BC-348 AVC

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WU2D:
Hi Guys,

I have found a ton of info on AVC/AGC performance and improvements on many other receivers but I can not find anything on the BC-348. I picked a Q model up at Spring Hosstraders and I replaced the suspect caps and any high value resistors and even got the crystal filter working nicely, but I am not convinced that I have the AVC working correctly. It works great on AM with mucho negative volts on strong signals and a nice slow AGC action. On CW however, it just plain don't work.. In MVC it works fine for CW. When switched to AVC however, the BFO seems to phase lock on strong CW signals and the receiver overloads. Turning down the gain control does nothing to help.

The manual claims that the receiver is specifically designed to handle CW in the AVC mode - any personal experience comments before I post the writeup and schematic details of the AVC section? 

Thanks,

Mike WU2D

WU2D:
No Bites so I will attach the schematics for your look see. This is a an old design, but it is one of the later models of the BC-348 and the circuits are a result of improvements on the series during the war years. There is plenty of gain with two RF stages and three IF stages! Again I am looking for some help on the CW AVC action.

Mike WU2D

W2JBL:
things get better if you divorce the RF and AF gain controls from the ganged setup they have, but still not very good for CW with the AGC on. might i suggest a product detector? that worked well for me. this is an old buzzard design, and ask any old buzzard how to operate CW it starts with turning the AVC off. great receiver pretty much as is, i have had many. i like to keep them stock as they are fine on AM. my father used one on CW from 1946 to his QRT about 8 years ago. never saw him use AVC.

WU2D:
Chris,

Thanks for the reply and the comments which come from experience with the old girl. My buddy had one of these as his novice rig and that was my only exposure before picking this one up. You are validating what I thought - that CW AGC was more wishful thinking than reality with the BC-348. No I'm not going to change or modify the radio - It works fine for what I use it for - checking into the OMRN Net once a week!

Thanks again.

Mike WU2D

w3jn:
Mike, are you sure the thing is wired to factory?  A typical mod was messing around with the RF gain pot, I can't remember what it dealt with, but a couple of the 348s I've had over the years needed de-hamming in this area.

Another area to watch out for is that hams loved to ground the B- from the power supplies they built in.  This prevents any negative bias (IIRC it only affects the audio stage though).  The B- should NOT be grounded,  per the skizmatic.

It sounds to me like the AVC is disabled with the BFO on.   The reason the BFO is phase-locking is that either the IF signal is very strong (as in no AVC action, or someone markedly increased the coupling from the BFO to the IF (also possible, but a high BFO signal usually increases the AVC voltage).  Further, you have a distorted signal which leads me to believe the AVC isn't functional with that BFO on.

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