[R-390] Image problem solved

[Tony Casorso]

No, I am not feeding the two balanced inputs the same signal. I tied the 
shield of the coax to one sine of the balanced input amd the center of the 
coax to the other side. I just don't tie either side to chassis at the same 
time. This seems to give the best results.

The field change connections look odd to me as well but I tried it vecause 
it was shown that way on the schematic.It does seem to work much better, but 
not as well as lifting the chassis ground from one side of the balanced 
input.

The schematic I am referring to is in TM-11-856a.

Tony


----- Original Message ----- 
From: "Jon Schlegel" <ews265 at rochester.rr.com>
To: <r-390 at mailman.qth.net>
Sent: Thursday, October 25, 2007 12:03 AM
Subject: Re: [R-390] Image problem solved


> Tony,
> Please see inline.
>
> At 11:23 PM 10/24/2007 -0600, you wrote:
>>I was using an BNC to twinax adapter which grounds one side of the 
>>balanced connection. When I remove this and run the signal generator or 
>>antenna two the balanced pins without tying one side to the chassis, I can 
>>see nothing on the spectrum analyzer.
>
> So it sounds like you are saying that you are feeding the two balanced 
> twinax pins in common mode, i.e. the twinax pins are tied together and fed 
> with the center conductor of the coax.  Coax shield to radio case/ground. 
> Is this correct?
>
>
>>  That is, With thereceiver tuned to 3 Mhz and the sig gen at 1.5MHz, I 
>> see nothing at the input to the RF amplifier.
>>
>>Looking at the schematic, they show a field change which grounds one side 
>>of the balanced input but SWAPS J105 AND J106.
>
> It sounds like the intent of the field change is to ground one side of the 
> balanced input and transfer the other side of the balanced input so that 
> it would appear at the single ended connector referred to at least on my 
> schematic as "J103 Whip Unbalanced".  Is this correct?
>
>
>>  They were not swapped in my receiver. Swapping them and then going back 
>> to my adapter gives much improved performance. I don't hear the 1650KHz 
>> broadcast station at 3.3MHz any more but I can again see some a 1.5 MHz 
>> signal at the RF amp input when the receiver is tuned to 3 MHz. But it is 
>> way down from before. The best performance for the preselector seems to 
>> be with J105 and J106 not swapped  and neither side of the balanced input 
>> grounded to the receiver chassis.
>
> I'm not getting this part because it sounds like the balanced input is 
> being fed common mode.  Maybe I'm misunderstanding what you mean swapped 
> and not swapped.
>
> BTW - I'm still waiting for the eBay seller to ship my radio. GRRR 
> &$@)&@!$%^.  If I had the darn thing to look at it probably wouldn't be 
> confusing!
>
> Jon
>
>
>
>
>
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