[Hallicrafters] SX-71 Bandswitch problem

[Phil Atchley]

Hello again 8^)
Well, I have nearly all the resistors replaced in the set.  I decided to
check it out and see if it helped the extremely poor sensitivy (local BCB
stations only) any.  Well the set is "better behaved" but still extremely
weak.

Then, I noticed that connecting and disconnecting the antenna didn't help
any. So, connected it to the grid of the RF amplifier (actually the tuning
capacitor stator).  Wow, reception GREATLY improved!  This problem was there
when I did a preliminary checkout before starting work on the set, so it's
not a problem I created.

Further troubleshooting indicates that the primary side of the RF coils are
all shorted to ground, this even when the back Panel jumper from A2 to
ground is lifted.  When that jumper is removed the inputs should "float" and
have no ground.  Examination of the circuit shows no shorted wires to ground
that I can find.

This wafer also was binding and caused the bandswitch to be very difficult
to turn until I loosened the two screws on the wafer and "centered it".
However I'm quite sure that those screws aren't the source of the "short" as
I loosened them and "wobbled" them  while observing the meter.

There are some ground connections to this wafer and I was wondering if it's
possible the wafer itself is bad?  Needless to say, I don't want to remove
it for inspection unless I have to.

I have located a junker set and if the wiring in that section is intact I
can do comparisons and perhaps even replace the wafer.  BUT, that's a last
resort!

Any ideas?  Do these sets have a known issue with the bandswitch "Antenna
coil" wafer?

73 de Phil  KO6BB
Loving home provided for wayward Boatanchor Receivers
ko6bb at elite.net
Merced, Central California
37.18N 120.29W CM97sh






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