[TenTec] Ig drift on amps

Bob McGraw - K4TAX RMcGraw at Blomand.net
Sat Feb 7 22:34:46 EST 2009

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, 

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.

Bob, K4TAX

----- Original Message ----- 
From: "Jim Brown K9YC" <k9yc at audiosystemsgroup.com>
To: "Discussion of Ten-Tec Equipment" <tentec at 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
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