By the time the R4B appeared, Drake had a solidly designed master
oscillator - PTO (permiability tuned oscillator). From that time forward
and with the exception of the TR5, Drake used pretty well the same PTO
in all their equipment. Largely, the major difference was in the dial
plates and gear box, but the *PTO* remained much the same. The PTO in
the TR7 is much the same PTO in the TR4 which is much the same PTO in
the RV7 which is ... you get the idea.
The great news about this piece of trivia is that you can swap the PTOs
around quite a bit, *providing* you can deal with the dial plates.
- PTO End Play
If the tuning knob can be wiggled from side to side, chances are the
end play needs adjustment.
End play can be adjusted by tightening the ball bearing on the PTO
tuning worm gear. Some Drake equipment might have an extra hole on the PTO cover for this purpose. For those that do not, you must remove the PTO cover. Use a long 3/32" allen (hex) wrench. The adjustment 'nut' in question is recessed below the coil form.
Do not overtighten or you will ruin the dial drive ball bearings and race.
Do not disturb any placement of components or you will affect the PTO dial tracking.
- PTO General Notes
Most of the PTO units are much the same throughout the 4 line, but
the drive mechanisms and indicator plates are not. The worst things
you can do to a Drake PTO is to continue tuning past the STOP indication or clean the dial plates with something that dissolves the plastic. Be very careful with cleaner on those Lexan dial plates! If
in doubt, use mild soap and warm water. Dial plate replacement is impossible (there are no dial plates to be had). Drake will service the PTO for you - they have the gears and other mechanical parts and can reset the PTO for drift and linearity spec. It would be most wise, however, to not lunch the PTO gears and dial plate.
- PTO Evolution
The PTO stayed basically the same over the years. It is a good,
solid design. Early model PTO assemblies had a brass pin that was
driven into a slot in the gear to provide a stop. As these assemblies wore, sometimes the pin would not extract itself and the PTO drive would end up in a locked state. If this was forced, the pin
will snap, leaving no dial stop at all and the dial plate will go around and around until the slug bottoms.
Later assemblies used nylon gears and dual dial plates on a concentric shaft. Some of these dial plates are 3 pin and some are 2 pin.
The number of pins refers to the brass rivets that hold the dial plates to the gear faces. There is no stop to speak of in these assemblies and turning past the mechanical resistance of the drive
assembly will lunch the gears. These PTO drive assemblies may have 'ears' and there may be 2 ears just behind the gear assemblies. The purpose of these ears is to allow the dial plates to be rotated for
proper mechanical orientation with the dial window gradical.
- PTO Mechanical Instability
This is usually caused by either the worm gear tension spring not hooked to the aluminium PTO cover or by the end cap on the end of the PTO coil form being loose. If the problem is not the tension
spring, remove the PTO cover and look at the end of the PTO coil form. You will see a cap on the end of it. It should not be loose.
If it is loose, carefully remove it, apply some glue and stick it back on. The coil form is delicate! The end cap just has a square hole through which the tuning slug brass rod goes through. For glue, I use GOOP.
See also PTO End Play above.
- PTO Warble
Sometimes the PTO will warble slightly while tuning. This is usually
caused by dried out grease on the drive mech ball bearings. This is
the ground path for the PTO slug drive which has a brass rod inside.
Fix the dried out grease problem first. Use Teflon lube or
Lubriplate. Run a flexible ground strap from the PTO drive yoke to
ground. Do not grease or lube the top guide pin for the PTO slug
- PTO Lockup - B Series
Sometimes the brass pin will insert into the gear at the 'STOP'
area, but it will not extract itself, causing the PTO assembly to
lock up. Wear will cause this, but in a lot of cases its caused by
the gear timing being off a little bit. What happens is that you'll
buy a used 'B' and use it. One day, you'll hit the stop, and the pin
will lock the PTO.
Unlock the PTO first by pushing the stop pin back and rotate the
tuning knob. Looking at the front of the radio, you'll notice a ny-
lon gear thats spring loaded. Gently push this gear back and rotate
the tuning knob ever so slightly (which way? take your pick). Now
run the PTO to the stop again and see if the pin extracts. No? You
went the wrong way or there are more serious problems.
- PTO Lubrication
Given the age of these unit, the grease is starting to dry out. Its
possible that it may even has run away slightly after seeing God
knows what use in a car or in a hot tent on Field Day many years
ago. Most important is to lubricate that worm gear. I use Teflon
spray lube. Just use the slightest bit. Too much is much worse than
too little. The dial mech should offer only slight resistance to the
tuning knob. You should be able to fast spin the tuning knob by
placing your index finger on the outside of the knob and rotating
your hand. If you cannot do this, then you have some kind of a prob-
lem in the PTO dial mech.
- PTO Backlash
Inspect the brass rod that extends from the rear of the PTO cover as
the unit is tuned. It may be discolored, but it should not be covered in grease and guck. This rod and the end of the PTO tuning coil comprise the end bearing. Clean with alcohol and a paper towel.
Sometimes 'junk' will accumulate in this area and actually cause
some binding in the tuning slug. You'll tune the PTO and in about 5
minutes, it will 'jump' frequency up to a few hundred cycles.
- PTO Drift
All Drakes drift. Once warmed up and settled down, they are satis-
factory for all modes but RTTY. For all practical purposes, the PTO
from the B series to the TR7 (excluding the TR5) are identical with
the exception of the dial plates and the markings on the aluminium
In very general terms, the PTO should settle down within about 5
minutes after turn on and be usable. It will still shuffle around a
bit after that, but you should not be chasing it continuously.
There are no PTO adjustments available to compensate for drift;
components were 'selected in production'. In extreme cases, you will
either replace the PTO from a junker or send the unit to Drake for a
rebuild. Expect to pay for 4 quarter hours labor minimum.
Drake PTOs are stable, but the temperature compensation is not per-
fect. The PTO in the SPR4 is quite a bit more stable than the R4C.
The reason is heat. The heat inside an SPR4 is neligible, but the
R4C heats the PTO slightly from the audio output transistor. Thr TR7
and R7 will shuffle around a bit from the heat from the dial lamp,
and the TR4any transceivers are much more stable when a fan is installed.
- PTO Skipping/Hysteresis - C series
Inability to have the C line dial plates to indicate exactly the
same frequency after moving away 100 kHz or so and returning to the
same frequency is usually caused by the rubber collar under tuning
knob and dish. After all these years, the rubber has hardened or has
worn. Replace the PTO rubber. The C clip should not be tight against
the aluminum washer and the washer should be installed such that the
groove around the perimeter is on the outside.
While you've got the knob and dish off, inspect the shaft for burrs.
Sometimes when you push a new rubber collar on you'll displace one
of the gear sets. This is easy to fix, but you'll have to remove the
top cover, push the left hand ear to the right while gently rocking
the tuning shaft and pulling out. Then align the dial plates again.
A new collar should last for years.
- PTO Seizure
I had one report of a PTO that locked up solid on a TR4. This apparently happened very suddenly. The clue is the knowledge that the transceivers run very warm, and the lubricant is 20 to 27 years old.
All the PTOs are essentially the same. There is a shaft that turns a
worm gear; the worm gear moves a yoke follower back and forth that
moves the tuning slug. Running off this shaft is a gearbox that
turns one (2 in the case of the C line) dial plate that has a bearing collar that is around the tuning shaft. The grease in that bearing area had cooked off. With the dial plate now 'locked' to the
tuning shaft the shaft is locked since the dial plate is driven by
the gear box and the gear box is driven by the shaft.
The cure is to flush out the grease and replace it with Teflon lube.
Teflon will not cook off, but the TR4 also needs a fan to resolve
the heat 'problem' caused by vaccuum tube density.
- 4 Line Dial Plate Dish, Knobs etc.
Personally, I don't like the plain dial skirt on the C line. I replace them with TR4 dial dishes. This does nothing except for appearance and is a matter of personal taste.
The TR7 dial dish is different than the 4 dial dish.
Some dial knobs are thicker than others depending on the PTO assembly. Most knobs are available for replacement except for TR7 band switch knobs. Unobtainium.
- Dial Plates and Plastic
These are not available from Drake any more. Use cleaning materials with extreme caution.
Most minor scratching of clear plastics can be polished out with
toothpaste, a touch of water and a paper towel. This works amazingly
well. If you use this trick on the dial plates, be careful you do
not rub the lettering off. Gel does not work nearly as well as
- C Line Dial Alignment
The C line allowed for dial indication alignment through the use of
idler gears in the dial drive mech. Just to the left and right of
the drive transmission you will find one or two 'ears'. Pushing
these ears towards the PTO shaft will allow the indicator plates to
be rotated such that the plates align to display the correct frequency. Depending upon the age of the C line unit in question, there
may only be one ear. In order to reseat the idler gear, push the le-
ver over and let go such that it snaps into place. If you do not do
this, it will take some rotation of the tuning knob until the idler
seats and your alignement will be off (again!).
On the B, you can rotate the Lexan dial plate a little with a touch
of brute force. Place a small screwdriver on the edge of the dial
plate and flick it the few necessary degrees. Do not to this with
the C line (see above).
- Dial Plate Scraping
The dial plates are fairly large diameter Lexan disks. On the C
line, there are 2 of them. One knob turn tunes the receiver 25 kHz.
Sometimes the dial plates will scrape as they are rotated usually
somewhere around the front panel. Over the years, some heat warping
should be expected. Quite often though the scraping is caused by
poor assembly after removing the front panel for cleaning. If the
whole PTO had been removed, there is a little positioning adjustment
available if the 3 PTO nuts are loosened.
Ensure the dial gradical plastic is on the outside of the sub chas-
sis with the red line on the inside of the window (C line).
On all radios, the blue filter mounts on the back of the white plas-
tic dial backing. Make sure the dial light wires are positioned away
from the dial plates.
If you have warped dial plates, I would not try flattening them by
any method. The simple, expedient answer is to go to a craft store
and buy some felt. Attach a strip of felt to the back of the sub
front panel with double sided cellophane tape.
There is no such thing as replacement dial plates except from a
junker. Every time a plate scrapes, you are grinding it away. Fix
the scraping as soon as it is noticed.