[Boatanchors] SX-88 oscillates.
Glen Zook
gzook at yahoo.com
Fri Oct 21 10:45:08 EDT 2016
Try tightening all of the machine screws / nuts that go through the chassis. These do come loose and / or corroded over the years and the grounds, that are made through these connectors, go bad and that can cause all sorts of problems.
Glen, K9STH
Website: http://k9sth.net
From: "McDonald, J Douglas" <jdmcdona at illinois.edu>
To: "boatanchors at mailman.qth.net" <boatanchors at mailman.qth.net>
Sent: Friday, October 21, 2016 9:24 AM
Subject: [Boatanchors] SX-88 oscillates.
I got my "new" old SX-88 working. But it oscillates in the two RF stages
at some frequencis in the bottom half of each of the top three
frequency bands. The first two stages is verified by removing the 6U8
first convertor tube, it still oscillated, but removong either
RF amp stops it.
I've tried the usual fixes and nothing does anything except connecting
a good antenna or a 200 Ohm or lower resistor to the antenna terminals.
It only oscillates with them open. I've tried extra bypass caps in the three [sic]
associated RF compartments as well as grounding various unused
variable cap section above the chassis, and adding extra grounding straps
between the various (except for straps) insulated boxes. It only oscillates
at certain frequencies, so I assumed it was parasitic tuned circuits
due to parasitic inductance, but these tests should find that and don't.
My next guess is that it only oscillates at certain low frequencies on each band
because only at those frequencies do all three tuned circuits match
frequency and phase "properly" (due to minor differences in the tuning caps),
Putting on the top and bottom RF area covers has only a small effect
on the frequencies.
Any more ideas out there?
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