[TenTec] Omni6+ (564) intermittent problem

[Jack Mandelman]

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