[Milsurplus] Test your repair skills

[Patrick Jankowiak]

Re:
> Subject: [Milsurplus] Test your repair skills
> 
> Occasionally there is a abrupt roughly 60 KHz transmitted frequency shift. This
> appears after the set has been on some time (> 20 min) and often goes away upon
> cooling overnight.

This kind of thing was caused in my SX-28 receiver by a few things. An
intermittently leaking capacitor (C70) in the screen grid circuit of
the LO (V4). The screen volts would change slightly, either due to the
cap itself or it's effect on the screen dropping resistor (R47), and
the freq. would jump 20-50KHz. A few minutes later, it would jump
back. Once the rig was hot, this would become very erratic. Replaced
both parts and fixed it, so I thought. 

The erratic part was gone, but the radio would jump frequency several
times in the same direction as it got warm, and once hot for a day or
so, stay till it cooled. I noticed that the tickler winding on the LO
coil was very sensitive to tapping. This was very hard to find because
with the radio's bottom plate off, it would not happen! If I waited
till it was hot and removed the plate, it would make several jumps in
the reverse direction. I suspected that when the radio warmed up, the
wire, otherwise undisturbed, would stretch just enough to finally 'let
go' of its cold position and change the LO frequency. Two beads of ye
olde cement fixed that problem. Of course the SX-28 still drifts, they
all do, but the abrupt change is cured. 

PJ