[Central-Electronics] 100V sideband suppression adjustments [bcc][faked-from]

[Dennis Petrich]

Hi Jon,

I use the 75a4 and switch back and fourth from usb to lsb until I get the suppression as good as it will go.  I could never get the scope method to work well for me either.

Did you ever look at the 4700pf caps in the audio phase shift module?  In my 100V one of them got leaky and unbalanced the balanced audio amp circuitry.  That made it impossible to get the side band suppression to null out properly.  It was later written up in ER 5 or so years ago....  You will need a very low current leakage cap tester to verify if the caps are good.  The problem is silver mica metal migration so over time, if there is any DC voltage on the cap, they will start leaking.  They should be dead opens or over 1000G ohms to be good.   Good luck,

Ciao, Dennis, k0eoo

 -----Original Message-----
From: 	central-electronics-bounces at mailman.qth.net [mailto:central-electronics-bounces at mailman.qth.net]  On Behalf Of K6JEK
Sent:	Wednesday, March 12, 2008 5:50 PM
To:	central-electronics at mailman.qth.net
Subject:	[Central-Electronics] 100V sideband suppression adjustments [bcc][faked-from]

Does anyone have a better procedure than the one in the book for  
adjusting sideband suppression on the 100V?

I used the one on page 39 even using the MM-2 as the AF signal source  
and adjusting for minimum ripple.   I got about 37 dB of opposite  
sideband suppression with a 1 kHz tone.   I ought to be able to do  
better.

Next I used my Flex Radio SDR 1000 as a spectrum analyzer and a DSP  
599 as the AF signal source.  The DSP 599  turns out to be cleaner  
than the AF generator in the MM-2.    Fooling around with the four  
controls, AF BALANCE, AF RATIO and the two RF SHIFT inductors I was  
able to do a little better, at least I thought so, 45 dB.   That's  
about the same as the carrier suppression.   But when I put real live  
audio into it, like a microphone, I don't see anything like 45 dB,  
more like 20 dB.  I really don't understand that.  It's a complete  
mystery to me.

Anyway I have twisted those four controls with abandon without really  
understanding what I was adjusting.   I was just watching the  
spectrum, trying to make the bad peaks small.   Now the control are  
in some random state compared to where they started.   I'm game for a  
more methodical approach and have access to modern test equipment.     
For example, is there a way to set the phase shift adjustments first  
to some appropriate spot?

Have any of you come up with a step by step you'd recommend, or how  
about just helping me guess at one?

I did replace the MOD CAPS a few years ago with matched 6 MFD  
aluminum electrolytics.   Perhaps I should do this again before I  
beat my head against the wall some more.   I also replaced the MOD  
DIODES.   The audio limiter is bypassed but the audio filter is in  
the circuit.   The other usual stuff has been done except for the  
mixer adjustments.

Any thoughts would be most appreciated.

Thanks,

Jon, K6JEK 
_______________________________________________
Central-Electronics mailing list
Central-Electronics at mailman.qth.net
http://mailman.qth.net/mailman/listinfo/central-electronics