[MRCA] BC-654 Project 1.7

[J Mcvey]

 It may be a s Mark said, someone did a very neat looking mod back in the day, for reasons that are lost in time.I would think that the audio bias is way too much negative bias for the AVC line anyway. It probably drove the tube into cutoff.
Hey, if it works better without the mod, go with it!

    On Saturday, January 25, 2025 at 04:14:46 PM EST, Ray Fantini via MRCA <mrca at mailman.qth.net> wrote:  
 
 #yiv1320182008 P {margin-top:0;margin-bottom:0;}yes, removing the modification gives clean undistorted AM regardless of carrier level. with the mod in place the receiver overloads on strong signals and distorts the audio. not so much that it's not useable but enough that I don't like it. With a signal generator if I drive it over a couple hundred millivolts you can drive the receiver to the point of motorboating but after removing the mod AVC action is solid from 2 microvolts to half a volt.I don't know, maybe it somehow improves the MVC function in the CW mode where the AVC source is disconnected and a manual gain control is used? will play around with the receiver in CW tonight.
Ray F/KA3EKH
From: MARK DORNEY <mkdorney at aol.com>
Sent: Saturday, January 25, 2025 3:56 PM
To: Ray Fantini <RAFANTINI at salisbury.edu>
Cc: MMRCG <MMRCG at groups.io>; mrca at mailman.qth.net <mrca at mailman.qth.net>
Subject: Re: [MRCA] BC-654 Project 1.7 

CAUTION: This email originated from outside of Salisbury University. Please exercise caution when clicking links or opening attachments from external sources.


Let me preface this by saying I’m no electronics guy.  Does the other BC-654 receiver work correctly?  If so, and the only thing different on the BC-654 receiver that isn’t working correctly is that one mod, I would think removing it and bringing the receiver back into spec would eliminate the problem. That mod might look factory, but that would only mean that whoever made it did great work. My two cents. 
73

Mark D. WW2RDO

“In matters of style, float with the current. In matters of Principle, stand like a rock. “.   -   Thomas Jefferson 
Sent from my iPhone
On Jan 25, 2025, at 2:46 PM, Ray Fantini via MRCA <mrca at mailman.qth.net> wrote:

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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