Hello, I have a 2A that I was unable to get the passband tuner setup so that the pitch of the noise was equal on both sides of the center.
I setup a sweep generator, 0.1v pk-pk, sweeping from 40 - 60 KHz, in on pin 5 of V4. I setup an oscilloscope on pin 1 of V5. A marker output on the sweep generator is used as a trigger. Doing this, I can clearly see a narrow passband that is shifted around by the twist of the knob. On the 4.8 KHz position, the passband is greatly widened, as expected. Now, back to the narrow mode. There is ALOT of ripple on that passband, and the sides are not symetrical. One side had a slow rolloff, while the other had a far faster one. That explains the greater amount of high frequency hissing in one sideband. I opened the tuner. Four slugs are ganged together and mounted on a spring machanism. Each slug is mounted on a coiled wire, which is soldered to the part that moves them all together. Unlike the 2B and 2C, the slugs are not adjustable. I really want the set to work correctly, so here is what I'm thinking of doing: Take out the passband tuner, mount it to a wooden board. Take the front cover off of the tuner, and install a simple nut/bracket to simulate the action of the tuning knob. Unsolder and free the slugs. Using the same sweeping mechanism, adjust the slugs for 2.4 KHz, 6dB voltage, and as flat on the top as possible. Solder back into place, reinstall into the radio. BUT! I remember basic troubleshooting. Failures have a cause. WHY did the passband tuner to out of tune? Caps in the circuit failed? A short somewhere? I'll find out, but I am wondering if my method here makes sense. Comments? -- David Goncalves W1EUJ
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