Universal Small Cathedral Restoration

The radio is a small 4-tube TRF circuit cathedral radio branded Universal.  There is no model number indicated.  No schematic has been found.  I drew a hand-drawn schematic for the radio before starting restoration.

The radio had seen some servicing in the past.  All paper capacitors and resistors were original.  The line cord and the filter capacitor had been replaced.   I decided to try to reverse these repairs and to restore the set to its original condition to the extent possible, yet get it working. 

My antique radio restoration logs

Condition As Found

This radio was purchased on the Antique Radio Forums Classified Site.  The cabinet was in good condition with its original finish, but a little faded.  The grille cloth also looked to be original.  The knobs had been replaced by the seller, but were appropriate to the radio.  The manufacturer of this radio is likely Universal Radio & Television, but this has not been confirmed.  According to The Radio Collector's Directory and Price Guide, by Grinder, Universal Radio & Television made radios only in 1931, including three cathedral models.  The model number of this radio is unknown, but it could be 42, since the "4" may indicate the number of tubes.  Other cathedral models listed in Grinder start with 5 and 8 (again, could be tube count). The date of manufacture is consistent with 1931 based on the circuit and the parts used.  A photo of this radio appears in the Stein book, listed as Universal, 1931 (it has a different dial escutcheon).

Previous Repairs


My usual restoration procedure is to first make a complete survey of the condition of all components.  The survey results guide my restoration strategy.  I never apply power to a radio before restoration, even through a "dim bulb tester" or variac "to see if it works".  If major and unique components are defective or missing and cannot be restored or replaced, I may elect to sell the radio for parts rather than restore it.  I always assume that all paper and electrolytic capacitors are leaky and thus should be replaced (I always "restuff" the original containers if possible).  Any mica capacitors are assumed OK until testing proves otherwise.  


Before starting repairs I made BEFORE photos of the chassis top and bottom.  I use these photos to ensure that replacement parts and wiring are placed as close as possible to their original positions.  Some radios are subject to problems (such as oscillation or feedback) if wiring is re-routed or lead dress is not the same as the original.  Plus, I prefer to keep the under chassis appearance as original as possible.

All tubes were removed.  The tuning capacitor and filter capacitor were removed.  The chassis was dusty, but had no rust.  It was first cleaned off with an air compressor.  The chassis top and power transformer were then cleaned using GoJo (white) hand cleaner and 00 steel wool, and then wiped clean using paper towels.  The chassis was then carefully checked for any steel wool remnants which could cause shorts.

The power cord was replaced using a reproduction cloth covered cord available from Antique Lamp Company.  The existing old style acorn plug was reused.  Several frayed wires were replaced, and others were insulated using shrink tubing.  The 3-lead speaker cable was hopelessly frayed.  A replacement was fabricated.

One dogbone resistor (40K 1 watt) was replaced by a NOS unit of the same type that was in tolerance.  I purchase all the NOS and used dogbone type resistors I can find just for this purpose.  The remainder of the resistors were within 20% tolerance and were left alone.

The ST type 24A tube was replaced by a globe type 324 so that all tubes were the same types.

Volume Control

The AC switch on the volume control was OK, but the control itself was open.  The control was a complex wire wound affair with dual tapers - there was coarse resistance wire on both ends and fine wire in the center 50% of the control.  The control also had a minimum resistance of about 180 ohms to provide bias for the RF amplifier at maximum volume.  The resistance was about 8K ohms.  The usual repair strategy is to replace the control with a modern 10K linear taper control with switch, or to use a 10K reverse audio taper if one can be found, and to add a fixed resistor in series for the minimum resistance section.  But if this is done, control of volume is difficult and most of the control is crowded on one end of the control and is not smooth.

The break was between two of the sections.  The wire had originally been welded, but the weld or the wire had broken.  Nichrome wire cannot be soldered.  So I tried another approach.  The wire on both sides of the break was cleaned and sanded until shiny.  A small piece of thin brass was then inserted to bridge the break.  As it turns out, the resistance element was pressed tightly against an outer insulation layer by a center insert.  The repair worked fine, but may fail in the future.  The finish resistance was about 7K ohms (originally 8K or so).


Since I did not know what the original power supply filter capacitor looked like, the existing capacitor was restuffed with new 10mfd 450 volt capacitors.  The tacked in filter capacitor was removed.  Another similar radio was found on the web with the same chassis, but this one was branded "Paramount".  Perhaps this chassis was used on many different radios.  In any case, the filter capacitor in this radio was likely what the original capacitor looked like.  The tall cylindrical shape matched the mounting holes in my chassis.

I attempted to reproduce the unique paper capacitors rather than pepper the set with new mylar capacitors.  The original capacitors appeared to be a normal paper-foil roll capacitor with attached end leads.  The leads were then wrapped around a slotted fiber board to reinforce them and to prevent them being pulled loose.  The entire assembly was then dipped in some sort of low melting point black wax.  I suppose the purpose of the wax was to prevent moisture absorption.  The normal method of manufacture would have been to insert the foil roll in a cardboard tube and then seal the ends with wax.  Unfortunately, the wax tends to crack with age, thus admitting moisture.  None of the capacitors had any form of identifying information, such as value or manufacturer.  There were about three different sizes used in the radio.  Most originals were too leaky to measure accurately.  Based on the measurements I could do, and circuit usage, I chose reasonable values - all at 630 volts.  The values used are documented in the hand-drawn schematic.

In order to reproduce these capacitors, I attached a modern film capacitor to the original fiber board and added longer leads (Step 1 below).  The assembly was then dipped in melted rosin/wax salvaged from RCA Radiola Superheterodyne catacombs (Step 2 below).  Finally, the assembly was painted using semi-gloss enamel (Step 3 below).  The result was a little to shiny - I should have used flat black enamel paint!

Here is a photo of an original and a reproduction capacitor:


The cabinet was vacuumed and then cleaned using GoJo white hand cleaner and 00 steel wool.  No further treatment was needed.  The knob set screws were changed to shorter screws.

Testing and Alignment

Once the radio chassis was reassembled and the tubes installed, power was brought up slowly using a variac in order to reform the filter capacitors and to check for problems.  The radio powered up and voltages were normal, but there was no reception.  The radio was completely dead.  The problem was traced to an open primary winding on the antenna coil.  This coil had tested OK during my initial survey, but was now open.  One end of the primary winding had broken at the terminal, and was easily repaired.  Movement of the terminal during cleaning and repair had likely caused the break.  Once this was repaired, the radio was working, but badly out of alignment.  There were only two trimmers, and these were easily adjusted.  The radio worked very well for a 4-tube TRF, with only a 25 foot antenna in my basement ceiling (and I am in a rural area).  The repaired volume control worked very well.

Restoration Results
Chassis Bottom Before and After Restoration

Hand Drawn Schematic (capacitor values as replaced)