PTOs and Corrosion
By: Lee Bahr W0VT Houston, Texas
PTO's are subject to corrosion. Many times a bad ground connection in the PTO is the problem. The
PTO is used on each band. Try cleaning up metal on the moving PTO parts.
Drake used to solder a braid wire from the ground solder lug on the PTO to the nut just forward of the PTO ferrite plunger, I believe.
Now that these rigs are old, more corrosion exists and more grease where it should not be. I would probably take out the PTO and clean everything up and then maybe solder on this braid wire if you have lots of corrosion.
When you remove the PTO, unsolder the black/white ground
wire, the red/white plus wire and the coax output wire on the bottom side of the chassis. Remove the 3 screws on the bottom to remove the PTO.
MAKE SURE YOU DO NOT SCRATCH THE DIAL WHEN YOU REMOVE THE PTO.
You will have to remove the knobs, dial lenses, and painted front panel before you do this so you can cock the PTO out of the chassis front panel.
WARNING: The front panel is mounted with 4 small black screws in the 4 corners of the front panel. There is a fiber washer as a spacer between the front painted panel and the chassis front panel. When you remove the 4 black screws, the fiber spacers can be easily lost, so be careful when you remove these screws.
While you are at it, clean the dial lenses with mild detergent and with nothing that will dissolve plastic. Do not take the two clear rotating dials apart. Clean them in
place using tissues and Q-tips. Relubricate the METAL BEARINGS because the grease dries up and adds drag and relubricate the metal screw drive that moves the PTO ferrite plunger. DO NOT GET ANY LUBRICANT ON THE PLASTIC GEARS, if you have plastic gears, as the plastic will swell and
the gearing ruined if you do this. Check the wabble and looseness of the front tuning shaft.
You can adjust for snuggness using a LONG allen
wrench or use a shorter one by removing the PTO cover after removing the spring by squeezing the 4 cover prongs on the sides of the PTO. Do not over squeeze as to bend the prongs permanently. You want to adjust the shaft to be snug but not over tight so you don't wear the rear bearing and add too much drag to the tuning.
LET ME WARN YOU ABOUT ONE MORE THING. DO NOT TURN ON YOUR R-4C while the PTO IS OUT OF THE RECEIVER, electrically !!!!! You will blow the tantalum cap in the receiver that
is after the voltage dropping resistor feeding the PTO because there will be no current load on the resistor when you remove the PTO and the voltage will then rise and go above the voltage rating of the tantalum. You will know you did this when the tantalum explodes like a cherry bomb!
All this seems difficult but it is fairly easy to do if you use caution and you take your time. The end result will be, a clean smooth running PTO with no jumping of frequency.
Drake made MANY variations of this PTO. When you reinstall the PTO you will have one job left to do. That is, adjust the dial calibration of the PTO.
Some PTOs have ONE lever and some have TWO levers on the top of the PTO. When you push the lever, one at a time, the gears disengage so you can put your finger in the gap on the right side of the PTO and front panel and move the disc
at will until the frequency is correct. (There is a cut-out on the side of the front panel to get your finger on the sides of the plastic dial discs).
If you have a second lever, you do the same thing for the
second dial. WE ARE TALKING C-line dials here as they have 2 dials and not one as are on earlier Drake models. You can set a transmitter to put out a signal at say 4 Mhz or use a frequency generator to give a say 4 Mhz signal, so you can set your dial to 4 Mhz on the receiver.
What frequency you use is unimportant as long as your receiver can tune that frequency and the signal is accurate for the calibration. The use of a frequency counter could check the calibration of the test signal for accuracy.
I hope this helps.
Lee Bahr w0vt Houston, Texas