My
antique radio restoration logs At the March 1995 AWA meet at Charlotte, I picked up what appeared to be a real
bargain: a Majestic Model 20 semi-cathedral. The cabinet had been refinished,
and the grille cloth replaced. Original knobs and tube shields were in place,
but it had no tubes. Cursory examination showed that the speaker cable had been
detached from the speaker - a bad sign. The line cord and plug were original -
good - it probably had not been diddled with! The price was only $40! This
looked like a good project, so I picked it up (it must have weighed 60-70
pounds!) The Majestic model 20 is an 8-tube non-AVC superhet, circa about 1931. It
calls for Majestic type tubes, which are quite rare: G-51 RF amp, IF amp, and
converter; G-27 oscillator and power detector; push-pull G-45's audio output;
G-80 rectifier. Per Riders I, the G-51's are equivalent to type 24A's. After pulling the chassis, I began my usual process of evaluation before
attempting repair. If major parts are bad and cannot be replaced without ruining
the appearance of the radio, I often will simply sell rather than restore. I
like to maintain the original appearance, both above and below chassis. The
first concern was the speaker, which was large and quite unique. The field coil
and voice coil were OK, and the cone was not torn. So far, so good. Now for the
chassis. The chassis is totally enclosed and very deep. The first challenge was
removal of the bottom cover. The first hint of trouble was that it appeared that
there were components attached! Removing the screws (mostly missing) confirmed
my worst fears - there were indeed parts attached to the bottom cover. I next
removed the side covers to confirm. Riders I mentioned that specific wires must
be disconnected before the cover can be removed, but the drawing was difficult
to read. I began tagging and removing wires until the bottom cover could be
removed. It turned out that the output transformer, filter choke, and
combination filter condenser block, detector RF choke, and interstage driver
transformer were mounted on the bottom. Obviously this radio was not designed
with service in mind! After the bottom cover was removed, the survey was continued. The large
wire-wound multi-tapped bleeder was the next worry - fortunately it was good per
Rider's resistance specifications. The power transformer had been replaced
(looks like an early Philco), and resistance measurements of the primary and
secondary were reasonable. There was no sign of burning or melting. Putting 10
volts on the primary from a Variac indicated equal secondary voltages across the
center tap - so far, so good. The filter choke was OK. Wiring protruding from
the filter block had been cut, and it was found that someone had tacked in a
small interstage transformer on the top of the chassis. The transformer was
removed. Resistance measurements of the existing transformer (inside the filter
condenser block) indicated an open primary winding - the usual failure mode.
This was not a showstopper, since appropriate plate to push-pull grids
transformer can be found or even purchased. But since the radio used a
"power detector" circuit with a plate current of 8 ma driving
push-pull 45's, a large unit with lots of iron and a high primary inductance
would be needed in order to prevent loss of bass response. The standard A-53C
type transformers would not cut it in this application. I found a reasonably
sized unit in my junk box. Since the transformer is mounted inside the condenser
block, appearance did not matter. The output transformer was OK. The paper filter caps showed high leakage
(about 500K each) and would have to be replaced. Other problems found included:
a bad line cord (plug OK), missing pilot like (#41), power switch non-operative
(suspect it just needed cleaning), volume control bad, most resistors high by
30-50%, bypass block condensers leaky, tone control condenser leaky, bad speaker
cable. On the good side, the IF, RF, and oscillator coils measured reasonable
resistance values. The pot-metal bushing holding the tuning drive shaft was broken. It appeared
that a new one could be fabricated and bolted to the tuning condenser frame. So
this was not a show-stopper. It now appeared that the radio could be restored,
so the first order of business was the replacement of this bushing. I found a
suitable piece of brass in my junk metal box (I NEVER throw anything away!)
which was the correct inside diameter (0.25") and had a flange on the end
which could be bolted to the tuning condenser frame. This was shaped up on my
Unimat lathe, the remnants of the pot metal removed with a Dremel moto-tool,
holes drilled in the condenser frame, and the existing shaft installed. With a
little cleaning and grease, the drive mechanism worked great! Next, the filter block was addressed. It also contains the detector RF choke
(good) and driver transformer (bad). Retaining lugs were pried back to release
the top cover, but it would not budge. I then unsoldered the wires from the lugs
on the cover. Removing the cover revealed that everything was embedded in TAR,
except the RF choke, which was still attached to the top cover. In order to
service the filters and driver transformer, the tar would have to go. So, I
placed the unit upright in a pan (to catch any tar drips) in a 300 degree oven
for 1 hour. Once the tar began to bubble and expand, cooking was complete. The
old tar was poured off, and the interstage transformer and filters removed (with
great difficulty). The can was cleaned up as well as possible (only on the
outside) using lacquer thinner. The replacement driver transformer was bolted to
the inside of the can, and its leads extended so they could be attached to lugs
on the cover. Riders listed the filters values as 1 and 2mfd units. For
replacements, I used 1 and 2 mfd 600 volt mylars I picked up at the Charlotte
meet. It is very important NOT to use large electrolytics when servicing early
AC sets - use the original values. Otherwise, the B+ will be much too high.
These sets were designed to operate on 110 volts - line voltage at my house is
122-124 volts! In order to reduce hum, there was a 0.07mfd condenser across the
choke coil, forming a resonant circuit. This was replaced by paralleling 0.047
and 0.022 Mylar caps. All the condensers were attached to the bottom of the top
cover. The tar was not replaced. The bypass blocks were serviced next. They don't make it easy in this radio!
The condensers were in metal cans which were both riveted and soldered to the
chassis! I drilled out the rivets and removed them. One of the units contained
four 0.1mfd units. The other (power detector cathode bypass) was listed in
Riders as two 0.4mfd units in parallel. These units were also embedded in tar. I
was able to coax the contents out by heating the container with a heat gun and
pulling on the lugs. Modern Mylar caps were substituted, the existing lugs
attached to a cardboard cover, and about 1/4" of the original tar melted
and replaced to maintain original appearance. The blocks were reattached with
screws and re-soldered to the chassis. Several resistors which were more than 20% high and in more critical
functions (such as bias resistors) were replaced with units of the same vintage
(dog-bone type). I have accumulated quite a collection of off-value old-style
resistors. These are measured and the correct measured (but not MARKED) values
are used to restore other radios. I make careful notes to warn future restorers
that the color codes of the replacement resistor will not match the schematic. I next removed the volume control rheostat and switch assembly for service.
The switch (a bat handled toggle actuated by a cam on the back of the volume
control switch) responded to cleaning with "Big Bath" (the non-CFC
equivalent of tuner cleaner). The back of the volume control was removed,
revealing the burned element (trouble!) Riders I showed the value as 4800 ohms.
The control is in series with the bleeder resistor, and operates by changing the
bias on the RF amp, converter, and IF amp. So the value would have to be fairly
close. This turned out to be a real challenge, since the shaft length was
critical, and would have to be able to be extended out the back of the unit in
order to operate the AC switch. The closest thing I could find in my junk box
was a 10K (linear taper) wire wound dual unit with concentric shafts. The rear
unit was removed, a 10K resistor installed in parallel to lower the maximum
resistance to 5K ohms, and the inner concentric shaft modified to operate the AC
switch. The rear of the inner shaft was turned down on the Unimat to accept the
switch cam, cut flush with the outer shaft on the front side, and then drilled
and pinned to the outer shaft so that both would turn together. The power cord was replaced using a piece of cloth covered wire of the proper
color, and the original AC plug re-used. The speaker cable was replaced using
cloth covered ("AK style") 8 conductor cable sold by several suppliers
- only 4 conductors were used. A found a set of tubes and installed them. It's
getting close to SMOKE TEST time! In order to be able to measure voltages, etc., I did not want to install the
bottom cover. So I used jumpers to connect the chassis wires to the components
on the bottom cover (choke, condenser, output transformer). Making sure no wires
were touching, I fired up the radio on a Variac, keeping a meter on the B+ at
the rectifier. The radio came alive on the first try and worked! However, there were more problems. There was a very bad rattle in the
speaker. Closer examination showed that the voice coil had mostly separated from
the cone, and the centering device was missing. The cone was removed and the
repair effected using service cement. The cone was then reinstalled and
centered. Most of the rattle was gone. The various voltages were then measured
and compared to Riders. Trouble! The B+ 180 line measured only 90 volts. B+ 90
(screen grids) showed only 40. B+ raw was also low. My first thought was that
the replacement power transformer did not deliver the correct B+ voltage. But
then I noticed that the first section of the bleeder was getting quite hot. I
then shut it down, disconnected the B+180 line (which feeds the RF amp, IF amp,
and converter plate circuits) and made resistance measurements. Bad news!
Resistance to ground was only about 8K ohms! Removing the tubes did not change
this. I had rebuilt the bypass caps, so what could cause this? Looking at the
schematic again, I found six 0.1mfd bypass caps listed. But I had replaced only
4. Where were the other two? No cans were in site. I traced out the wiring and
was able to mark off the ones I had replaced. The two remaining were
suspiciously close to the IF transformers - could they be INSIDE? If these were
leaky, that would certainly account for the low B+. Disconnecting the wires on
one IF transformer confirmed that the leakage was inside the can. So, next I needed to remove the IF cans. Each was bolted to an upright
bracket which was riveted to the chassis. Once the cans were removed, it was
found that the top cover holding the trimmers and terminal lugs was RIVETED to
the case. I pulled the wiring back through the holes in the rivets and drilled
out the rivets, but the cover could not be moved. A careful examination
indicated that even the IF transformers, along with the bad bypass cap, were
potted in TAR! So back to the oven. The can was cooked at 300 degrees for about
45 minutes until the tar began to liquefy and expand. The expansion is the
signal that the tar has liquefied enough to remove any components - if you leave
it cooking too long or at an excessive temperature, it may overflow, smoke,
burn, or all of the above! If you attempt to remove components from tar that is
not liquid enough, the components may be damaged. The transformer, trimmer and
bypass cap were removed and the tar poured off. The defective bypass was removed
and a modern 0.1mfd unit installed. Fortunately all this did not hurt the unit -
it still could be adjusted to 175Kc using a signal generator and 'scope. The
transformer was reassembled using screws and nuts. Questionable lead wires were
replaced with vintage type wire. The tar was not replaced (the IF transformer
coils still had a healthy layer of tar to protect them). Both transformers were
repaired and reinstalled. After this, the B+ was found to be right on specification. The radio proved
very sensitive on my 50' attic antenna. The linear taper replacement volume
control, along with the lack of AVC in this set, made tuning a challenge -
definitely a two-handed operation. Hum was almost nonexistent despite using only
2mfd filter condensers. Small filters are compensated for by having a tuned
filter choke plus using the speaker field as an additional choke. This was probably the most difficult restoration project I have ever
undertaken, but there was some satisfaction in getting the set working yet
maintaining original appearance both above and below the chassis.
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