Steve, I haven't worked or owned a 2-C in a long time but Drake was fond of very high impedance AGC circuits.??
Many, many issues with the AGC come down to one or more gassy tubes.? The circuit is so sensitive that a lot of times, tube testers won't catch them.? What I usually do, after checking for other obvious faults, is swap ALL of the tubes at once and seeing if the problem is now fixed.? If it isn't, put all the old tubes back in and re-box your spares.? If it fixes the problem, start putting the old tubes back, one at a time.? continue through all of the tubes 'til you're done.
Why do I do this?? Because I have had receivers that have had more than one gassy tube.? Each gassy tube will hose the AGC buss.
73,
Steve Wedge, W1ES
Time flies like an arrow.? Fruit flies like a banana.
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On Sunday, April 6th, 2025 at 12:06 PM, Steven KW4H via groups.io <reedsteve@...> wrote:
All,
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I have a very early (S/N 260) Drake 2-C on the bench with an AVC issue I need a head check for.? The symptom is that the idling AVC voltage, which should be adjustable during alignment to -1.35V will only adjust around the -0.4V to -0.6V range.? Troubleshooting has revealed that the negative voltage supplied to the circuit is lower than expected -- it's riding around -25V, instead of an expected -70V (although some schematics show a -50V).? I've traced this back to the power supply and confirmed that, at the junction of C53 and D5, the voltage is indeed about 25V.
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Does anyone know if, in the very early versions of the 2-C, the AVC voltage was significantly lower, or is there likely an issue that needs to be further diagnosed?
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What started all this was that while the slow AVC works fine, if you select fast AVC with the RF gain wide open, the AVC rapidly oscillates unless you back the gain down.? Voltage checks have led me down this path, but I don't want to jump to conclusions due to the extremely low serial number involved.
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73 - Steve, KW4H
