[MRCA] BC-654 Project 1.7

[B. Smith]

Every receiver I have worked on needed more BFO injection for CW ops.
http://k4che.com/SCR-284/SCR-284%20%20Part1.htm#bi
On 1/25/2025 4:22 PM, J Mcvey via MRCA wrote:
> 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:
>
>
> 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
>
> ______________________________________________________________
> MRCA mailing list
> Home: http://mailman.qth.net/mailman/listinfo/mrca
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:MRCA at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
> ______________________________________________________________
> MRCA mailing list
> Home: http://mailman.qth.net/mailman/listinfo/mrca
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:MRCA at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>
> ______________________________________________________________
> MRCA mailing list
> Home:http://mailman.qth.net/mailman/listinfo/mrca
> Help:http://mailman.qth.net/mmfaq.htm
> Post:mailto:MRCA at mailman.qth.net
>
> This list hosted by:http://www.qsl.net
> Please help support this email list:http://www.qsl.net/donate.html
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.qth.net/pipermail/mrca/attachments/20250125/36a953c0/attachment-0001.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: semk4che.gif
Type: image/gif
Size: 9511 bytes
Desc: not available
URL: <http://mailman.qth.net/pipermail/mrca/attachments/20250125/36a953c0/attachment-0001.gif>