Saturday, June 18, 2016

1948 Zenith Trans-Oceanic radio

A friend gave me a 1948 Trans-Oceanic radio. It was in poor condition on the outside, and while it looked good inside, the radio did not work. While I have considerable experience with conventional tube radios, the battery Zenith stymied me at first.   Troubleshooting these is complicated by several things.  Here is my list of what makes a Trans-Oceanic more difficult.

1:   1.2 volt tube filaments do not exhibit a visible glow. This is by far the easiest thing to check on an conventional radio.
2:   The tube filaments are in a series string.  You must have a complete set of tubes to do any troubleshooting, and removal of an individual tube to isolate a problem is not possible. In fact, removing and replacing a tube may blow the filament if the capacitors are still charged.
3:   Being directly heated tubes, the filament circuit must also create the proper grid bias voltages. Being directly heated and series string makes this even more complicated.
4:   Since the tubes operate as such low voltages and currents, supplying proper voltages to the tubes is critical.  They are much more intolerant of low heater voltages than conventional tubes, especially if the tubes are weak.
5:   The above difficulties are compounded by the added complexity of the shortwave circuits and the AC/DC/Battery power supply.

To gain a better understanding of the radio, and to better appreciate the fine engineering that went into it, I decided to study it and create separate schematics for each major circuit.

Here is the B+ supply:  Included in this schematic are the plate and grid bias circuits powered by the B+ supply. Only the components needed to deliver the B+ voltages to the tubes are highlighted. There is a high B+ of 90 volts, and a low B+ of 76 volts.
 The filament circuit. Included are the grid bias circuits, including the grid biases derived from the AGC circuit. Included are the filament voltages you will see at each tube when using a 10.5 volt source.

Here is the AGC circuit by itself.  The AGC controls the gain of the mixer tube in the Broadcast position only. In any of the shortwave bands, the mixer operates at a fixed gain. The band switch is shown in the Broadcast position. 

 The RF, IF and Audio signal circuits, stripped down the the bare minimum to illustrate how the signal passes through the radio.  Only the Broadcast position is shown.
The Oscillator circuit is by far the most complicated, and the most intolerant of weak tubes, batteries or other components.  I believe all troubleshooting should be done using batteries or a very good bench top power supply. The radio's AC supply should be used sparingly, and only after replacing the capacitors, and adding Zener diodes to protect the tubes. The original AC supply simply is not very good. In addition, it is not the safest, for there is no isolation from the mains. A shock hazard is present, especially if there are leaky capacitors. 

Please note that the tuner has its own isolated ground circuit.  The paper capacitors in my radio were leaky and the tuner ground was elevated a few volts above the chassis ground. 

Here is the Oscillator in Broadcast mode.

The Oscillator in SW mode.  The other SW positions are similar.

The complete circuit:
  The separated circuits are good for illustrating that many components are part of multiple circuits. The IF transformers, for example, carry not only signals, but also B+ voltages and AGC voltages. Here, the signal paths are given priority over the other functions.

The case restoration:

 The cabinet looked like a mouse chewed on it.  Not only was the vinyl coated cloth gone, some of the wood was chewed too.

I patched the chewed areas with nylon cloth and JB Weld epoxy. After the epoxy cured, I trimmed the cloth and sanded the patches.

After the epoxy cured, I painted everything with satin finish black paint.  The patches are now only visible under close inspection.  The most noticeable difference is that the texture of the patches does not match the original finish. But it looks good.

The radio is an excellent performer on batteries, and just OK using the AC supply.  When on AC, the filament voltage is less than 10 volts, and the B+ is greater than 90 volts. The low filament voltage is enough to make a noticeable difference in the gain of the tubes.  Occasionally, the oscillator quits when using shortwave and the AC supply.  It is very sensitive to changes in voltage.
I tested the radio at various line voltages, and here is what I found:
Minimum reliable working voltage on shortwave bands: 124.5 vac

Voltage at which shortwave does not work at all:  121.2 vac

A and B supplies at 124.5 vac:  9.7vdc, 94.5vdc (high B) and 88.8vdc (low B)

A and B supplies at 121.2 vac:  9.43vdc, 93vdc (high B) and 86.5vdc (low B)

Voltages on 1LA4 tube:

On batteries:                                                            At 124.5vac                                               

1: 6.2vdc (1.55v cathode voltage)                      1: 5.7vdc (1.4v cathode voltage)

2:90.1vdc                                                             2: 89.9vdc

3: 81.1vdc                                                            3: 80.8vdc

4: 1.3 vdc (-5.4v relative to cathode center)     4: 4.6vdc (zero relative to cathode)

5: 53.9vdc                                                            5: 52.7vdc

6: 4.65vdc                                                            6: 4.3vdc

7: 4.65vdc                                                            7: 4.3vdc

8: 4.65vdc                                                            8: 4.3vdc

In the above test, the oscillator is not working on the shortwave bands when using the AC supply.  Oscillator operation can be verified by looking at pin 4 of the tube.  If the oscillator is working, pin 4 is negative compared to the cathode voltage.  When the oscillator quits, the voltage on pin 4 rises to equal the cathode voltage.

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