Notes from the January 17, 2009 Meeting
President Jim Sargent opened our well-attended meeting, held at our old location at Irving's Senter East meeting facility. Jim was able to reserve it for our group for all of this year. He announced that he would be conducting, on February 28, a good-sized auction of some large consignments as well as some items to be surplused from his own holdings. Jim reminded us that the Houston club would be having a show/sale in two weeks. A number of VRPS members regularly attend their affairs, as they do ours.
Our program director, Mike Grimes, told everyone to start thinking about what to bring to a "free-for-all" show-and-tell for the February meeting and also items for the specialized topic of "Batteries" and related items for the April meeting. Mike introduced the day meeting program. The program consisted of a presentation intended to explain the function of each and every part of a typical AA5 radio. The program was given in two parts, the first (presented by myself, the author) covering the RF and IF components up to the Detector. The second part, presented by Cleo Cherryholmes, covered the Detector, Audio stages and the power supply components. The "typical AA5" radio chosen is the SENTINEL Model 329 (Rider's Vol. 20). Most of its features are common to about 90% of radios made in the late 50's to the days of the transistor radios. Starting with the loop antenna and ending with the power supply and filament string, each part was identified as to its name and function. There was some discussion about each part as to what happens if the part is bad, what to replace it with, how to test it to see if it's bad, how critical is the value (e.g. resistance, capacitance), and what damage will take place if the part is off value, etc, etc. In the time available, it became difficult to complete the original mission of the program, which was simply to identify each part and its function. Troubleshooting tips were much in demand. It was noted that terminology can vary widely, even for the same part. For example, the AA5 first stage (the 12BE6, in this case) can be called a "mixer", a "mixer/oscillator", a "first detector", a "converter" or an "OSC-MOD" (for Oscillator/Modulator) - the designation on the Rider's schematic diagram. There were many questions from the group and a lot of input from experienced restorers. During the discussion of the oscillator circuits, the author mentioned a good method of determining whether or not the oscillator is running. The oscillator grid should have a substantial negative voltage if it's running. (Mike McCarty reminded us that the meter may actually stop the oscillator from running, if the meter doesn't have low enough input capacitance. An old VTVM, with its usual one (or more) megohm probe resistor, is not likely to stop it from oscillating. Some of the new digital meters are more likely to be a problem, but placing about a � megohm resistor between the probe and test point, at the probe end, will avoid this problem.) Naturally, the question of IF alignment came up. But that is a subject that deserves its own special time, and Cleo Cherryholmes is planning to cover it in a near-future presentation.
During Cleo's presentation, he discussed the diodes in the 12AT6 Detector/First Audio (aka "second detector") and the development of the Automatic Volume Control (AVC) voltage, used by the RF/IF stages discussed earlier. A question came up about there being two diodes in the 12AT6, and it was pointed out that some circuit designs used one of the two as a separate AVC detector. This was used in sets featuring so-called "delayed AVC" to provide a better (slower) response at low frequencies to improve fidelity. Cleo put a strong emphasis on the function of the capacitor that couples the audio signal from the plate of the First Audio (the 12AT6, in this case) to the grid of the Audio Output tube (50B5). It needs to block all DC and should always be replaced, unless it is (rarely) a mica capacitor. Otherwise it will create a positive bias on the grid of the 50B5, making it draw more current, shortening its life and causing distorted sound. (Here is a good place to point out that the molded Bakelite capacitors with the trade name "Micamold" are really paper/foil capacitors molded into Bakelite. The trade name is misleading, and they are often bad.) Cleo discussed two "mystery" parts, which are two capacitors that do not have an obvious function. One is the capacitor from the plate of the audio output tube to its cathode (or sometimes to the other side of the output transformer primary). Although the radio will play without it, the sound can be harsh and have too much treble response, depending on the speaker size and the output transformer characteristics. The capacitor bypasses the higher audio frequencies. Its value varies and was chosen during the radio design process to create a more pleasant sounding radio. The other mystery capacitor is connected across the AC power when the power switch is on. The radio will play without it, too, but will be sensitive to all sorts of noise from interference coming in on the power cord. Cleo pointed out that more complex radios still work with the same basic principles, circuits and parts as the example that we discussed, but they become more complicated when they are multi-band, with all the additional coils and switching. As an example, he showed the schematic diagram for a very complex high-end set - the Zenith 11S474.
