Many RF gain controls work by a voltage divider setup, a pot
with the wiper going to the cathode of a stage, one terminal going
to ground, the other to a positive voltage source.
This allows the cathode voltage to go to ground (rf gain max),
to a small positive voltage (rf gain minimum).
Other circuits use the AGC setup on multiple stages.
Don't assume the pot is good, or the correct value.
The pinched wire could have ruined the pot, some PO
could have changed the pot because of the pinched wire, etc.
You need to reference the diagram and check values and connections,
many older receivers were modified for better SSB reception (yuk!).
If a pot went open, or was too high a value, the rf gain will
be normal when the wiper is grounded, but as soon as its off the
end point, its open...
Many older receivers (not the SX28 I think) did different
things with the gain control in SSB/CW mode, on my G76, the audio
gain control turns into an RF gain control...
On my SX17, the bfo is separate, and does not interact with the
RF gain circuit.
I think the RF gain control is the voltage divider type.
I think there is also an AGC on off switch.
Great sounding receivers, all those that Hallicrafters made
with the nice push pull audio output, puts receivers
that use a 6aq5 or some such to shame, and people don't
know what they are missing, listening to good AM
on the typical receiver, even high end modern stuff.
It sounds better than side band, but it can be as much of a difference
between a good and bad audio amp as between ssb and AM.
The Scott SLRM receiver I have seems to have a real hi fi
audio amp, with no driver transformer in the audio stages, and even
a position on the selectivity control marked hi fi.
Broad as a barn door!
Push pull 25L6 tubes!
Brett
N2DTS
> -----Original Message-----
> From: [EMAIL PROTECTED]
> [mailto:[EMAIL PROTECTED] Behalf Of w5sum
> Sent: Tuesday, December 30, 2003 5:47 PM
> To: [email protected]
> Subject: [AMRadio] another SX-28 question
>
>
> well 24 hours of work and she is coming along nicely. The
> audio section is
> fine now. Sound greeeeaaattttt..
> She is not working on the top two bands but we'll cross that
> bridge later.
>
> the RF gain control was not working at all. I found a pinched
> wire that was
> grounded. This was the wire going to the cathode resistor of
> V-1. I fixed
> this, and even put a new pot in there for the RF Gain. It
> works BUT.. all you
> have to do is just barely turn it and it blanks out the front
> end. Could one
> of the cathode resistors have changed value, canceling out
> the work of the rf
> gain control? R-3 to V-1 or R8 to V-2? Or, possibly, C14 on
> V-1 or C18 on V-2?
>
> looks like they will be tough to get to if that is the case.
>
> your ideas gentleman?
>
> your
> The "AM" voice of Shreveport, Louisiana USA
> formerly WN5AIA and WB5AIA
> I got my Extra Class the old fashioned way... I earned it!
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