[R-390] Re chirping CW and low audio plus strange PTO problem.

[Bob Camp]

Hi

You don't mention who's PTO you have so this may not be exactly right 
for the PTO you have. The simple answer is that yes you have a simple 
mechanical problem with the PTO.

With the original design the PTO is linearized by a bunch of little 
sliders that make up a zig zag track that runs the length of the coil. 
There is a follower that runs along this track and it drives the 
compensation process. If you have a steep spot on the track around the 
100 KHz point and a weak follower spring then it will run "up hill" 
just fine. When you come back and try to go "down hill" the 
compensation will not be repeatable.

On a totally unrelated note the set up of the sliders on the track was 
still remembered twenty years later as "the worst job they ever made me 
do here" by the line workers who built the Motorola PTO's .... Talk 
about hard to explain your hobby to somebody ....

	Take Care!
		
		Bob Camp
		KB8TQ

On Jun 16, 2004, at 6:57 PM, Forrest Myers wrote:

> Hello All,
>  Found the problem causing the low audio in my Capehart SN 557. It was 
> C537
> that changed from a capacitor to a 33k resistor. It was in the cathode
> circuit of the limiter. I didn't have a direct replacement for it so 
> put in
> one, temporarily, about ten times larger than the original 1800pf. 
> Audio is
> great and it seems to have helped the CW chirp too, don't ask me how. 
> Have
> ordered replacement capacitor of the proper size, actually a 1000 pf 
> and an
> 850 pf which will be paralleled.
>
>  Now, I have a strange problem with the PTO. If I tune up from 0 to 
> 1000, it
> is very linear, within a couple hundred hertz all the way. If I tune 
> down
> from 1000 to 0, it is very linear, within a couple hundred hertz 
> except at
> 100khz where it is suddenly about a kHz off. Going on down to 0 it is 
> back
> on. If I approach the 100khz mark from about 10 kHz below, it's right 
> on the
> money. If I approach it from about 10 kHz above, it's about a kHz off, 
> low.
> Scratching my head for a while, I figure there must be some mechanical
> problem inside the PTO but can't imagine what it is unless the core is
> dragging on the coil and causing it to move at around the 100 kHz mark.
>
>  There is no play in the gear train or the PTO shaft coupling.  When 
> tuning
> at the 100 kHz mark, any slight movement of the tuning knob will cause 
> a
> change in the frequency of the PTO.
>
>  Any ideas? Should I just live with it?
>
> Forrest Myers
> -------------------------------------------------------------------
> "Security is mostly a superstition.  It does not exist in nature...
> Life is either a daring adventure or nothing."  -- Helen Keller
> -------------------------------------------------------------------
>
> _______________________________________________
> R-390 mailing list
> R-390 at mailman.qth.net
> http://mailman.qth.net/mailman/listinfo/r-390
>