[R-390] Image problem solved

[Tony Casorso]

Now that I look at it more closely, I see that the field change just moves 
the whip input to the balanced connector while grounding one side of the 
balanced input and connecting the other side of the balanced input to the 
unbalanced connector on the back of the radio. A mouthful. The net of it is 
that swapping the cables and using my twinax-BNC adapter is exactly the same 
as using the unbalanced input without swapping any cables. Also, with the 
cables swapped, the unbalanced input becomes the same as using my twinax-bnc 
adapter when the cables aren't swapped.

I recall that when this all started, using the unbalanced input seemed to 
work better, but I never made any measurements on it.

As for overloading, I guess I don't see why you say that. From the 
schematic, both antenna inputs end up at the grid of the RF amplifier. The 
antenna trimmer is in the exact same place in the circuit. Unless the 
preselector RF transformers have a step-down turns ratio, I wouldn't expect 
the unbalanced input to exhibit any more tendency to overload than the 
balanced input.

>From this, it sounds like I am saying that grounding one side of the 
balanced input is a bad thing. This contradicts conventional wisdom. So, 
either conventional wisdom is wrong, or there is still something going on in 
my radio and all of this is just another clue.

One other thing, the neon bulb is only connected to the unbalanced input.




----- Original Message ----- 
From: "Bob Camp" <ham at cq.nu>
To: "Tony Casorso" <canthony15 at msn.com>
Cc: <R-390 at mailman.qth.net>
Sent: Thursday, October 25, 2007 5:24 AM
Subject: Re: [R-390] Image problem solved


> Hi
>
> Swapping around the cables on the back of the radio can have you  routing 
> the signal into the "whip antenna" input when you think you  are running 
> with the normal antenna input. You *will* overload the  radio if you put 
> much of an antenna on the (unbalanced) whip antenna  input.
>
> Bob
>
> On Oct 25, 2007, at 1:23 AM, Tony Casorso 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. 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. 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.
>>
>>
>>
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>