R-390 RF DECK - SOME NOTES AND TIPS
The R-390 RF deck is very different from the R-390A and in many ways is much more difficult to handle. Here is a list of the main differences and some suggestions as to how to handle them.
1. Mechanical Layout.
In the R-390 the racks and associated coils are in one line abreast whereas in the R-390A the two IF racks are placed at the rear of the deck. The calibrator is in a separate sub assembly mounted underneath the main frame and adjacent to the PTO. The crystal oscillator unit is mounted in the rear of the RF deck and is driven through a shaft from the gearbox and a small Oldham coupler similar to the one used for the PTO but smaller. Don't lose this - they are very hard to find. All the crystals, including the first crystal oscillator crystals, are contained in the same oven. This is only accessible after removing the unit so changing a crystal is quite a chore. The crystal adjustment caps are accessible from the rear apron, after the removal of a covering plate. This plate is often missing as is the lid covering the tubes. The gearbox and the drive mechanism from the front panel tuning knobs are separated so that removal of the RF deck presents some problems. If the RF deck is simply removed loss of synchronization will inevitably follow. To recover this is a major and time-consuming chore. To avoid this a synchronization wheel is provided. (The magic GREEN WHEEL). This is a stamped and dished gear wheel and may be found in one of two places. In early models, mostly Collins, this will be to a spindle at the bottom of the front of the gearbox in the center. It will be attached with the dished part upwards so it does not engage with anything. In later models, mostly Motorola, this wheel will be attached to the top left-hand edge of the gearbox. For more information on the use of this wheel see below under synchronization. The two front panel tuning knobs and associated mechanisms are attached to a heavy bronze web going the entire width of the frame. This remains in place when the RF deck is removed.
To check the synchronization of the racks is difficult. It is most easily done by removing the RF deck but this is quite a chore. This is because the alignment marks are on the rear panel of the deck and are difficult/impossible to see while the deck is mounted in the frame. A slightly easier way is to remove the crystal oscillator unit at the rear. You can then see the alignment marks. The use of a dental mirror makes this fairly easy. If you have to remove the RF deck then make special note of the following.
First remove the front panel. This requires the removal of the two handles as well as several other screws. There are four captive screws and two green head regular screws on the side panel. DO NOT REMOVE OR LOOSEN ANY OF THESE AT THIS TIME. Now for the green wheel. Remove it and attach it to the spindle at bottom center of the gearbox. Be sure the dish is down so that it engages with the appropriate gear on the gear box. Make very sure it is correctly mounted (the spindle is oblong not round, as is the hole in the green gear). Also make sure the mounting screw is very tight and will not loosen when the mechanism is turned. Now you can remove the gearbox. The process of restoring the gearbox to the frame is well described in the manual.
3. RF Coils, racks and slugs.
Here are more differences with the R-390A. Please note that the slugs and coils are NOT interchangeable with those in the R-390A. The slugs are not always interchangeable with each other. This fact is not mentioned in the manual but is spelled out in a later amendment. If you have a problem aligning particularly the IF slugs or those in the 15-31 MHz rack then the first thing to check is the slugs. Low sensitivity in the high range is often caused by someone swapping the slugs. I don't have the details of how to identify these slugs. Hopefully someone reading this may provide that information to be included here later. The way I do it is to maintain a set of known good racks and slugs and swap until I find the bad one(s). Finding a replacement is another matter!
The coils have different contact pins and are different electrically also. In the R-390 these are a type of banana plug. The contacts are a push fit and sometimes they simply pull off when you remove the coils for cleaning. While cleaning these contacts be sure all these are firmly in place.