[ARC5] OT? bc-348 bfo question

[John Hutchins]

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
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