SB-300 Receiver problem

[Mark V. Johnson VE3DJU]

Hello All...Once again I'm turning to the collective wisdom of the team to
offer an ideas regarding a problem I'm having on my SB-300 receiver.

The rig is in fairly good shape...well built...no mods of any consequence.
The problem..when tuning on some CW, RTTY or even the odd SSB signal, the
signal sounds distorted/unstable or jumpy, when tuning it in..Almost like
an unstable BFO on a cheap receiver. When you stop tuning (turning the tune
knob) the instability settles immediately. When you turn the knob to get
the right pitch on a CW signal, it gets jittery again.  It happens in the
USB and LSB mode, it is not dependent on the AGC setting, cutting back on
the RF gain does not effect it, and it only seems to happen on some
signals. Not all!! Not a big problem, but something's not right and it s
gonna drive me nuts until I find it. Unless someone can tell me its an
inherent design issue, which I doubt.

Someone has been into the LMO as it is a 6CB6 tube( I believe a 6AU6 was
std) and the screws are no longer sealed. . I put my scope on the LMO
output and it appears very clean and stable. As I tune it, it changes freq
very nicely..no apparent distortion.

The problem occurs with the receiver in standalone mode and when mated to
my  SB-400.

Any ideas are welcome and appreciated!! TNX

Sponsored by the City of Tempe 

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