[ARC5] That "deaf" receiver of Brian's - and that 12FS7

[Leslie Smith]

Hello All,
This topic - a 12FS7 in a BC-454 - has grown legs without reason.
Here is a message I sent to Brian recently.  I think it makes the
situation clear.



Hello Brian,
I'd like to clear up the matter of the signal "on pin 4".
Looking at AN_16-30 the 2nd IF 'tube' is a 12SF7.
BUT YOUR BC-454, and like the ARA sets.  The second IF is a 12SK7. 
So pin #4 is the grid #1 (of the 2nd IF).  This is why you say YOU fed
the signal to "pin 4".
Tell me if I  understand what's going on here.  (or not).

Incidently, in Table 6-7 in AN_16-30, the 12SF7 is listed at "RF amp".  
This is clearly a mistake.   It should read,  "2nd IF amp".

Scott is correct when he says that pin 4 isn't used on the IF
transformer, but I'm looking for an understanding that allows you to be
both correct.  I suspect you report pin #4 for the reason I gave above; 
I know you; every-one knows you're thorough.   (But any-one can make a
mistake.)

Next point.
The 400Hz "blips" at 20mSec intervals.
(1)  You replaced all tubes, one at a time.   - No change.
       Conclusion.  Tubes probably OK.
(2)  You don't mention checking tube base voltages.  
        See Table 5-1 in AN 16-30.  (This is only a 'useful guide'. 
        Your set belongs to the SCR- series but the AN document helps).
       Knowing you, I think you have done this and all is per spec in
       AN_16-30.
(3)  You checked the caps; you replaced leaky caps.
       Conclusion:  Taken with (2) the caps are OK.
(4)  You set the supply voltage to 170V and the screen divider gives
about +70V on the screen supply.
        The set will easily work with this supply.  Every-one knows
        this.
   
However, your report that the 400Hz tone appears as a "blip" at 20mS
intervals suggests some link to the (50Hz) mains.
I don't know if you are feeding the heaters with DC or AC.

I don't know if your B+ supply has lost a filter cap.  (Feeding the B+
supply with an unfiltered supply.)
That would allow the 400Hz modulation to 'sync' with 20mS.
I can't think of any other way the problem you report could be
'synch'd"to 20mS, (because if there is only DC feeding your set, then
where does the 20mS come from).

I would like to know what you see when you inject 900-1000Hz onto the
grid of the 12A6 - pin #5.

Next point.
I have a cheap and nasty "tracer" probe.
These were common in the 50s and 60's.  They are a high-gain AF amp
(with diode detector sometimes).
If this will help I can post one down to you at Veronica Pl.
Let me know about this, pls.

Look forward to your next posting.

    
   73 de Les

-- 
http://www.fastmail.com - Or how I learned to stop worrying and
                          love email again