[Hammarlund] SP-600 and Drift

[Richard Knoppow]

----- Original Message ----- 
From: "Bob Young" <bobyoung53 at hotmail.com>
To: <hammarlund at mailman.qth.net>
Sent: Sunday, July 07, 2013 8:53 AM
Subject: Re: [Hammarlund] SP-600 and Drift


>I have my SP-600 going through a good Hammond tranny and an 
>old hifi speaker it definitely sounds better than an 390-A 
>but then again it doesn't sound as good as previous super 
>pros. In the 8 kHz and especially 13kHz position it does 
>sound pretty good though, the highs sizzle but it lacks 
>bass (I know that can be fixed with different bypass caps) 
>but overall the audio is decent. I think my tap is on 130V 
>but haven't checked in a while, when I get the gumption to 
>pull it out of the case I'll check it but it doesn't run 
>hot.
>
> Bob Young
> KB1OKL
>
    I think you mean coupling caps but its not them so much 
as the DC in the output transformer.  I think its much more 
profitable to use an outboard amplifier driven off the 
detector than to try to modify the receiver's amplifier. 
You also have to eliminate the IM from the AVC by running in 
manual RF gain or putting a cap across the AVC terminals on 
the back. Running in manual will eliminate it altogether.
    To run from the diode load terminals connect a 
decoupling cap, say about 0.1 uf in series with about a 1 
meg resistor to one end of about a 50K pot, then connect the 
wiper of the pot to the input of the amplifier.  You may 
have to experiment with these values as I am going by 
memory.  The main thing is to have a large enough resistor 
from the diode terminals so that the external circuit will 
not load it.  The cap is not critical as long as its large 
enough to pass the low frequency audio.  The audio voltage 
out of the diode is quite large, many volts, so you need 
some sort of attenuator for most audio amp inputs, that's 
what the pot is for.  This will work on any receiver that 
has access to the diode ouput.  On the older Super-Pro the 
diode audio shows up on the PHONO terminals on the back. 
While the amplifier in the Super-Pro is a pretty good one a 
reasonably modern hi-fi amp will be better.
    Modern broadcast transmitters are capable of quite high 
fidelity but most stations process their audio so much that 
you may prefer low-fi reproduction. Also most AM stations 
now introduce a high frequency pre-emphasis to try to 
overcome the limited IF bandwidth of most receivers, on a 
wide-band receiver it sounds quite shrill and usually 
requires a lot of clipping or compression to avoid 
overmodulation.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com