Identifying and correcting a common type of PTO instability:

Ever tune your R390A through a carrier with the BFO on and get a "warble?" Does the PTO seem unstable while tuning?

If the answer is yes, this fix may apply:

First, test for the problem by turning the radio on and allowing it to warm up about 30 minutes. Tune in a calibrator point and turn on the BFO. Tune the radio through the calibrator signal. The BFO note should change smoothly and without any warble or instability. If this is not the case, the problem may be as below:

There is an angled piece of metal fastened to the front of the PTO. It makes a 90 degree bend then engages the tuning shaft just in front of the forward PTO shaft bearing. This piece of metal serves as the ground return for the PTO. The points where it engages the PTO tuning shaft and its connection point on the housing can become contaminated causing unstable PTO tuning. I have even seen well meaning but uninformed owners incorrectly apply grease or oil to the point where the return strap engages the PTO shaft.

After unplugging the radio from the AC mains, try this easy fix:

Remove the PTO by first removing the shielded cable (center plug next to J-208 on the left rear of the RF deck). Turn the receiver upside down and unplug P109 on the back of the PTO. Loosen the two screws holding the rear PTO bracket. Do not completely remove them. Loosen the 3 captive, green headed screws which hold the PTO to the chassis. Carefully remove the spring on the Oldham coupler and set it in a safe place. Carefully maneuver the PTO from the main chassis being careful to guide the shielded cable which was attached to the RF deck past the antenna cables and through the hold in the main chassis. Place the now completely free PTO assy in a clean, well lighted place so it can be worked on.

Its a good time to clean out the PTO cavity in the main chassis with some WD-40 on a rag and apply a few drops of oil to the tuning shaft at the bearing in the main chassis located in front of the PTO. Because it will never harden, I use synthetic Mobil-1 90W rear end lube.

Remove V-701, the 5749 tube from the PTO.

Using plenty of light, you should now be able to see the ground return strap. There are 2 small screws holding it on the the PTO housing. The opposite end of the strap engages the PTO tuning shaft after making a 90 degree bend. On some PTO's, this end of the strap rides in a small groove machined into the PTO tuning shaft.

Remove the 2 screws holding the strap to the PTO housing. It is not as easy as it looks due to the bracket being located directly above the screws which holds the PTO body to the mounting assy. Be sure not to loose the small screws and their lock washers.

Using De-Oxit or Flux-Wash, thoroughly clean the curved end of the strap where it touches the PTO shaft. Work you way slightly up the sides of the strap to accommodate the sides of the groove. Next, use a toothbrush with DeOxit and clean the spot on the PTO housing where the strap attaches. Finally, clean first the side of the strap where it mates to the PTO housing then clean the other where the two mounting screws seat.

Turing you attention to the PTO tuning shaft, clean area or groove where the strap rides. Do NOT TURN THE PTO SHAFT or you will be realigning the radio due to the cam positions v/s the PTO frequency being changed. Be very careful not to get any DeOxit or Flux Wash into the front bearing. To best do this, put some of the cleaner on a "Q" tip or small toothbrush first then clean the contact/groove area thoroughly. DO NOT GREASE OR OIL THE CONTACT POINT.

Reinstall the ground return strap on the PTO. Be careful to insure that the turning shaft end is riding in the groove machined into the tuning shaft.

Reinstall the PTO in the radio chassis, grease and install the Oldham coupler and reconnect the 2 plugs which were unplugged to remove the PTO. Make sure the PTO is mechanically aligned to the tuning shaft then tighten the 3 green captive mounting screws then finally, the 2 loose screws on the PTO rear chassis bracket. Install the anti-backlash spring on the Oldham coupler. Reinstall V-701 after first tightening the screws in the tube socket and giving the tube pins a shot of DeOxit.