R-390 RF Deck problems and Solutions
While working on an R-390 (Motorola) recently I ran into some especially irrritating and difficult problems. I know others have had the same experiences.
These particular problems were:
1. Erratic function of the bandswitch. Required jiggling of the knob to get corredt operation.
2. When switching on the calibrator the B+ fuse blows.
The causes for 1. were twofold in this case.
a. The xtal oscillator module is driven from the gearbox via a small Oldham coupling.
The fixed disk on the RF deck is pinned to the relevant shaft. I found there was some
slop on this in that it was not firmly affixed to the shaft. The cure was to drive out the
pin. This is a split pin and very carefuuly widening it and driving it back firmly cured
this problem. This is not the easiest thing to do. You need a very small punch and
quite a bit of manual dexterity and a lot of patience.
b. The alignment of the xtal oscillator shaft can be quite critical. You will find that it is
not quite linear throughout its range. In other words if you line it up exactly at
00 it will be found to be out a little at 31. You need to find a compromise so that the
xtal switch connects properly at all positions. This has also been noted with R-390A
c. The alignment of the bandswitch is also a critical issue. The manual is quite explicit in
this procedure and should be followed carefully.
In the matter of item 2 above this was puzzling but simple once the problem was found. It was noted that the B+ fuse did not blow if the connector at J124 was not inserted. Measuring from the center pin of the BNC plug to ground revealed a short. To find the cause of this required the removal of the RF deck which is a real chore in the case of the R-390. On inspection the problem was quickly found. connected to the grid of V-201, the first RF am, there is a small metalised capacitor. This had been displaced so one end shorted to the frame. Some R-390 RF decks have a different type of capacitor which is better insulated. This is the coupling capacitor to the Calibrator.
I hope thus information might help others in the future who work on this radio.