capacitors across the power line for years, probably because it could do double duty as a convenient terminal strip for the power cord. I have heard that the potting in them is tougher to get out then the 1930s blocks, but not too hard.
Regards
Arran

[morzh]

They did change the tar type, more than once, and yes, the later ones are a bit tougher. Also flooded grommets they used for some time did not help
I am sure they were re-using old stock in the later radios.

[radiorich]

Hello Arran
yes, I am sure that is one the reasons they kept using that item way past it's prime ,
If it works why change it !

Sincerely Richard

[jrblasde]

Does anyone know what kind of paper is used to line IF cans? There is a tube of paper inside all three IF cans on this radio, and it is preventing me from removing the internals. I had to sacrifice the paper to remove the internals.

To replace it, can I use regular paper or will I need insulating paper?

[jrblasde]

Here’s the paper I am talking about. Seems to be glued into the can.

   

   

[jrblasde]

Disregard, I found an answer. As I suspected, that paper is placed around the innerworkings to insulate them from unintentionally contacting the metal can (since that is grounded). Given that regular paper has an unspecific insulation rating, we can't say whether it will be adequate. It probably would be fine, but let's use insulating paper. I've discovered it is readily available on Amazon for roughly $12.

[morzh]

I was lucky and bought a large roll of fishpaper on eBay.
Fishpaper is the one to use.

[jrblasde]

Thank you, Mike. I will order the fish paper. I saw that on Amazon, and admired the odd name.

    I should provide an update on the project. This week was fairly busy with work, so I felt that I wasn't able to spend as much time working on the radio in the evenings. Nonetheless, I finished disassembling the main and the RF chassis. Those were some very tough rivets to drill out! I knew that it would get up to about 50 degrees on Wednesday, and then it was 61 degrees on Thursday. I read that it needs to be between 50 and 90 degrees to use naval jelly, so I rushed to get the two parts of the chassis ready to clean on Thursday. It was about 7:00 PM by the time I was ready on Thursday evening, and the temperature was cooling down and hovering right at 50 degrees. I got it done, though! I applied the jelly, let sit for 10 minutes, and then scrubbed it with steel wool. I let it sit for another 5 minutes, and then wiped it clean with a wet rag. There was still some remaining jelly, so I sprayed the chassis clean with a garden hose on my driveway.

    There was still just a bit of rust, so I applied a bit of baking soda and scrubbed it into the metal. One more rinse, and it was looking pretty good! There's a couple of remaining spots of rust, but they look more black in color to me (that is, they don't look like normal red rust).

   

   

    Aside from that, I have been cleaning up the components I'll be reusing (hardware, trim caps, the band wafer switch, the IF cans, etc.). There is an assembly of three trim capacitors named C404 A/B/C, and it is a Philco part number 31-6464. One of the three trimmers had a cracked housing. I was able to repair it with some super glue and a clamp. I then was able to reinsert it into the assembly and re-solder one side of it to the common frame. I made sure that it still adjusts (it does) and that it still appeared as an open circuit to DC.

   

   

   

    I used an LCR meter to start filling in some missing values on the schematic. C402A and C402B (both trimmer caps) both measure 10 pF. C404A and C404C measure 20pF, whereas C404B measures 600 pF (this unit is larger than the other two). As for inductors, L403, L406, L407, and L408 all measure 2.0 µH with a DC resistance of 0.1 Ω.

    Lastly, I've removed the cans from all three IF cans. I cleaned them up in an ultrasonic bath of hot vinegar. They are looking nice! I've cleaned all of the trimmer caps in them with some electrical contact cleaner, and I have measured and cut 9" leads of new 20 AWG stranded wire for them. I need to order new capacitors to replace a few mica capacitors in two of them (might as well, while they are open).

   

Lastly, I have finally disassembled the wafer switch. That is to say, I've removed all of the wires and passive devices connected to it. It's out in my garage, and I've been in the process of cleaning up all of the solder lugs so that it may be reused. I will also need to clean up the air gap tuning capacitor a bit, and then I'll reinstall it. But the chassis is now fully disassembled, is clean, and is looking pretty sharp with a few fresh rivets on it!

   

[radiorich]

Hello Joseph,
nice Job on your chassis and I too use fish paper "Fibroid Fish Paper Electrical Insulation" made by GC electronics .
Sincerely Richard

[jrblasde]

I've made a disappointing discovery this evening. That is, my 3rd IF can is a Meissner part #16-6660. This is not the original unit (I don't care about that part), but it is strictly an AM IF can (456 kHz intermediate frequency). That's not going to cut it for FM demodulation!

    I've already been down this road with my 49-906, but these Philco IF cans are impossible to find for the FM sets. Philco used 9.1 MHz as their FM intermediate frequency at the time, and that's a very unusual value. I'm going to have to build my own IF can, just as I did for the other set. But last time I built a 1st IF can, so this will be slightly different. This also appears to be quite the complex IF can to reproduce!

   

I suppose I'll search the internet to see if there happen to be any similar options available right now, but it may be time to put back on my PCB design hat!

[morzh]

Yeah...fun.

[jrblasde]

It's been a long week of pulling out my hair, trying to decide what the engineers at Philco were thinking when they designed the 32-4074 IF transformer. But, at long last, I understand the design.

    I have looked, but I could not find a single other model which uses this IF transformer. The only other 1946 model with FM reception, the model 46-1213, was different. I also looked into the 47-1227, because it was supposed to be included in the 1946 model year line-up but was delayed. It had a completely different design as well. I've looked at the 48-482 and the 49-906 as well, but the circuits they implement are quite different.

    I'll open this discussion with a screenshot from the first page of the 46-480 service manual, which provides an explanation of the circuit. I'll refer back to this periodically.

   

    In short, there are two factors which make this design very unique. First, there is no switching in any of the IF stages. The only switching between bands occurs just before the first IF stage. Second, AM and FM detection are accomplished with a total of two diodes via a 6H6GT/G tube. Most AM/FM sets at the time used other means, such as a double-diode/triode tube where the two diodes could be utilized for either a Foster-Seeley discriminator or a ratio detector for FM demodulation, and the triode could be used for either plate detection or grid leak detection of AM. Here's a screenshot of the aforementioned AM/FM detection portion of the 46-480.

   

Notice that the third IF transformer is very strange. For one thing, AM and FM are crammed into a single IF can. It was very common for the first few years for AM and FM detection IF transformers to be combined into one single can. After a few years it became more common to separate AM and FM IF transformers into different cans. But, that said, the AM portion of this IF transformer is strange because it isn't a double-tuned resonant circuit. Pictured below is what I was expecting, one screenshot taken from the Philco model 49-906 schematic (because I also happen to have restored one of these) and the next from vintage-radio.com (specifically, https://www.vintage-radio.com/repair-res...tages.html)

   

   

    What I am trying to point out is that most AM/FM sets will stack two double-tuned resonant circuits, one for AM and another for FM (be it in one can or in separate cans). AM will be closest to the plate of the IF amplifier tubes, and FM will be closer to the B+ voltage source. I have spent the week trying to comprehend the 46-480's circuit, but here is a summary of what I have come to understand. I will split this into a summary of AM detection and a summary of FM detection. In AM operation, we have the following circuit.

   

    Right away I noticed that this is different from a "textbook" envelope detector circuit, seen in the below excerpt from one of my college textbooks (page 247 of "Introduction to Communication Systems", Third Edition by Ferrel G. Stremler). 

   

The circuit in the 46-480 is different, because the diode is not part of the signal through-path. It instead acts as a pull-down switch. I finally found such an arrangement on page 168 of "Old Time Radios!: Restoration and Repair", seen below.

   

    Having read up on the theory of this particular detector, I could return to analyzing the 46-480 circuit. Essentially, we have a diode half-wave detection of AM, with AVC feedback. As I have already mentioned, you will notice that the primary and secondary windings of T1 do not have capacitors connected in parallel to create resonant circuits. I was baffled for the longest time, but I now understand that it is because the secondary winding also functions as the RF choke coil for the FM circuit; if a capacitor were connected in parallel, it would disturb FM functionality. Thus, in lieu of parallel LC circuits on primary and secondary, capacitors C302C and C302D are inserted as "overcoupling" capacitors. C302D is a trimmer capacitor, so that the circuit can be tuned to couple at precisely the AM IF frequency of 455 kHz, while decoupling at the higher FM frequencies.

    A couple of other things happen in AM operation. First, the diode between pins 3 and 4 of the 6H6GT/G is biased to cutoff (confirmed by reading the "General Information on Model 46-480" section on the last page of the attached service manual for this radio). This is accomplished by opening the plate connection to ground, instead forcing the plate to connect to ground through parallel resistance and capacitance. Second, R309 (27 kΩ) is connected to ground. R309 connects between the audio frequency output and ground. When it is grounded on one side in AM operation, it is essentially connected in parallel to R311 (6.8 MΩ), reducing the output resistance to just shy of 27 kΩ (confirmed in the circuit description from the first screenshot). Third, T2 features primary and secondary capacitors connected in parallel--these resonant circuits are not selected to resonate at 455 kHz, and are thus negligible to AM operation. I should also note that there is a small capacitor connected across the detection diode. After some research, I learned that this was often done for diode detectors with common cathodes because the diode is used as a pull-down switch and is thus subject to rapid fluctuations in capacitance compared to envelope detector circuits where the cathode outputs to a smoothing RC circuit. Below is the simplified circuit, showing only the components necessary for AM operation.

   

    Now on to FM operation. Here's a screenshot of the FM detector circuit.

   

I can tell by the opposing diodes that this is a ratio detector circuit. But it's not quite the textbook version of a ratio detector. Again referencing "Introduction to Communication Systems", Third Edition, here's a picture of page 331 showing the typical ratio detector.

   

Notice that, to the right of the diodes, there are two capacitors connected in series with a center tap connecting to the output of the RF choke. These series capacitors are connected in parallel with the "very large" capacitor. They provide a sort of ratio, or voltage divider, allowing the voltage of the demodulated output to be a ratio of the total voltage across the "very large" capacitor. When analyzing the ratio detector circuit in the 46-480, I notice that there is no voltage divider created with two capacitors. There is only the single, "very large" capacitor (C314, 5 µF). Furthermore, there are also not two resistors connected in series between the two diodes; there is only a single resistor (R310, 33 kΩ). There are some variants of the ratio detector which have only one resistor. The only difference in functionality is that the reference voltage cannot be centered. In the case of the 46-480, the reference voltage is ground. But the lack of two series capacitors, connected in parallel to the "very large" capacitor really had me stumped.

     Enter a 1951 copy of a Philco service manual (attached to this thread and screenshot below this paragraph). It detailed a few variations Philco used of the Foster-Seeley discriminator and the ratio detector, and actually included this exact arrangement. The ratio detector circuit can still function if the output voltage of the RF choke is solidly referenced to the cathode of the diode with the grounded cathode via a resistor and capacitor. And, what would you know, the 46-480 does exactly this (C302G, 1,200 pF and R311, 6.8 MΩ)!

   

    Now that I established what variant of the ratio detector circuit was being used, it was time to think about how the IF transformer came into play. The primary and secondary windings of T2 each feature a capacitor connected in parralel to create a double-tuned resonant circuit. Contrary to the AM transformer, the FM transformer can be arranged this way because the windings of T2 have no function to AM detection. From there, the ratio detector outputs a signal into the RF choke (Secondary winding of T1). Below is the simplified circuit, reduced to showing only the components necessary for FM.

   

    And there we have it. I needed to understand the functionality of the AM and FM detector circuits before I could understand the 32-4074 IF transformer, but now I comprehend the inner workings. I can now select coupled inductors and capacitors which should provide the proper functionality to replace the missing original part.

    I will say that I have a bug in Mark Oppat's ear about this. He's been busy lately, but said that next week he can make it to his junkyard to see if he can find a spare 32-4074. But, if not, I am prepared to produce one. I may still do it anyway, just to see how it functions and so that I'll have a backup part.

[Arran]

With regard to the fish paper, I have a roll of automotive gasket paper, I think it was treated with rubber or some similar substance to make it resistant to various fluids, like oil, or antifreeze, and would need to be somewhat heat resistant as well. would that work as a substitute for fish paper in an IF can?
Regards
Arran

[jrblasde]

My gut feeling is that the automotive gasket paper would be adequate, but I hesitate to provide a definite "yes". Gasket paper has a similar composition to fish paper (i.e., it is a fibrous material which is impregnated with rubber), but the manufacturer intends for it to be a pressure seal with good thermal insulation rather than electrical insulation. Thus, I doubt that the dielectric strength per mil of thickness has been quantified.

Now that I know that the purpose of this paper is to shield against accidental contact between the can (which is tied to the chassis ground) and its inner components (connected to B voltage), that's the only reason I hesitate to say yes. But, you can certainly run a test! You might try using the high voltage output of a transformer, with an ammeter in series. Do not short circuit this setup, but rather place a piece of your automotive gasket in the circuit (transformer high voltage -> ammeter -> gasket paper -> return to transformer). Make sure to press the leads onto opposite faces of the gasket paper. You need to be making secure contact with the paper, so tape the leads down to it. Energize the circuit and let it run for 10 or 15 minutes under supervision. If it is adequate for electrical insulation, then you should see no measurable current on the ammeter and the gasket paper should show no signs of melting, burning, or darkening in color. If it passes the test, then I'm sure you're fine to use it for this application. Just don't do it for commercial purposes.

This is all coming from a transmission power engineer, so I am likely providing worst-case insight. We must be very careful about insulation and grounding at those sorts of voltages.

[jrblasde]

Actually, let's safe-guard that experiment a bit further. Suppose someone used that setup and the gasket paper were inadequate. The transformer and ammeter would be toast!

Okay, new plan. Insert a 1 MΩ, 1 W resistor in the series circuit. Should the gasket paper fail, this resistor will safely absorb the current of a transformer with a 640V high side. Most Philco models have B voltages less than 640V, but I know that my 46-480 has 640V. If your transformer produces a lower voltage, then this 1W resistor will still be adequate.

We might also think about testing DC current, as that's what the gasket paper would be exposed to in an IF transformer. Go ahead and rectify and then filter that transformer voltage, and then use it to test your gasket paper.

The circuit would now look something like this:

Your test power supply:
    Transformer high side -> rectifier -> ripple current smoothing RC network (the output of which we will call the DC power supply)

The test circuit:
    DC power supply -> ammeter -> 1M, 1W resistor -> gasket paper -. return to DC power supply