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.






Wednesday, December 2, 2015

My Robotic Arm



I designed and built a robotic arm for a friend who had built a clever telepresence robot based on a Roomba vacuum.  His software development was brilliant, but he lacked the facilities to construct the mechanical parts, like the arm.  A primary requirement for the arm is for it to be lightweight, for the Roomba lacked the weight and power to maneuver with a heavy arm.  It was also critical that as much of the mass as possible was located close to the center of gravity so that the lightweight robot did not lose its balance.  Therefore, I located all of the heavy servos in the shoulder, and made the wrist and claw as light as possible.  Strong Kevlar strings transmit the motion from the servos to the joints.

The shoulder:  5 servos and several pulleys packed tightly together.
Servo 1: (large one center left)
Arm shoulder joint up/down
Servo 2: (large one top left)
Elbow joint up/down.
Servo 3: (small one bottom right)
Claw open/close
Servo 4: (small one above Servo 3)
Wrist clockwise/counterclockwise.

The wrist and claw.  Everything possible was done to reduce its weight. The aluminum parts were lightened by by milling large holes in them, and by keeping the parts count low.  The hinge halves and the wrist housing are one piece.  The wrist pulley and lever are also one piece.
 This was made on my Harbor Freight mill using a rotary table, since turning this piece on a lathe would not be possible.
This pulley/lever combination opens and closes the claw using a stiff wire which passes through the hollow wrist axle.


 The wrist is rotated by a pulley driven by one of the Kevlar strings. This string pair, and the claw string pair run through the lower PVC pipe.  There are pulleys in the hinged pipe joints to ensure that the string tension and position remains constant throughout the range of elbow movement.

The wrist pulley is held to the wrist axle by a wire pin to make removal for servicing easy.




The new robotic hand was far smaller and lighter than the original hand, which had its servo coupled directly to the claw. The original hand did not have an articulated wrist.   Since the original arm was being left on the robot, I copied the style by also using a pair of 1/2 inch PVC pipes for the "bones"


The lower elbow joint has 3 pulleys inside a very small space.  One bronze pulley guides the wrist strings, and the other bronze pulley guides the claw strings.  The center pulley is locked to the outer half of the joint, and carries the string which is attached to the elbow assist spring.   Top left in picture: The center pulley being turned from 1/2 inch diameter steel stock.  Top center: the steel pulley and one bronze pulley.  There was not enough space for a V groove in the bronze pulley, but since the bronze pulley extends into the relief cut in the sides of the steel pulley, the string stays on the pulley, and does not get caught in the space between them.   Note that the center of the steel pulley is threaded.  This pulley does not turn relative to the outer half of the elbow joint.  

The upper elbow joint is a completely different design, for its sole purpose is to move the elbow.  The pulley in this one was machined as one piece with the hinge half, and designed to take the large amount of torque required to actuate the elbow while having a lever arm length of only 3/8 inch.  The joint has a huge mechanical disadvantage of about 20:1.  As a result, about 20 foot pounds of torque is required for the arm to lift a 1 pound weight.  This was a consequence of choosing styling over  engineering.  I wanted to create an arm which moved without any external mechanisms visible.  However, it also met my requirement that the weight be kept very low.  The arm worked well, and met its design goal of being able to lift a cup of water.  Designing an arm which could lift more weight would have been pointless, because the robot would have lost its balance if it tried to lift more weight.

 The string was tensioned by turning this nut.  The string had to be kept very tight for proper operation. That was easy to do with the Kevlar string. I used multiple strings to achieve a tensile strength of about 200 pounds.  Manually moving the arm would not break the string.
The underside, or "armpit" or the arm.    There is very little wasted space.  Note how closely packed the  shoulder rack and pinion is.  The end of the rack is contoured to clear the curved end of the shoulder servo. It appears that there is no room for it to move, but it can move  to its full travel limits.  This gives the arm about 10 degrees of left/right motion. I chose this range of motion to keep the design simple. 
This photo shows how the 10 degrees of movement was easily done.  The arm can move that much while retaining a simple belt drive for the up/down movement, and all the servos can be mounted in the same frame.  This is only slightly more complex than not having any left/right movement at all.  A lightweight, compact design was more important than having a larger range of movement. 
Bottom view of the belt drive.  The servo pulley is also custom made. The splines in the servo pulley were filed by hand. 


The robot I bult the arm for is called MAYA, and it is the brainchild of Ben Hylak, who came up with the idea of hacking a Roomba robot and using it as the motive power for a low cost telepresence robot. It is Ben's telepresence robot concept and software development that is the reason for the national recognition he has received and his subsequent invitation to the White House.  My arm merely went along for the ride.  While I have never received, nor expect to receive an invitation to the White House, at least I can say that something I had a part in making was a guest of the President!


Tuesday, November 10, 2015

Stancor Ultralinear Amplifier

The Stancor original Williamson and Williamson Ultralinear amplifiers were kits marketed by Stancor to showcase the performance of their transformers.  They used a separate power supply chassis connected by a 4 conductor cable. A pair of these makes a great sounding stereo power amplifier.  The Ultralinear is much preferred because the power output is much greater. 25 watts vs. 8 watts. 

I rebuilt a set of these, and can attest to their fine performance. 
The number one problem with any old electronic equipment is the electrolytic capacitors. I removed the metal can capacitors and replaced them with modern ones.  The modern ones are much smaller than the originals, and are designed for mounting on a printed circuit board.

I cut pieces of circuit board material into the shape of the original capacitor bases, and drilled holes for the new capacitors.
At Right:  The new next to the old.

Below: An amplifier chassis with the new capacitors.
One of the originals was a dual capacitor, so my replacement has 2 capacitors, too. 
 The power supply chassis has 3.   The 3 capacitors combined with one of Stancor's chokes do a great job of filtering hum from the 440 volt supply.
 I made new cables to connect the power supplies to the amplifiers.  The plugs and sockets are the same as early 4 pin tubes, like the type 80 rectifier.  I needed a replacement plug, so I took the base off an old Philco 80 tube, and made an aluminum cap for it.
The chassis had no bottoms, but they have threaded holes to attach bottom covers.
I made covers from sheet steel and attached rubber feet to them
I modernized the chassis by adding power sockets and replaced the original 2 wire lamp cord with 3 wire grounded cords.
 The modern cords and the steel bottom covers make these amplifiers much safer.
The amplifier schematic.
  Ultra-linear circuits are easy to identify by the number of transformer leads going to the output tubes.  Ultra-linear circuits have 2 wires to each tube, while other circuits have just one.  If you find a Stancor chassis with the labels missing, this is how you can tell which version you have.

The amplifier has sockets to measure and balance the plate currents on the 807 output tubes.  This is important for two reasons. One, balanced current is important to achieve the lowest distortion. Another good reason is to verify that the tubes are not using too much current.  This happened to me. C4 was bad.  If C4 or C5 are leaky, the grid bias voltage will go positive, and cause the tubes to draw excess current.  Drawing only a little too much will cause the plates to glow red to an excessive degree..
In this picture, the tube in the foreground is drawing too much plate current.  The one in the background is ok. Its plate is slightly red, that is acceptable.  The blue glow on the glass is acceptable, too.  A gassy tube has a glow inside the tube.  These are factory new Raytheon tubes.


The power supply is simple.  The supplies are only large enough for a single amplifier, and will overheat if two amps are connected to one power supply.