The other day I decided to check for propagation on 15M, and was
surprised by how unusually quiet the band sounded; even the infamous
21.333MHz birdie was not to be heard! I then discovered that the
maximum key down power output was about 2 watts, with very little
sensitivity to RF output control setting. 12M exhibited the same
symptoms, but the 564 operated normally on all other bands. After about
15 minutes the receiver abruptly returned to life and TX output was
normal, on both 15M and 12M. This intermittent was repeatable for every
one of the several times I checked the rig from a cold start. The
problem has not been observed after the rig is warmed up.
Based on the symptoms and examination of the schematics, I suspect an
open in the bandpass filter for the L.O. output on the XTAL osc/L.O.
mixer board. 12 and 15 meters share the same filter for the L.O. output
before L.O. signal is sent to the BPF/front end board. The symptoms may
indicate an open in this filter. I expect that the switching diodes
(D38 and/or D43) to the filter are not blown, since diodes usually fail
permanently in an open or shorted condition; an intermittent internal to
a diodes is very rare. I would like to reflow all the solder
connections associated with the L.O. filter served by these diodes.
My question is how to best access the XTAL osc/L.O. mixer board. Has
anyone had experience with accessing and removing this board? I believe
it is under the FM and LL driver/NB boards. Haven't yet removed the
covers to look at this problem. When I do, I will take many photos of
the disassembly along the way.
All comment/suggestions are appreciated.
Jack, K1VT
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