The
restoration
of the electronics on this radio began with a twist,
but ended up being nothing more than routine later on. After a good
cleaning of dust and loose debris from the chassis, the first repairs
included replacing the filter capacitors and installing a new power
cord, which I opted to use reproduction cloth-covered cord. On the
first power up, I was greeting by a 60 cycle hum, and the B+ voltages
were way off. As some point in time, the power transformer had been
replaced, and it was a sloppy job at that. Many other capacitors,
controls, and resistors had also been replaced over time. With a
schematic in hand, I was quick to notice that the center tap on the
power transformer was not connected to the proper place. The way
everything was connected, the filter capacitors were essentially doing
nothing at all!
With the nasty looking repairs of the past, I
decided to clean the wiring up and check out the rest of the power
supply section. On the second power up, the hum was gone, but after
several seconds of warming up, the ammeter on my power supply would
begin to jump around. That is when I noticed arcing inside one of the
6V6 power output tubes. Testing both of the 6V6's on my B7K 747B tube
tester revealed shorts in both of them. Further testing of tubes
revealed three more with internal shorts, and the 6C5 inverter showing
no grid emissions. All of the tubes in this radio were RCA branded with
matching date codes. The large number of bad tubes seemed rather odd to
me, the most that I have ever encountered in a radio. The mis-wired CT
of the transformer must have been the cause. It had been connected to a
point that shared a connection with the cathodes of the 6V6 tubes.
Perhaps this created some kind of crazy voltages inside of the tubes,
thus causing arcing and eventual shorts.
With the replacement of the bad tubes, on the third
power up the receiver was working much better. The paper capacitors
were then replaced, testing out the receiver after replacing two or
three at a time. This way if I make a mistake, it is much easier to
retrace my work and find the problem. Once the capacitors had all been
replaced, the controls were cleaned and the alignment was checked. The
resistor in the eye tube base was way out of tolerance, so it was also
replaced. Two
small modifications were then made to the radio circuitry. A fuse was
added to the hot side of the AC line voltage going to the primary of
the power transformer. I also added a thermistor on the 120V AC. The
thermistor is around 200Ω when cold, and once warmed up, drops to just
a few ohms. With the addition of the thermistor, this supposedly
decreases "thermal impact" of the tube filaments and other components
by letting the voltage slowly rise. Another result of using a
thermistor is that it knocks down the AC line voltage by a few volts,
effectively causing the power transformer to run a little cooler.
As you can see
in the above photos from before the restoration, the cabinet was in
several pieces and needed quite a few repairs. Luckily, all of the
pieces were accounted for, including some of the larger veneer chips.
The cabinet restoration began by disassembling the remainder of the
cabinet, and building it up from there. The sides, with a top layer of
veneer and several sandwiched layers of a cheaper wood, had to be
completely re-glued. The base was reattached/ glued to the front panel.
The repaired sides were then attached, followed by the top.
Below photos: the cabinet during reassembly