[MRCA] BC-654 Project 1.7

[Ray Fantini]

OK, explain this to me. Been working on this dam BC-654 receiver for a while now trying to get the AVC to work right. It overloads on strong signal and if driven hard enough will motorboat. The symptoms of week or leaky AVC capacitors. I had changed them all, checked the resistors and everything twice over, but the AVC bus was still not good. It would distort on strong AM stations on 3885 and that won’t do. Looking at the receiver discovered that the factory installed a line to pin eight on the first IF tube that’s unused by that tube to pin 8, 2K1 the audio bias point. The other end of this line connected to the AVC Bus via a 1.0 meg resistor. If you have a 1.0 meg resistor between pin 8 of the first IF to the cold side (AVC input) of the junction of 2R8 and 2C14 you have the same mod. Although this is not shown on my manual or schematic, I have every reason to think this was installed at manufacture. So, I can see them doing modifications for improvements, but this mod screws up the AVC line and causes distortion on strong signal, poor sensitivity by two or three microvolts in AM and has no beneficial effect that I can see. What am I missing? This is on radio SN# 35911 and I looked on another receiver in the shop, SN#9680 and it was not modified in that way. Was it thought that early production runs were too sensitive or sounded too good?


Ray F/KA3EKH

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