Gear Train Rebuild

Truly outstanding gear train assembly instructions and well labeled photos have been done by Scott Seickel.  <Click on the link above>

This above geat train initially looked like this:

After many years of use and abuse (dirt, dust, sand, over-lubrication), the R-390A gear train can become rather stiff.  When properly cleaned and re-lubricated, the difference can be startling.  At one time I simply cleaned most gear trains and only disassembled the really bad ones.  But after a few I noticed that the ones I had disassembled were typically smoother and tighter than the ones I had only cleaned.  I now disassemble all gear trains, separate the split gears, clean, lubricate, and reassemble with correct split gear tensions.

The rebuild of the gear train is time consuming.  There are a couple of "traps" that can spoil the enjoyment of a good afternoon with your radio.  Before proceeding, become very familiar with the procedure and understand what your main goals are:

CAUTION:  I recommend you completely understand the above objectives before you begin any disassembly of the gear train.  This is not necessarily a step-by-step procedure.

The gear train rebuild should always be started at the 7+000 KC position.  You will be doing a mechanical alignment as it goes back together, and the alignment marks are all set for 07+000 KC.  That's one KC above 07 999 KC.  Also make sure the zero adjust is centered in its range of adjustment.

I always start by removing the veeder root counter and the two gear wheels in front of the aluminum plate.  Removing the gear to the right of the KC shaft will require loosening the gear clamp behind the aluminum panel, and pulling its shaft through the panel bearing.  It looks to be a simple matter of removing the few screws that hold on the plate.  But before removing the screws, see if you can tap the pin on the KC shaft (see photo) far enough into the shaft to give clearance through the bearing.  This pin will have to clear the bearing as the shaft pulls through.  If you can't tap the pin into the shaft to get it out of the way, you will have to remove the 10-turn counter from the back side and keep the KC shaft intact with the aluminum panel.

Now remove all the screws holding on the front plate, and remove the front plate.

The following series of photos show the disassembly step-by-step.  Reassembly will be a simple reversal of this procedure, with a couple of adjustments and checks along the way.

Did you make it this far?  If so, now it's time to separate all the split gears.  You'll need a pair of snap ring pliers to remove the snap rings which hold the gears together.  Just remove the snap ring, and leave the springs attached.  You can use a paper towel to wipe out any grit or grease between the gears, then re-lubricate with a 50/50 mix of Mobil 1 10W30 and Marvel Mystery (penetrating) oil.  Put the gear back together with the snap ring, and move on to the next one.

Now clean everything thoroughly.  I spray WD-40 onto a rag or paper towel, and then use the rag to wipe and clean.  You can also use brass polish on the brass gears if you like.  Give it a good cleaning and oil all the rotating surfaces well (but not the gear teeth themselves).  You can take apart the multi-gear if you feel brave by loosening three screws.

Reassembly is just a reversal of disassembly.  First, study the drawing of the intermittent gear position shown in the manual.  You must have this gear set correctly before proceeding.  Getting it wrong can cause great weeping and gnashing of teeth.  The intermittent gear causes the RF bandswitch to turn as you move up past the 1, 2, 4, 8, and 16 MC marks. 

 Lubricate where the gear shafts seat into their brass bearings as you go using synthetic grease.  As you put each gear back on, line the camshaft pointers and alignment holes over the alignment marks.  If you reassemble in reverse order as shown, you will not have to make too many split gear teeth line up at once (two as you move from step 2 to 1, one elsewhere).  In step 2, where the big multi-section gear goes back in, have the big gear to the right on the megacycle shaft pushed all the way back.  Also, temporarily remove the detent spring piece which is above the MC shaft and held in place by two screws.  This keeps these out of the way while the multi-section gear goes in.  After the multi-section gear is in place, you can pull the MC gear forward into position.  To set the split gear tension, start with the spring at resting position (slack out but no tension), and then move the teeth about two teeth marks to stretch the spring.

One gear that many have tried to mate together is shown below.  They will mesh together with some effort.  Do not mesh these gears together, or nothing will move.

If you kept the camshaft pointers aligned as you went, you should be in pretty good shape by now.  You can still make minor adjustments to the cam positions by loosening the gear clamps and rotating the cams.  With the cams aligned, put the veeder root counter back on.  You'll want to make sure the KC shaft is about 1/3 turn away from the 10-turn stop which occurs at full clockwise rotation.  Make sure to return the KC shaft pin back to it's original protruded position before putting the KC shaft gear back in place.

It is now important to get the bandswitches aligned correctly.  The crystal oscillator bandswitch has a label printed on the drum.  It should be at the line between the 6 and 8.  I also visually check the wafer contacts to make sure it is truly centered on the contact.

The RF bandswitch is a little more tricky, as it's proper alignment is integral to the mechanical gear train.  Since you never moved these gears from the 7+000 KC position, the RF bandswitch is still set at the correct position.  Now move the MC wheel to 8MC, and watch the rotation of the RF bandswitch from underneath.  If you got the intermittent gear in the right place, you should see the bandswitch move to the next set of contacts.  Move it back and forth a time or two between the 6MC and 9MC positions.  It should begin moving immediately after 7 and stop at 8, with no movement below 7 or above 8.  This usually requires some amount of adjustment.  Look at the picture below.

See that little gear clamp that is highlighted?  That's the one we want to work with at this point.  It is this gear shaft that couples the rest of the gear train to the RF bandswitch intermittent gear.  You want motion between 7 and 8 MC, with no motion below 7 or above 8 MC.

OK.  Your gear train is back together with the bandswitch aligned.  Be sure and put some synthetic grease (see Hints and Tips for type) on the detent wheel and the veeder root gears.  If your gear train is stiff after reassembly, adjust the veeder root gear clamps to make sure these are not too tight.  It may take a few adjustments to find the "sweet spot" for these.

As you reinsert the slug racks back into the RF deck, be sure and lubricate all the bearing slides and ALL surfaces where the slug rack fingers slide against metal with synthetic grease.  Move the MC and KC knobs to where each slug rack is lowest, loosen the three slug positioning screws to allow the slug to re-position itself, and retighten the screws.

You'll have to realign the PTO to the RF gear train properly, getting 2455 KC from the PTO at the 7+000 KC mark (and of course 3455 KC at 7 000 KC).

Checkout The Quest Online to see where I now spend much of my time.  God Bless!

© 2005 by Walter Wilson, KK4DF.  All information is offered for non-commercial use without warranty, expressed or implied.