RCA Model 100 (Late) Cathedral Restoration

My antique radio restoration logs

 The RCA Model 100 (Late Version) from 1933 is a 4-tube AC non-AVC superhet circuit radio.  It receives the standard broadcast band and the old police call band.  The only schematic I have been able to find for this radio is at Radio Museum.  Membership is required for access. The Riders Manuals and Nostalgia Air include the early version schematic listed under RCA 101 and information on a chassis similar to the Late Version under the RCA 301. All parts references are to the Radio Museum schematic (most of which apply to the Early Version in Riders).

Overview and Prior Servicing

The radio was purchased on eBay.  It was said to power up and receive static but no stations. I slowly powered up the radio using a fused variac and watt meter in order to verify the eBay seller's claim. It did indeed power up OK and there was some noise from the speaker. I normally never apply power to a radio until it is first restored.  But in this case it was known that the seller did apply power (likely full line voltage).  

I always attempt to avoid purchasing radios that have been "restored" by collectors or flippers, and am looking for either all-original examples or those which have been "lightly serviced" in the distant past by radio service shops, rather than peppered with new film capacitors. Service shops would typically only replace the minimum number of parts. Based on the eBay photos provided, all of the top chassis parts were still in place and there were no signs of any restoration or repairs.  The line cord was original and the cardboard cased filter capacitor was still in place. But when I pulled the chassis, I was disappointed to say the least! All except one wax/paper capacitor had been replaced with modern film capacitors. And one of the original  capacitors had been replaced by a disc ceramic. A tubular filter capacitor had been tacked in under the chassis, although the original filter capacitor was still connected. All the original resistors were in place. The volume control and pilot lamp socket had been replaced. 

On thing was very worrying: there were a couple of power resistors (5 and 10 watt) plus some diodes found connected to the 1-V rectifier socket. I was very concerned that the power transformer had failed and that this was some sort of work-around. But after analyzing the circuit as wired, it was discovered that the added components and modifications were simply a replacement for a burned out 1-V rectifier tube!  This was a lot of work to replace a very common tube. Whoever did this substituted a 25 ohm 5 watt resistor for the tube's filament, and a 120 watt 10 watt resistor in series with two diodes (in parallel!) for the rectifier function. This radio's circuit is quite unusual in that the 6A7, 6F7, and 1-V tube filaments (all 300ma) are in series and connected between a 6 volt and 24 volt tap on the power transformer's single secondary winding.  The 6 volt tap supplies power to the 41 output tube filament and the pilot lamp.  So all I had to do to reverse this "repair" is to remove the added components and plug in a good 1-V tube.

I decided to try to reverse any previous servicing to the extent possible and to restore as much of the original  chassis appearance as possible. My objective was that any repairs or restoration would not be obvious, and no new parts visible. All of the missing original capacitors and replacement volume control and pilot lamp socket would limit success in restoring originality. Even if original capacitors could be found, their exact locations in the chassis was unknown since all but one had been replaced.  The provided RCA connection diagram is helpful, in that at least connecting points are documented. But the only such diagram is for the early version radio, and there are differences.  It is not known if the schematic and connection diagram for the RCA 301 Duo (in Nostalgia Air, ignoring the phonograph connections) is the same as the RCA 100 Late Version.

This same or similar chassis is used in other RCA and GE radios. The Riders manuals contain the early version schematic, parts list and connection diagram. That information is available on-line at Nostalgia Air under the RCA 101. Radio Museum has both the early and late version schematics. The main difference between the early and late version is that the early version uses a type 38 output tube.  The late version uses a type 41 output tube. In the early version, since all tubes are 300ma, they are all in series and connected to a 24 volt tap on the power transformer high voltage secondary.  The 6 volt tap is only used for the pilot lamp.  In the late version, the 41 output tube (400ma filament) and pilot lamp share the 6 volt tap.  The three remaining 6 volt tubes are in series and connected between the 24 volt and 6 volt taps. One other difference I noticed is that the tone capacitor C20 is 0.025mfd for the early version and 0.017mfd for the late version. The output transformer is also likely different due to the different audio output tubes used (different DC resistances on the schematics). This radio uses half wave rectification for the B+. The power transformer has only a single secondary winding with multiple taps.

Survey

My usual restoration procedure is to first make a complete survey of the condition of all components.  The survey results guide my restoration strategy.  If major and unique components are defective and cannot be restored, I may elect to sell the radio for parts rather than restore it.  I assume that all original paper and electrolytic capacitors are leaky and thus should be replaced (I always "restuff" the original containers if possible).  In this case, most of the original paper capacitors had been replaced. Since this radio actually powered up and made some noise, it was assumed that all major parts were OK. 

Repairs

First, all non-original parts were removed after taking careful notes of where they were connected.  

Paper Capacitors

Only one original paper-wax capacitor remained: C1, 0.01mfd.  But this told me what the remaining (now missing) capacitors should look like.  Early RCA/GE paper capacitors of this era were simple cardboard tubes with each end sealed with tar.  No value or voltage rating appears on the part - only a 5 digit number such as 69031 (sometimes there is a suffix also).  This number bears no apparent relationship to the RCA part numbers listed in the parts list.  I maintain a collection of dud capacitors just for this situation. My collection includes branded (Zenith, Philco, RCA/GE) as well as generic (Sprague, CD, etc.) parts.  I had previously restored an RCA 301 which has a similar chassis.  In my restoration notes for that radio, I had noted the part number for each of its capacitors (which were restuffed).  This gave me the part numbers for most (but no all) dud capacitors to look for in my collection. I had appropriate sized RCA/GE duds to replace the capacitors that had been removed, but only had the exact part in one case. 

The one original and all replacement paper capacitors were rebuilt using modern 630 volt axial film capacitors in order to restore the likely original under-chassis appearance. These types of capacitors are very easy to restuff.  First the component leads are pulled out of each end, which also takes much of the tar with them.  Then the foil roll is pushed out one end using a screwdriver.  The cardboard tube is then cleaned of any remaining tar.  I also clean the outside of the tube by heating with a heat gun and wiping away the wax, tar, and dirt. A new axial film capacitor is then inserted.  Before restuffing, the component's original lead length and any insulating sleeving use is noted. In some cases longer leads must be first attached to the replacement capacitor before restuffing (I use #20 or #22 solid tinned buss wire to extend the leads). Here is the method I use to rebuild cardboard tube type capacitors.  

One capacitor C20/C21 was a dual capacitor (0.017mfd and 0.005mfd), with wire leads.  One of the existing leads from the output tube plate was still in place. I chose a suitable sized RCA/GE cardboard tube from my dud stock, which was restuffed using  .015mfd and .0047mfd 630 volt film capacitors.

Capacitor C15 (2400pf) had been replaced using a 2200pf disc ceramic capacitor.  It is unknown what type of capacitor was originally used. It could have been a metal clad mica type, since there are quite a few of this type used in the radio, and the early version connection diagram hints of this type.  I also have chassis photos of an RCA 301 Duo and a GE M-49 both of which use an almost identical chassis.  One such chassis used a normal flat brown bakelite mica capacitor with flat leads terminated in a ring.  Another radio used an RCA/GE tubular capacitor sealed with tar, like all the other paper capacitors in my radio.  Since the capacitor had been REPLACED, and since no other mica capacitors were replaced, then I had to assume that the original must have been a tubular. In yet another RCA/GE restoration, in the process of restuffing this very same capacitor, I discovered that inside was a metal clad MICA capacitor rather than the usual foil/paper roll!  It was tested and good, so I simply resealed the tube.

Two capacitors, C10 and C18 remain a mystery at this writing. The values, types and placement used vary depending on the model. And I have never seen an actual original example of C10 as a 4mfd tubular.

I decided to replicate the configuration used by the RCA 301 and GE M-49.  I installed a single lug terminal strip similar to the original seen on an RCA 301 on the rear chassis and used a restuffed 0.25mfd capacitor for C18.  I had an original part to restuff in this case - which looked like a block of tar wrapped in thin paper. Here is how I restuff this type of capacitor. For C10 I used an RCA/GE dud paper capacitor which was the appropriate size and restuffed it using a 4.7mfd/450 volt axial electrolytic capacitor. Here are the restuffed capacitors:

Filter Capacitor Block

The original cardboard cased filter capacitor C22/C23 (2 x 8mfd high voltage) and C13/C19 (2 x 5mfd low voltage) was opened up and restuffed using two 10mfd/450 volt  and two 4.7mfd/450 volt electrolytic capacitors. The rivets holding the capacitor box to the upright metal mounting bracket were first drilled out. Then the backs of the hollow eyelets holding the cardboard box together were ground off using a Dremel Mototool with a grinding stone.  The eyelets could then be removed from the top with minimum damage to the cardboard case, and could be reattached later.  The stiff bottom cardboard base was then removed using a heat gun to melt the glue and wax.  The capacitor case was then carefully opened, again using the heat gun.  The capacitor inside as well as the original wire leads were then removed after severing the foil connections to two internal terminal rivets.  Two 4.7mfd at 450 volt capacitors and two 10mfd at 450 volt capacitors were then mounted inside. All of the original wire leads were re-used. The two 5mfd (4.7mfd) and two 8mfd (10mfd) capacitors have separate common negative leads.

The capacitor was then reassembled and held together using hot glue.  The bottom cardboard base was reattached using hot glue.  The two eyelets were then reinstalled.  They are held in place by the capacitor's mounting screws.  

Resistors

Three original resistors significantly more than 20% out of tolerance were replaced.  I used dogbone type resistors as were used originally. I purchase all the reasonably priced NOS and used dogbone resistors that I can find on eBay or at antique radio swap meets.  I picked out NOS and used dogbone resistors from my collection that were either in tolerance or had drifted to the correct needed resistance and then repainted them to match the original resistor's color codes using hobby enamel paint. Another consideration is lead length. The replacement resistor's leads would ideally be long enough to reach each connection point without having to splice the lead.

Here are the replacement dog-bone resistors, ready for installation in the radio.  

Volume Control

The volume control had been replaced.  The replacement was likely a 5K potentiometer with switch (it measured 5.6K).  The original was 4K according to the schematic. My IRC Volume Control Replacement manual called for an IRC type D11-113 which is 4K ohms with an A taper (linear) and no series resistor (minimum resistance). Whoever installed the replacement control added a 680 and 100 ohm resistor in series.  So perhaps the original control had a minimum resistance of 750 ohms or so. My plan was to try the existing control (5K) without the series resistance. My concern was that the taper of the replacement control was correct.  In this application, a potentiometer with a linear or normal volume control type taper may not work well in spite of the IRC recommendation. The original part was likely a reverse taper part. If the incorrect taper is used, all of the control of volume will be crammed into the last 25% of shaft rotation.  I attempted to measure the taper by attaching a chicken-head pointer knob and mark points on the chassis at 0, 25%, 50%, 75% and 100% rotation. I then measured the resistance at each of those points in order to plot the taper.  Rough measurements were:

Knob Position    Linear    Installed Replacement Control Resistance
0                         5670    5670
25%                    4252    3820
50%                    2835    1970
75%                    1417    443
100%                   0         0

So it appears that the installed (replacement control) has some sort of reverse taper, which is needed in this application.  Only testing will tell if it works OK!

Other Repairs

Testing and Alignment

Once the radio was reassembled and tubes installed, power was brought up slowly using a variac.  AC power was monitored using a watt meter, and a DVM monitored the B+.  The radio came alive immediately and worked.  The set was then aligned. The oscillator could not be set to scale. With the oscillator trimmer completely loose, the dial frequency was about 50kHz too high, and the tuning knob could not be moved on the tuning capacitor shaft to compensate due to the flat portion of the shaft. The dial pointer (plastic) was attached to the back of the knob and also could not be moved. I assumed that this frequency error was due to changes in the oscillator coil due to age and perhaps absorption of moisture. The radio picked up lots of stations using my basement ceiling mounted 30' antenna.  It even picked up some stations on the "police" (low short wave) band in the middle of the day. 

The volume control provided a good adjustment range near maximum volume, so the taper of the replacement control was acceptable but not ideal. If the volume was set close to maximum (with no station being tuned) the set would break into oscillation. I assumed this meant that the original volume control likely had a minimum resistance.  And in fact, the previous "restorer" had installed 680 and 100 ohm resistors in series with the control. When I checked for oscillation at various points on the dial, I found that about 400 ohms was needed as a minimum resistance. I installed a 390 ohm 1/2 watt resistor in series, hidden inside black spaghetti tubing.

Restoration Results

I was not able to successfully reverse all previous repairs and to restore the radio to its un-serviced appearance.  No replacement parts are visible except for the volume control, grid cap leads, and some wiring including the antenna and ground leads.  It is likely that some of the components were not in exactly the same position as installed at the factory, since the radio had already been "restored" (recapped) when I received it.  I did not have an unrestored under-chassis photo to refer to - only some photos of RCA/GE radios with similar chassis. The paper capacitors which were restuffed were not the correct RCA part numbers as used originally. Only one original paper capacitor was intact - the remainder were from my dud capacitor stock. 

Chassis Before Restoration

Chassis After Restoration

Similar RCA 301 Chassis For Reference