Collins 75A4 Tuning drive

[w6xr]

Recently I acquired a refurbished Collins 75A4 and all is well except the tuning dial feels as if its binding.  I would attempt to disassemble, clean and lubricate if I could find a reference source for that action.  Can anyone refer me to a source of information including a step by step procedure to service the tuning mechanism?

Thanks all!

Natan W6XR/2

w6xr@frontiernet.net

What is the size of the Bristol wrench used to remove the tuning knob?  Neither of the little blue packages purchased has one that fits properly.  I think they are numbered.  Once I get the knob off, I'm sure I'll solve the problem.

Thanks all!

Natan W6XR/2

[k4kyv]

I have two 75A-4's, and I have replaced one of the gear reduction drives with a new one, and disassembled, cleaned and relubricated the other.

First of all, every one of the set screws in the tuning knob and gear mechanism takes a spline driver, aka Bristo or Bristol wrench.  NEVER attempt to turn the set screws with a hexagonal or Allen wrench.  This will strip the splines in the heads of the set screws and turn the screw and driver into a pivot-bearing assembly, impossible to remove without special tools.  Wait until you obtain a set of Bristo wrenches before you even think about trying to disassemble the drive.

Once you acquire the appropriate drivers, take off the main tuning knob, then remove the vernier drive assembly.  Once the main tuning knob is off, it should be perfectly obvious how the rest of the assembly is taken off the receiver and comes apart, if necessary.  With the vernier assembly removed, test how the shaft turns to see if the binding is in the gear mechanism, or in the PTO itself.  It could be that the "dial drag" or friction mechanism is binding the kilocycle dial.  If the PTO moves freely and the dial drag functions normally,  disassemble and clean all the parts in the gear mechanism and re-lubricate.  It should work perfectly once  reassembled.  Again, never attempt to loosen ANY of the set screws with an Allen driver.  One more thing, if you disassemble the gear mechanism, be sure to  make drawings and take notes and  make sure you remember exactly how everything is to be assembled together.  Be very careful not to lose or damage the thin nylon washers that go under the brass bushings, and to replace them exactly where were originally located.

I had to replace the entire gear drive on one of my receivers because someone had tried to take it apart using an Allen driver and had ruined all the set screws, then apparently tried to grab what was  left of the set screws with a diagonal cutter, further trashing the set screws.  I had to file through the brass bushing to remove the mechanism from the tuning knob shaft, ruining the mechanism.  Luckily I happened upon a classified ad for n.o.s. virgin dial drive mechanism and tuning knob from the Collins factory, that had never been opened, and purchased the whole thing for $35.  Once installed, the receiver tuning knob felt just like brand new.

[N3DRB The Derb]

ifn you got a hi res digital camera, take multiple hi res (5mb or better) B4 pix and print em out for reference. I cant do anything anymore unless
I do this - I used to be ok fine with just taking crap apart, now invariably I cant figure out where something went upon reassembly unless I do it.

This is whats wrong with the maul at the moment.  I have a black wire to connect somewhere on the terminal board strip and I didnt take any b4 pictures for reference. I have not a clue where it is supposed to go. 

[Todd, KA1KAQ]

Stepping back a bit, you didn't mention if it was an early model with the standard Dakaware tuning knob or later example with the 4:1 vernier tuning. The early models used a thin metal strap with a simple bushing held in place with 2 screws for the PTO shaft to pass through. I've seen these slightly bent enough to bind the shaft, probably by an impact to the tuning knob at some point. If you have one of these, simply remove the tuning knob and check the strap for binding by loosening the 2 screws slightly and rotating the shaft. If it moves easier, you have an alignment problem. If it doesn't, the PTO shaft is binding and will likely require removal of at least the front panel and perhaps the entire PTO to repair. Could just be gummed up from decades of old crud buildup.

If your receiver has the 4:1 vernier drive and 'spinner' knob, Don's info covers it pretty well. The gear drive does mount to the same holes as the original bushing strap, so it's possible but unlikely that there's a simple issue of misalignment. Usually not the case since the holes in the panel are factory drilled, and the gear mechanism mounting holes have little, if any slop, and the knob doesn't mount directly to the main tuning shaft. This was more often an issue when someone added the 4:1 gear set to something like an earlier A Line receiver, requiring a template for drilling, and the possibility for an alignment error.

Chances are, it's just built up crud in the gear set from years of use, excess lubricant, etc. The 4:1 knob takes a tiny Bristol spline wrench, and can be annoying to loosen and tighten due to its size. Be careful not to exert much pressure on the tuning spinner while installing or removing the knob as they tend to snap off easier than you'd think.