[Heathkit] SB-200 Input Inpedance.. Frustration Continues...

[Larry Babb]

Hi 

I suggested to ken that he disconnect the wire to the cathode of the tubes,
terminate in 50 ohms non-inductive, and check input SWR.
In this configuration, he tells me that the SWR is less than 1.5:1 on all
bands.

The tubes are cathode driven, which should be a low impedance.  Next step in
my mind is to confirm that the tubes are conducting.
I asked Ken to confirm, that in transmit mode, without drive from the
exciter, the plate current rises slightly to a resting value,
What ever Heath designed the tubes to do.  If the tubes are ready to
conduct, the input cathode circuit should be a low impedance.


I am posting so your expertise can be utilized to help Ken resolve the
problem.  

Larry N7LB 

-----Original Message-----
From: heathkit-bounces at mailman.qth.net
[mailto:heathkit-bounces at mailman.qth.net] On Behalf Of Kenneth G. Gordon
Sent: Tuesday, August 14, 2007 1:43 PM
To: KBØNLY
Cc: Heathkit at mailman.qth.net
Subject: Re: [Heathkit] SB-200 Input Inpedance.. Frustration Continues...

On 14 Aug 2007 at 12:32, KBØNLY wrote:

> Unless i find someone to buy the POS for what i have into it i need to 
> fix it.  I installed new caps as suggested by many reflector members 
> and as shown on the website by NE7X, and it didn't make one damn 
> difference.

Strange....

> 
> Sure, i can get 50 ohms at the input according to the MFJ-259, but the 
> SWR meter still shows infinite.

REALLY weird!

>  So how is that possible?

Open cable between SWR meter and one of your radios? Shorted cable between
one of your rigs and the SWR meter? Bad connector on one end or the other of
one or both cables? Bad connections at one or all four ends?

>  I have
> 50ohms but infinite SWR on the 259 that is?

>  I have never seen it do
> that.

Look on page 13 of the on-line PDF manual. Here is what it says.

"An IMPEDANCE of 50 ohms can be composed of both resistance and reactance.
If the impedance is 50 ohms (or whatever the meter measures), but the SWR is
not 1.0 to 1, reactance is probably making up part or all of the impedance. 

Contrary to popular misconception, it is impossible to obtain a 1:1 SWR when
the load is reactive. This is true even if the complex impedance is exactly
50 ohms. 

A good example is a 50 ohm load with almost pure reactance and almost zero
resistance. The MFJ-269 LCD will indicate R=0 X=50 while the impedance meter
reads 50 ohms or the Z display indicates a 50 ohms impedance. 

The SWR would overflow (SWR>25) because the nearly-pure 50 ohm reactance and
impedance load absorbs almost no power from the source.

It has a nearly infinite SWR, despite having an impedance of 50 ohms.

On the other hand if resistance is near 50 ohms and reactance near zero, the
impedance would remain 50 ohms. SWR would be 1:1 in this case, since a
dissipative resistance readily accepts power from the source."

What this is telling you is that SOMETHING is shorted, or possibly the
length of coax you are using is long enough that an OPEN looks like a SHORT
to the meter.

Why your system "sort of works" on 40 meters may have something to do with
the length of the interconnecting cables.

The intermittant activity of your transceiver's SWR indicator may ALSO mean
some thing is not right with the switching network in one end or the other.

I really suspect your interconnecting cables or connectors at this point.

As I said, when I first got my SB-200, I had a cable problem that took me
several hours to discover.

Ken Gordon W7EKB
_______________________________________________
List Administrator: Duane Fischer, W8DBF        
** For Assistance: dfischer at usol.com **         
$$ See the vintage area on the HCI web site - http://www.w9wze.org $$
Heathkit mailing list Heathkit at mailman.qth.net
http://mailman.qth.net/mailman/listinfo/heathkit