[R-390] PTO problem, part II

[Tisha Hayes]

Maybe it is the "DO NOT OPEN" warning on the can but the PTO is just a
precision tuned variable inductor (Permeability Tuned Oscillator) there is
a threaded rod that drives a slug into a coil and a few other compensating
devices in there (caps and inductors). It definitely sounds like a
mechanical binding issue if you are getting drag in one direction or
springiness when tuning it. The electrical connections to the PTO are
minimal, it is just an oscillator, you could rig it up on the bench and
connect it to a good frequency counter to see it in action.

Maybe the threaded rod is all gooed up with residue of 60 year old grease
or someone else had been playing with it or the rod is bent or the carrier
for the slug is canted out of alignment (from someone forcing it past its
stops).

Carefully cleaning it out. using a little bit of mild solvent to wash away
the crud, cleaning up the o rings (on the case and the tuning shaft) and
being very sparing in putting lube on the right spots is usually all it
takes. Sometimes you need to remove a turn on one of the variable inductors
if you cannot reach the end-point (there is an entire procedure out there
that shows you how to do that with pictures and step by step instructions).
As it has been said, that thing should tune very freely and if done right,
the entire mechanical tuning assembly (racks, slides, gears, shafts, PTO)
can be worked on so you can tune the radio with the tip of your finger on
the knob.

If you were enterprising you could take out the little desiccant capsule
and bake it under low heat to dry it out completely (the color will change)
and put that back in right when you are ready to button the thing back up.

I would dread the idea of adjusting that swash-plate compensating disk.
There is a special type of masochist who will tackle that. Fifty-million
little screws to adjust linearity... euck!

Tisha


On Thu, Jul 11, 2013 at 1:20 PM, Norman Ryan <nnryann at yahoo.com> wrote:

> Tisha's mention of "stuck-on heaters" reminds me that it is a good idea to
> disconnect the heater wires permanently while one has the PTO open.  There
> have been instances where the PTO heater's thermostat gets stuck with
> disastrous results to the innards.
>
> A special foray into the PTO isn't necessary just to disconnect the heater
> wires;  a simple fix is to remember to keep the Heaters On/Off switch
> located behind the receiver in the "Off" position.  At normal room
> temperatures the receiver is exceptionally stable after a thorough warm-up.
>
> Check behind the receiver and see where the heater switch is set and try
> operating with it "Off" if it isn't already;  the difference likely will
> not be perceptible in most instances.
>
> If you operate with the heaters off, change the main 3 amp fuse to a 2 amp
> fuse for maximum protection of the B+ circuits.
>
> 73 de Norman, KG4SWM
>
>   ------------------------------
>  *From:* Tisha Hayes <tisha.hayes at gmail.com>
> *To:* R390A <r-390 at mailman.qth.net>
> *Sent:* Thursday, July 11, 2013 12:11 PM
> *Subject:* Re: [R-390] PTO problem, part II
>
> Since it appears there is some sort of internal binding you may want to buy
> a spare PTO and put it in service. They are not unavailable; even on eBay
> you can get one for $30-$80. Then you will feel less hesitant about opening
> up the funky one you have to see what is binding.
>
> You can probably fix it but it is going to be a "learning experience" and
> may be like the wire reattachment saga on the mechanical filters a few
> weeks ago. If you could not get replacement parts (all of the spare PTO's
> in the world suddenly vanished) then you could go to heroic lengths after
> you taught yourself to be a machinist and metallurgist.
>
> If they had been "cooked" by stuck-on heaters or turned way past their end
> limits you can have a bent shaft, stipped parts or a melted plastic piece
> (if you saw the inside it would be immediately obvious)
>
> I would rather listen to the radio than to spend weeks working on one item
> where I have spares. The PTO would become a rainy day project but the
> problem would not keep me from having the receiver on the air.
>
> That is just my personal opinion, YMMV (your mileage may vary) but I am
> being a bit pragmatic about it.
>
> --
> Ms. Tisha Hayes/ AA4HA
>
> "Objective considerations of contemporary phenomena compel the conclusion
> that success or failure in competitive activities exhibits no tendency to
> be commensurate with innate capacity, but that a considerable element of
> the unpredictable must invariably be taken into account." -- George Orwell
> ______________________________________________________________
>
>


-- 
Ms. Tisha Hayes/ AA4HA

"Objective considerations of contemporary phenomena compel the conclusion
that success or failure in competitive activities exhibits no tendency to
be commensurate with innate capacity, but that a considerable element of
the unpredictable must invariably be taken into account." -- George Orwell