SB-102 problem

[Brian Wood]

The S-meter *now* does go to almost full scale with RFG ccw, not completely
but close enough. It used to stop at S8. That seemed to be fixed by simply
pulling tubes and testing them and putting them back in, so it was probably
corrosion on the tube sockets.

 

Audio is loud and strong now that I found the open resistor. It was pretty
good before, which is amazing given that the plate voltage on two tubes was
coming from an open screen grid on another tube!

 

Resistors in the metering circuitry all seem to be about right. I pulled one
meter lead off, and the meter resistance and the other resistances in the
area are all about what you'd expect.

 

I think the S-meter is now working OK. I have managed to get strong signals
to read as high as 20 over 9. I think that the definition of S9 is different
in the SB-102 than it is today; I think I read in the manual that 100uV ->
S9, whereas today it is 50uV. There's also just the one AGC setting for both
SSB and CW (fast attack, slow decay). So I think I'll call it good and put
it back together and do what we always used to do anyway when giving out
signal reports -- wing it instead of telling people what the S-meter
actually reads!

 

Thanks for all the suggestions!

 

73,

Brian

 

From: G3OOU at aol.com [mailto:G3OOU at aol.com] 
Sent: Sunday, July 12, 2009 09:06
To: wood at lpbroadband.net; HEATH at LISTS.TEMPE.GOV
Subject: Re: SB-102 problem

 

Hi Brian

 

Two things to try:

 

1. When you turn the RF gain control fully anti-clockwise does the S meter
go to full scale deflection? If it does then there is nothing wrong with the
S-Meter circuitry and the DC control of the IF amplifiers as this is what
drives the S meter.

 

2. With the RF gain fully clockwise and the AF gain control turned up to
about 30% of full scale is the background noise clearly audible or very low?
If it is low then you have a problem with the RF/IF gain and I would switch
the 102 off and measure all of the fixed resistors. These rigs run fairly
warm and after 30+ years you will probably find one or more that are way out
of specification or have actually failed. I have never yet had a silver mica
or disc ceramic capacitor fail but  have had some electrolytics fail.

 

There are some shared and some separate circuits between Rx and Tx so you
may find that the Tx power is also down but do one thing at a time. I assume
that all of the rails from the power supply are correct?

 

73

Bob 
G3OOU
G-QRP 6907
Joint site with Crystal Palace Radio & Electronics Club: www.g3oou.co.uk
<http://www.g3oou.co.uk/> 
My technical site: http://www.qsl.net/g3oou/


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