[R-390] aligning the balanced input

[Bob Camp]

Hi

There is nothing wrong with the procedure you use. Provided you shield 
the setup and are careful about lead lengths and stray capacitance it 
will work and you are doing it correctly. If you want to go a little 
more nuts on the circuit then put a 50 ohm resistor from the center tap 
of the two resistors to ground. That will give the signal generator 
something to run into. It won't have any effect on the outcome of the 
test but the signal generator police will not give you a ticket next 
time they cruse by.

The only problems with the process are that it uses yet another custom 
test rig and it only makes sense on a 125 ohm balanced input radio. 
Since you need a 125 ohm to 50 ohm adapter to set up the balanced radio 
in the first place that makes two odd setups to keep track of. I have 
enough trouble just finding my soldering iron from time to time .....

On a 50 ohm unbalanced radio you are going to short out the balance 
adjustment cap as soon as you hook up the coax. Once you short out the 
balance cap the tuning of the first RF circuit changes. That pretty 
much eliminates any benefit from doing the balance adjustment.

If you are running a true 125 ohm balanced setup then the balance cap 
adjustment is a very necessary part of the alignment. If you don't do 
it then the input simply isn't set up right.

If you use it as a 50 ohm radio tune it up as a 50 ohm radio. If it's a 
125 ohm radio tune it up as a 125 ohm radio.  If you are running 300 
ohm balanced line into the radio then you should set it up for that. In 
each case the radio will run a whole lot better if you match things up.

	Take Care!

		Bob Camp
		KB8TQ


On Jun 27, 2004, at 10:50 AM, Steve Hobensack wrote:

> I align the balanced input by selecting two matched 68 ohm resistors. 
> I solder them in series and push the wild ends into the balanced input 
> connector. I attach a signal generator to the center of the two 
> resistors, and apply a signal. There is a deep null at the balance 
> point while adjusting the balancing trimmer. Inject only a small 
> amount of signal.
> 73...Steve...KJ8L
>
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