Regarding the 238 tuner caps, I experienced some SWR drift with mine at high
power and on certain bands. I one case I could use the same antenna and
feedline on the lower end of 80M with no change in SWR. However, at the
upper end with the same antenna and feed line on 75M there was SWR drift. I
could always tune it so the SWR was low but then when the caps cooled the
SWR had changed. Of course the SWR drift caused the tuning on the Centurion
to change.
I used a laser temperature meter to see what was heating. It was the ceramic
caps located at the back of the Low Z / High Z switch. I replaced the 220
pf caps units wired in parallel with some Russian made 470 pf 16KV RF caps
purchased on E- Bay. There's a couple of websites that show how these can
fit in. It takes drilling about 4 holes in the bottom for mounting and
wiring them in place. Problem solved. As to a source of caps, should you
not like Surplus Sales of Nebraska, try Fair Radio Sales. Or as I did,
E-Bay.
One other problem I found was the accumulation of paint, dirt and grime on
the insulation of the window/ladder line that runs along the side the house
and up the tower. Yes, the fellows that painted the house actually sprayed
the ladder or window line section that runs under the eve to the tower.
Again, this was a heating issue causing "what ever Z" change to take place.
I cleaned the paint off of that section of line using lacquer thinner and
the problem was solved.
As some of you may or may not know, I use a couple of retired BC
transmitters for 160M and 75M AM operation. In running these at a legal 375
watts of carrier output with 125% positive modulation, I use a Tentec 238
tuner for the matching chores. Trust me, that application will separate
the good tuners from the poor tuners. I don't care what the PEP rating of
the tuner is. I have one of the popular 3 KW PEP rated tuners that will
flash over on modulation peaks and finally the "heavy duty" internal balun
smoked, while under the exact same conditions, the Tentec 238 stays cool
and performs exactly the same tasks as needed. Oh yes, the match values for
the tuners are always set using a antenna bridge. I find that method
absolutely necessary when using a tuner with a T network configuration.
73
Bob, K4TAX
----- Original Message -----
From: "Jim Brown K9YC" <k9yc@audiosystemsgroup.com>
To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>
Sent: Saturday, February 07, 2009 4:49 PM
Subject: Re: [TenTec] Ig drift on amps
On Sat, 7 Feb 2009 16:09:07 -0600, Duane Calvin wrote:
Yes, Ron, I've experienced the same symptoms in my 238B
Ten Tec uses N1500 caps in the 229 and 238 tuners. Some matching
conditions cause these caps to carry a lot of current, enough to
cause them to change value by a fair amount. There are also matching
conditions that cause them to burn up. I run 229s and 238s with my
Titan during contests at the 1.2 kw level. I own six of these
tuners, and I've fried at least one cap in all but one of them.
On the oldest of my tuners, a 229B, I had enough heating around the
front end of the rotary inductor to toast the spring that holds the
traveling tap in contact with the inductor. I managed to repair it
with a replacement spring that I found in a surplus house.
73,
Jim K9YC
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|