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.

Friday, January 22, 2016

My precision alignment tools for aligining optical waveguides

One of my jobs is assembling polymer optical waveguides in packages which will then connect accurately to MT format fiber optic ferrules.  Below is one example.  I installed 4 waveguides on this board with ferrules to connect to ribbon fibers with MT ferrules.  A ribbon fiber is installed on one of them.
I often make small waveguides with fibers connected on both sides.  Below are some of those wave guides.  They are 10mm long, but will be trimmed to 8mm when complete. The extra 2mm aids in assembly and alignment.
To work efficiently, the waveguides must be accurately aligned between the pins to ±1 micron. In large volumes, we laser cut the parts to fit with micron precision.  However, in small volumes, it is more cost effective to cut them manually, and then align them under a microscope.  In the picture below you can see the waveguides inside the plastic sheet.  I designed an alignment tool to position the waveguides and hold them while the epoxy cures. 
The tool works by gripping the waveguide film with a pair of sharp points which are then moved to position the film accurately.
After the epoxy is cured, the pins are removed and the ends of the waveguides are trimmed and polished on machines I modified to better suit my requirements.
Here is a close-up view of the end of the waveguides. This is from a part that I connected fibers to using an MT ferrule.  I epoxied the parts together and then separated them after testing was complete.  The circles were created by the fibers, and serve as proof that the alignment was good.
Here is the tool. In the first picture, the clamp is lowered to hold the waveguide assembly in place, and in the second picture the clamp is raised so that the finished part may be removed.
The tool has 4 adjustments.  The two knobs on the sides move the points in and out to grip or release the film, while the two micrometer knobs are used to position the film in between the pins.
Alignment is done visually using a Wild macroscope equipped with a translation stage with digital readouts.  With care, accuracy of ±1 micron can be achieved.
The alignment tools fit in a base designed to hold the tools accurately, even when removed and replaced. This permits a higher volume of parts to be assembled, as multiple tools can be placed under the microscope, aligned, and then removed while the epoxy cures.
Daily throughput is increased by making multiple copies of the tools, which are easy to reproduce. 
 These are a good example of the accuracy that is possible on a modified Harbor Freight (Seig) Mini Mill.  Each bottom plate has 18 counter bored 4-40 holes, and the parts attached to the bottom plates have threaded 4-40 holes.  All of the parts are accurate to within ±.025mm, and are interchangeable. 
I developed another set of tools to align optical waveguides to VCSELS and detectors. Here is an array of 4 detectors coupled to a polymer waveguide with a mirror on the end. This technique uses a unique active alignment scheme that did not require the circuit to be powered up.