[Swan] 250C

[Howard Weeks]

Collin,

I may be about to repeat what you have already done but hope not!

The mixer input wants to see the RF signal before mixing and 
conversion.  Local oscillator has to get in there somewhere as well.
Sometimes, you can apply the expected IF output freq to the input
and get a good response on the output.  Sometimes, you do not get
anything on the output.  If your LO is working on the correct 
frequency and the mixer is otherwise okay, a signal of the correct RF 
frequency on the input will go through.  You may or may not see much 
gain through the mixer.  

I just rehabed an old 1937 Philco for a guy.  After replacing nearly 
every cap and most of the resistors (including power xmfr and audio 
output xmfr, she played.  One half of the dial worked fine, the other 
end produced whistles and pops, etc.  Voltages on the mixer plate 
were high - nearly up to the B+ source which was 250.  Should have 
been 100 or so.  Changed the tube and all is well.  Voltage is 
roughly correct now and it works across the dial.  

I have good results with injecting a signal from the signal generator 
to substitute for a bad local oscillator.  

Be sure to isolate the signal generator through a small cap.  
Anything else will probably load a grid too much and keep it from 
working.

Good luck with it!

Howard K5JCP

On 3 Aug 2007 at 21:48, n4tua at aol.com wrote:

> Dear Friends,
> May be getting closer but seems like I am going in circles. I went back 
> and did sig gen testing on the 250C again starting from the mixer plate 
> working back toward the antenna. At the mixer plate I get a good S9 
> signal with 5uV at 10.9mhz. Then when I moved to the grid of the mixer 
> expecting a stronger signal indication with less sig gen input. But no 
> just the opposite. For some reason the mixer does not seem to be 
> amplifying. I have checked all voltages and resistances associated with 
> this triode tube (6HA5) which has been subbed with a 6HM5. These are 
> the same electrically, but??? Once again I am scratching my head.
> Collin
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Howard Weeks
Harlem, GA 
K5JCP