[ARC5] OT? bc-348 bfo question

[Geoff]

For those with already modified sets the BFO can be injected into the last 
IF which will allow AVC action on SSB and CW. Adding a pot to control the 
injection level helps a lot.
This method also gives pseudo synchronous detector AM using the exhalted 
carrier method which goes back to the early 30's at least and resulting in 
minimizing the selective fading effect.

Carl



----- Original Message ----- 
From: "John Hutchins" <jphutch60bj at gmail.com>
To: <arc5 at mailman.qth.net>
Sent: Saturday, September 14, 2013 7:18 AM
Subject: Re: [ARC5] OT? bc-348 bfo question


> Fernando
> If you have a previously hacked set go for all kinds of modifications, in 
> fact I would encourage it.  Otherwise I suggest only making modifications 
> that are easily reversible.
>
> I have listened to SSB for years on MVC using the R series of the BC-348 
> no problem.
>
> Suggestion if your set does not have an Antenna trim control suggest 
> mounting a variable capacitor 75 ~150 pf with an isolated shaft/ or using 
> an  isolator coupler in a box and connect your receiver through this to 
> your antenna.  Trimming helps a lot to get the signal up.
>
> Next the next check the AGC/AVC timing capacitor (123-C on HKLR series, 64 
> JNQ series, 101 EMOPS series) .05 uF  I see ~100 V PP some times on real 
> strong signals.  The cap typically is 400V paper.  So draw your 
> conclusions there.  In the HKLR series it is mounted in the transformer 
> CAN!
>
> Also check the existing cathode bypass capacitor of the 2nd IF/Detector 
> circuit (9-16 EMOPS series, 70-B JNQ series, 11-16 HKLR series) .  This 
> also is a paper cap, same conclusions here.
> Next I suggest a mod that is easily reversible: add  5uF ~200V capacitor 
> across the cathode bypass capacitor of the 2nd IF/Detector circuit this 
> may help.
>
>
> Hutch
>
>
>
>
>
>
> On 9/13/2013 10:46 AM, Joe Connor wrote:
>> Fernando:
>>
>>      I can't specifically answer your question but I suspect this is the 
>> reason a lot of people modified
>> BC-348s to separate the RF gain from the AF gain. The Surplus Radio 
>> Conversion Manual by Evenson and Beach explains how to do this. If you 
>> run a google search, you can find it and read the appropriate section.
>>
>>                                       Joe Connor
>>
>>
>>
>>> ________________________________
>>> From: Fernando - LU2DFM <lu2dfm at gmail.com>
>>> To: arc5 at mailman.qth.net
>>> Sent: Friday, September 13, 2013 11:28 AM
>>> Subject: [ARC5] OT? bc-348 bfo question
>>>
>>>
>>>
>>> Sorry if this is OT here.
>>>
>>> I'm repairing and aligning a BC-348 (E, M or P variant, don't know
>>> exactly which).
>>>
>>> The set is going on very well on AM so far, however the BFO injection
>>> level seems to be too low for SSB on strong signals. It's possible to
>>> get SSB on the headphones with MVC and _very_ low RF gain.
>>>
>>> All voltages and resistors values are ok around that stage with and
>>> without BFO on. The xtal filter is ok too (bandwith with xtal on is 
>>> about
>>> 1-2 kc).
>>>
>>> The 2nd IF/CW osc. is the 6B8/VT93.
>>>
>>> Anyone knows what the level of the BFO is supposed to be?
>>>
>>> Thanks.
>>>
>>> -- 
>>> 73 de Fer, LU2DFM
>>> ______________________________________________________________
>>> ARC5 mailing list
>>> Home: http://mailman.qth.net/mailman/listinfo/arc5
>>> Help: http://mailman.qth.net/mmfaq.htm
>>> Post: mailto:ARC5 at mailman.qth.net
>>>
>>> This list hosted by: http://www.qsl.net
>>> Please help support this email list: http://www.qsl.net/donate.html
>>>
>>>
>>>
>> ______________________________________________________________
>> ARC5 mailing list
>> Home: http://mailman.qth.net/mailman/listinfo/arc5
>> Help: http://mailman.qth.net/mmfaq.htm
>> Post: mailto:ARC5 at mailman.qth.net
>>
>> This list hosted by: http://www.qsl.net
>> Please help support this email list: http://www.qsl.net/donate.html
>>
>
> ______________________________________________________________
> ARC5 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/arc5
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:ARC5 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>
>
> -----
> No virus found in this message.
> Checked by AVG - www.avg.com
> Version: 10.0.1432 / Virus Database: 3222/6164 - Release Date: 09/13/13
>