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Old September 9th 09, 04:12 PM posted to
Richard Knoppow Richard Knoppow is offline
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First recorded activity by RadioBanter: Oct 2006
Posts: 527
Default SX-28 image problems

"COLIN LAMB" wrote in message

That sounds like a cross-modulation problem, rather than
images. Images would appear 910 kHz away.

Cross-modulation would occur in the rf stages or in the
first mixer. To determine if that is the problem, reduce
the signal strength and the cross-modulation will go away.

The SX-28 should not be cross-modulating under those
conditions and should be able to handle fairly strong
signals. Often, the culprit is that someone has modified
the set for "improved sensitivity", in which case, they
made changes to make the S-meter read higher. Frequently,
this involves changing rf amplifier tubes from remote
cutoff types to sharp cutoff types, or changing resistors
or bias voltages.

Check the circuit to make sure it is stock. Also, check
the caps and resistors in all of the circuits, especially
in the rf, mixer and avc circuits.

Also, realignment of the rf stages may help, especially if
the tuned circuits amplify the offending signal more than
the desired one.

73, Colin K7FM

I think checking the tube types is a very good idea. If
someone replaced the originals with later high gain types it
may be the cause. These tubes look attractive because they
have lower noise and higher gain than the originals but do
not work well in circuits not designed for them. The SX-28
uses a 6SK7 in both RF stages. The later but similar SX-32
uses a 6AB6 in the first RF position. This was a remote
cut-off tube designed for TV RF and IF use and has much
higher gain than the 6SK7. Another possible tube is the
6SG7, also a high transconductance tube, similar in
performance to the 6BA6 miniature tube. This one was used in
the S-40 series. Someone may have been inspired to stick one
of these guys in. Beside the higher gain these tubes require
a different bias voltage so the AVC won't work right.
Actually, there are other tubes which can cause even
more trouble such as the 6AC7, a very high transconductance
sharp cut-off pentode meant for video amplifier use. A tube
like this will tend to cause the AVC to operate at a slight
AVC bias which will cause overloading in the other
controlled tubes. Even a 6SJ7 in the first RF stage will
cause this trouble.

Richard Knoppow
Los Angeles