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The 2011 Radio Workshop

Another standing-room-only crowd was awarded for their trek to the Museum on Super Bowl Sunday with a great presentation by Charlie Bradley chronicling his twelve-year restoration of a 1936 Zenith model 16A61. They were also treated to some great home- cooking by the Wagar sisters, Kim and Lynn.

The picture above shows the disassembled Zenith, complete with the custom-made base.

Not only did Charlie have the foresight to take pictures along the way, but he was kind enough to answer a variety of woodworking questions afterward.

Among the myriad of questions asked was, “How does one remove water stains from the top of furniture?”

If the stain is white, a “blush eraser” can be used without stripping the cabinet. Simply spray the blush eraser on the old finish and it will soften it up enough to allow the moisture to escape.

If the stain is black, you must strip the old finish, then apply a warm solution of Oxalic acid and water to bleach out the stain.

Both “blush eraser” and Oxalic acid can be purchased locally at specialty stores like Rockler or WoodCraft.

Thanks Lynn, Kim, and Charlie for a great Sunday!


Restored Zenith 16A61

Curtain-Burner Cords

I owe the world an apology. For years I’ve been telling people to use a diode to drop half the line voltage when replacing resistance line cords.
Now I find out that the effective voltage at the output of the diode is not half the RMS line voltage, but half the peak. In other words, if the RMS value of the line is 120, the peak is 169 volts. With the diode, your filament string sees half of that peak; 84.5 volts, not the 60 volts we hoped for; we’ve posted the math below.
To further complicate things, both my Simpson 260 and our bench Tektronix VOM measure the AC drop across the filament string as 60 volts AC. My GenRad 1800 shows the correct voltage of 84 volts AC.
So how do we replace that resistance line cord? Some of these sets only need 30 volts at 300 ma. across the filament string. As much as I have resisted, the answer seems to be a series dropping capacitor. The good news is that small, reliable AC components like Cornell Dubilier Type DPM Polypropylene film capacitors are finally available. Never use electrolytic capacitors to drop the voltage.


Here’s the latest solution for the line-cord replacement. A one-amp fast-blow fuse (F) should be fine for most applications. Calculating the value for the dropping capacitor (C) is not as easy as the familiar 1/2πfc. We use a program called Electronics Genius. It can be purchased online for $16, from http://www.electronicsgenius.com/ or you can call the Museum and we’ll provide the values for your radio. The surge resistor R1 is optional. Five-watt, back-to-back zeners across the dial lamp should be rated at a couple volts above the lamp voltage. For a #47 bulb (6.3 V at 150 ma.) in a 300 ma. circuit, R2 would be 42 ohms at 2 watts. For a type 44 bulb (6.3 V at 250 ma.) R2 would be 126 ohms.



Be sure to use a polarized plug on any AC-DC set.

I have a feeling that this is not the last I’m going to hear on this subject. I invite your comments. To see the calculus, go to curtainburner.html.


Gain Per Stage Measurements

Although the Rider Chanalyst and Meissner Analyst are among the most unwieldy and unnecessary examples of test equipment ever built, their application data is quite useful. The gain-per-stage measurements published in late thirties’ and early forties’ service manuals were a direct result of these devices.

The Chanalyst and Analyst were simply well-calibrated TRF receivers whose design promoted signal tracing instead of signal injection. Neither device had a signal generator. The following information from the RCA Chanalyst manual can be extremely useful.

USING THE RCA SIGNALYST IN CONJUNCTION WITH THE CHANALYST

The RCA Signalyst is designed as a companion piece for the Chanalyst. The Signalyst is a modern signal generator with metered output in microvolts. It is accurately calibrated and covers the extremely wide frequency range of 100 kilocycles to 120,000 kilocycles (0.1 megacycle to 120 megacycles) which includes the bands assigned to frequency-modulation and television.
The Signalyst and Chanalyst make an unbeatable combination for radio service and engineering.

AVERAGE GAIN-PER-STAGE VALUES

The figures on gain-per-stage listed below are based on the assumption that the receiver a.v.c. system is not operating. A.V.C. action will reduce considerably the r-f, mixer, and i-f stage gains. For comparison purposes, a weak signal should be used, or the a.v.c. circuit temporarily shorted out.
In the a-f sections for resistance-coupled amplifier, the lower gain figures represent average gains for ac-dc receivers while the higher gains apply to a-c operated receivers.

RADIO-FREQUENCY SECTION
Antenna to grid of first tube ……………………………….......1.5 to 10
Antenna to grid of first tube (auto- radios) ……………….....15 to 50
R-F Amplifier …………………………………………............. 2 to 40

MIXER SECTION
Converter grid to i-f grid (1 stage i-f amp.) ……………….30 to 60
Converter grid to i-f grid (2 stage i-f amp.) .......................... 5 to 20

INTERMEDIATE - FREQUENCY AMPLIFIER SECTION
I-F stage (1 stage i-f amp.) ………………………………...40 to 150
I-F stage (2 stage i-f amp.) ……………………………….... 5 to 20

BIASED DETECTOR
Types 57, 6C6, 6J7: A 1.0 volt r-m-s signal (20 percent mod.) at the
grid will produce approximately 10 volts r-m-s of a-f at the plate.

AUDIO-FREQUENCY SECTION
RCA-75, 2A6 …………………………………………...40 to 55
RCA-6F5 ..........................................................................40 to 55
RCA-6Q7 .........................................................................30 to 45
RCA-6N7, 6C8 (each section) ......................................20 to 25
RCA-2B7, 6B7, 6B8 .......................................................50 to 80
RCA-6F6, 2A5, 47 (grid-to-plate) .................................10 to 20
Triode output tubes ........................................................... 2 to 5

A.V.C. VOLTAGE - A.V.C. voltage may run as high as 40 volts, depending upon the strength of the input signal and the number of tubes under control. In general, the greater the number of tubes controlled the lower the a.v.c. voltage for a given input signal.

I hope you will find this information useful. Thanks again to everyone who helped make this another successful event for both the Northland Antique Radio Club and the Pavek Museum.

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Museum of Broadcasting