[R-390] COSMOS PTO EP ADJUSTMENT

[David Wise]

Disclaimer: I may have missed a previous message.

I have concerns about your series cap approach to
endpoint adjustment.  But first, let's make sure
I understand.  You propose adding a small cap in
series with C702 or C703, right?  If so, there
are two problems.

First, those caps are for temperature compensation.
All three caps (C701, C702, and C703) are
negative-TC ceramics.  Their decrease as the PTO
warms up balances the increase in L701 and L702 as
their forms and wire expand.  C702 and C703 were
selected at the factory to trim the overall TC.
Unless your cap is just right, it will throw off
the TC, which increases drift.

Second, you may go outside the linear range of L702,
or even run out of slug travel.  Reducing C requires
a corresponding increase in L, effected by running
the L702 (main tuning) slug deeper into the coil.
With L702 now making a larger percentage of the
L701+L702 total, the frequency range swept by ten
turns becomes wider.  This gets your endpoint back,
but does not correct the root cause, namely,
increase in L701's inductance.  The cure for
that is cutting off a turn, or (perhaps)
shaving the slug.

Regards,
Dave Wise

-----Original Message-----
From: r-390-bounces at mailman.qth.net [mailto:r-390-bounces at mailman.qth.net] On Behalf Of Pete Williiams
Sent: Tuesday, April 20, 2010 7:41 PM
To: wa3frp at aol.com
Cc: r-390 at mailman.qth.net
Subject: Re: [R-390] COSMOS PTO EP ADJUSTMENT

G'day Russ. You will have to take off the outer  and inner covers. The
threaded slug in the EP coil is easily screwed back in again if you were
unlucky  enough to screw it right  through.

No other disassembly is  required.

To make the  EP  adjustment approximately correct on the  bench, I suggest
you make up/acquire  apower supply to give you the 150  and 250 V plus 6.3
for heater . Make the appropriate  connections. Just put the inner cover on
pro tem and do the 10 turn trick checking both points.

When you find you have  sufficient adjustment range by doing the  1 turn
removal  or a series capacitor on the EP coil, a more deliberate /accurate
setting  can be got  when it's all back together.

I am fortunate  that I have a dual adjustable  2 voltage  supply and have
made up a test cable with a  spare plug to do EP  and  linearization.

Let the PTO warm up  at room temp for  an hour or  so..DON"T use the oven -
you'll go ballistic  trying to keep the  freq from moving  around ( lots of
HZ )

Best of luck

Pete  VK3IZ