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Philco made this model for the 1931 season and it was Philco's
first superheterodyne. It has eleven tubes, four 24's, four 27's, two
45's in the pushpull output and an 80 rectifier. The cabinet design
is by Norman bel Geddes (1893-1958) one of the great industrial
designers of the twentieth century. (bel Geddes also designed the IBM
THINK sign among thousands of other items!)
This Philco 112 was an accidental Ebay win. I bid the starting price plus a dollar and no other bids were forthcoming. The seller dissasembled the radio and packed it in three well padded boxes. It arrived undamaged! The cabinet was in a large box with the speaker packed inside in another box. The base was removed and packed in another box. The chassis was in another. All were double boxed. UPS would have had to REALLY try to break this radio!
The chassis electronic restoration was
fairly straight forward. I restuffed all nine bakelite blocks, the
two metal capacitor cans, the tone control and the main filter
capacitor box. The bakelite blocks can be difficult to restuff. I
used a modified version of Chuck
Schwark's procedure. I did not unsolder or cut any wires. The
tiny wires that connect the internal components to the lugs were
broken by twisting a drill just slightly larger than the rivet hole
by hand. Then the hole was cleaned with a drill that would pass
through the hole. The hold down screw was removed, the block tilted
away from the chassis and a hair dryer used to heat the block for a
couple of minutes. The smaller drill was then used to push the tar
block out of the bakelite block in one piece. The new caps and
resistors were then placed in the block and the leads soldered to the
lugs. It takes about 10-15 minutes per block using this method.
The filter choke was open so I rewound it using some number 32 magnet wire. The wire I had was larger than the wire originally used so the number of turns that could be wound was less. This made the inductance and resistance smaller but it works fine since the original 2uF filter caps were replaced with 10uF 450 volt electrolytics.
I then applied power and started the troubleshooting. Audio was very weak but some strong local stations could be heard. Bias, B+ and filament voltages all were reasonable. Using the scope, I determined that there was a good IF signal into the last 24 plate circuit but almost nothing on the grid of the detector. I powered down and did a little checking with the ohmmeter and found the secondary of the last IF transformer was open. The IF transformer could be repaired I suppose, but instead I chose to couple the primary signal to the detector via a 100 pf cap and a 150K ohm resistor. That brought the audio up nicely.
I left the radio playing while working on the cabinet. Suddenly the audio became distorted and weak. A quick check with the voltmeter indicated something very wrong in the output stage.The grids of the 45's were 100 volts positive! They should be 45 volts negaive. I unplugged the chassis and did a few checks with the ohmmeter and determined that the connection to the center tap of the interstage audio transformer was open. The transformer had continuity end to end so it was still delivering a signal. However the minus 45 volt bias is applied to the center tap. That left the 45 grids floating and they had assumed a potential somewhere between cathode a plate voltage. As a quick repair, I connected a 100K ohm resistor from each 45 grid to the minus 45 volt bias terminal and tried it. The radio sounded great and all voltages were normal. Rather than risk destroying the delicate interstage transformer, I chose to leave the resistors. A schematic, with the changes highlighted is here.
Here is a very complete explanation of the change by Don Black
as posted on the Antique Radio Forum.
"This will work and the center of the winding will be at the same potential as the resistor center tap provided there's no significant grid current. It forms a bridge circuit with the two halves of the secondary as one arm and the two resistors as the other. They will remain at the same voltage provided nothing else unbalances them. If you're confused by the apparent voltage division, consider that there's twice the voltage across the full winding as each side so the voltage divides equally over the winding and also the resistors. Just another way of looking at the bridge balance. The resistors will load the transformer somewhat and reduce the stage gain but they will also damp the windings reducing peaks and extending the bass response. It's similar to reducing the plate resistance of the driver tube as far as damping is concerned.
I used builders nylon twine to restring the dial cord and a faucet spring to replace the rusted out originals. The cord must be very tight to prevent the weight of the capacitor rotor from detuning the radio. I found this out the hard way. I originally had used some springs that were too weak and the tuning was very 'iffy'. I even considered using car wheel weights to balance the rotor! There were two springs installed originally. I used one strong faucet spring that had been intended to be in compression.
The final step in the chassis restoration was alignment. The alignment trimmer capacitors are accessable through holes in the top of the chassis. I made an alignment tool from polybutylene plumbing tubing by heating a 1/4 inch Allen wrench and forcing it into the tubing. It made a good insulated adjustment tool. There is B+ voltage on the trimmer nuts! Don't try adjustment with a metal tool!
The cabinet base was in several pieces and had to be
reassembled, glued, sanded, stripped and refinished. I bolted it back
on to the cabinet and stood the cabinet vertical for the first time.
It took some time to decide what course to take in the restoration.
One thing was sure. The joints needed to be reglued. The old hide
glue had lost much of it's 'cling'. I removed and reglued all of the
gussets and forced glue into all loose joints. After several stages
of glueing and clamping over the course of about a week, the cabinet
was solid once again. I added two additional gussets at the top rear
since that is a classic weak point in most cabinets. The top can pull
off easier than you think!
I chose to strip the top because of flower pot water damage. After striping, the top was sanded with 200, 320, 400, 600 and finally 1500 grit sandpaper. I then applied many (I did not count them.) coats of clear lacquer sanding between coats until smooth then sprayed one light coat of Mohawk Perfect brown toned lacquer. I did pretty much the same to the cabinet front but chose to just sand the original finish on the sides and respray with toned lacquer. The grain pattern on the front was almost completely hidden until stripped. I can't believe that the cabinet manufacturer would hide a beautiful grain like that but I suppose that was the way they did it. The factory lacquer could have darkened or maybe there was just a buildup of seventy-five years of wax and dirt. After the last lacquer coat was applied all surfaces were sanded with 400, 600 and 1500 grit wet sandpaper. The last step was to apply a red mahogany stain to all surfaces. The stain filled all of the tiny scratches left by the 1500 grit paper and left a very smooth finish that almost appears three dimensional. After the stain was throughly dry I used clear coat auto polishing compound for the final buffing.
The grill cloth is original. I thought it was a replacement until Ron Ramirez (Mr. Philco) saw a photo and told me it was original. It was faded but undamaged so I reused it. To clean the grill cloth I used Arm and Hammer powdered upholstery cleaner and a vacumm cleaner. A restorer from South America (don't know which country and lost the URL) had a writeup on his website that discussed using lacquer on a grill cloth after dyeing it. I was afraid to wet the cloth or even pull it off the backing so skipped the dye step. I have had cloths come apart too many times. I sprayed the cloth with clear medium gloss lacquer and, after drying, it tightened on the backer and was strong and stiff yet still was porous so sound could pass freely. The lacquer should last a bit longer than starch and also not attract vermin.
Update 04-21-2005: I gave this radio away.
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