Rebuilding The Vernier Tuning Shaft on Philco Models 38-8, -9 and -10
Tom Burke, December, 2004
Vernier Drive Assembly, Philco Part No. 31-2072, Used on Models 38-8, 38-9, 38-10
During the rebuilding process, it was discovered that stations would drift off their setting. Upon closer examination, one could watch the fine tuning knob move off the position it was set to. Also, the coarse tuning wasn’t smooth. As the coarse knob was rotated, you could feel significant intermittent resistance to the coarse tuning, in pulses, as the larger knob was rotated.
I had no previous experience with this type of mechanical issue so decided to share my problem with other fellow Antique Radio Forum Members. I was immediately assisted by a host of members as to what the issue could be. There was a mechanical issue with the tuning vernier. So it was time to do a little mechanical exploration.
Here are the steps to start disassembly:
- Remove both fine and coarse knobs from the tuning shaft assembly.
- Remove both screws that hold the crescent shaped keeper against the chassis.
- Gently pull the tuning vernier straight out of the chassis. A small gear on the end of the assembly will slide
away from the anti-backlash gear connected to the radios main tuning capacitor.
Vernier Assembly Examination
Examine and clean the assembly with a little propanol or ammonia and water mixture. Use an old toothbrush to clean the geared-end and perform a visual inspection. My gear and the mating tuning capacitor anti-backlash gear were in good condition. Check to see if the main tuning capacitor turns freely.
Just behind the geared end, there is a C-clip that fits into an annular groove on the coarse tuning shaft. Upon removal, the entire assembly will be able to be taken apart, so now is a good time to be sure you are working over a shallow pan or other container that will catch and retain the number of small parts that will come from the disassembly procedure. It is a good idea to fold a couple of paper towels in the bottom of your catch pan, as there are ball bearings that could fall out. There are nine parts to deal with, as is shown in the diagram.
The vernier assembly will be locked together as a unit, even after the C-clip is removed. The entire assembly is, in effect, a planetary gear system, with the ball bearings acting as the gears. Pressure on the end of the ball-ended Fine Tuning Shaft forces the three ball bearings housed by the Coarse Tuning Shaft, to be forced out against the Outer Race Assembly. The pressure comes from the spring that is buried within the Coarse Tuning Shaft. This pressure is transmitted to the Fine Tuning Shaft by the Single Bearing. Be careful not to mix the different bearings up, as they really need to stay in each of their original positions.
- Remove the C-clip.
- Gently push the Fine Tuning Shaft towards the geared end. This will reduce the pressure on the 3 Main Vernier Bearings and allow you to slip the Outer Race Assembly off the Coarse Tuning Shaft. You should now be able to pull the Fine Tuning Shaft out from within the Coarse Tuning Shaft.
- Keeping all the component parts separate, clean all the old lubricating grease from the parts. The existing grease should help keep the bearings in-place somewhat, but once that is cleaned up, all the parts will fall out and no longer be easily retained.
Examine all your parts carefully. You will be looking for wear. My assembly only had wear on the Fine Tuning Shaft Ball. It appeared as slight scalloping on the portion of the ball that meshes against the three Coarse Tuning Shaft Bearings. It is subtle, but any visual discrepancies in this area need to be corrected. If you can see something, that has to be repaired. This left me with two options:
- Repair the existing ball-end.
- Replace the entire Fine Tuning Shaft, either from another radio or build an entirely new shaft.
Perhaps, if you have a parts radio, the vernier components could be swapped out. We didn’t have this option. Also, we suspect that the ball end was hardened, which would make replacing it with a newly manufactured piece difficult.
Reconditioning the existing ball-end
Having a small Unimat lathe, it was decided to turn down the ball-end that had the scallop marks. Evidently there is significant pressure on this ball to cause this. The shaft was chucked in the lathe and ended-up having good luck stoning the area smoother with the dressing stone that is part of a Dremel Tool kit. Take care to follow the same contour. Finish with fine emery and then a finer crocus cloth at high speed.
You may also refreshed the inner surface of the Outer Race Assembly using emery and crocus cloth. The ball-end will end-up being slightly oval, but that doesn’t appear to adversely effect smooth operation. If you didn’t have a lathe or a friend with access to one, then I’m sure that a small machine shop could assist for a nominal charge.
After examining all the components and applying fresh lubricant, the reassembly is just the reverse of disassembly. The end of the Fine Tuning Shaft needs to be pushed in, so the three bearings can be relieved, in order to get the Outer Race Assembly back onto the Coarse Tuning Shaft.
The key to success is to clean the parts well enough so that all the components may be examined for wear. It is also imperative that the parts be examined under magnification. The magnifying binoculars worked well for me, as it allowed the hands to be free.