[Amps] HL2K high input SWR on 21 MHz

[Jim Thomson]

Date: Fri, 4 Mar 2016 19:34:07 +0100
From: Patrick EGLOFF <pegloff at gmail.com>
To: "amps at contesting.com" <amps at contesting.com>
Subject: [Amps] HL2K high input SWR on 21 MHz

Hi all,

I have a HL2K amplifier (pair of 3-500Z) for several years now (got it
second hand) and it always had a high input SWR on 21 MHz. I matched it
with my TS850 antenna tuner, so it was OK.

During last contest i had a problem on 40m (500pF/2kV in output tank
shorted). So i had to open the amplifier and while being there i wanted to
see what's wrong with this SWR.

I couldn't reduce the SWR in adjusting the capacitor.

I removed all components used in the input tank and checked them. They are
all perfect.
Coil is 0.34uH and caps are as marked and on diagram.. Input C = 150 pF and
output adjustable up to 200 pF.

So now, i'm stucked....

On all other bands, the input SWR is just fine, so i suppose there is no
problem with the tubes or anything else in the amp. I have full power and
everything is stable.

Does anyone have a HL2K and can confirm that it is OK (or not) on 15m ?

Any idea on what could be wrong ?

73, Patrick

-- 
Patrick Egloff - TK5EP
email : pegloff at gmail.com
Web page : http://www.egloff.eu

##  Did you actually check the tuned input coil, and the 150 pf fixed cap and the 
adjustable cap with a digital LCR Meter ??     
My drake L4B also uses a pair of 3-500Zs, and the caps are  both  fixed. 
The coil is adjustable....slug tuned.  Input swr is  1:1  across the 15m band. 

##  The L4B  uses  paralleled  100 pf caps  ( total = 200 pf)  on the   xcvr  side......
and paralleled 65 pf caps  ( total = 130 pf) on the cathode side. 

##  I suspect that either your fixed 150 pf cap is out of spec, or your fixed .34uh
coil is out of spec.   Two paralleled  75 pf caps  ( total = 150 pf)  will handle  more RF 
current vs a single  150 pf cap.  Your 150 pf cap may well have been out of spec since day
1. 

Jim   VE7RF