[R-390] Image problem solved - Not.

[Tony Casorso]

Hi Mark,

Thanks for asking around for me. I did check the AGC (-11 volts to 1st RF 
amp grid on a strong local station). The thing is, the actual signal at the 
frequency I am tuned to is not that strong. I am tuned to 3.3MHz where there 
is actually no signal being transmitted. The signal that I am hearing is 
being transmitted at 1.65MHz. Normally, AGC does not even operate in this 
situation because there is no signal at 3.3MHz where I am tuned. So, once I 
hear this harmonic, it's already too late for AGC to prevent it.  I don't 
think I am explaining this very well.

Suppose the 1.65MHz signal was just below the threshold of causing this 
problem? I would hear nothing at 3.3MHz and there would be no AGC at all. 
Now turn up the 1.65MHz signal a little bit and I have the problem. How 
could AGC have prevented it? I'm not sure thats much better.

Anyway, I still say that the front end preselctor is way too broad in this 
situation. Talking to Bob gave me an idea that is probably all wet. If the 
impedance of my antenna system at 1.65MHz were really low, say a few ohms, 
would this kill the Q of the preselector at that frequency? Putting a 220 
Ohm resistor in series with one side of the balanced input reduces the 
1.65MHz signal at te input to the RF amplifier enough that I no longer hear 
anything at 3.3MHz. Increasing it to 2200 Ohms reduces the 1.65MHz signal at 
the RF amp grid when tuned to 3.3MHz by 30db while only reducing the signal 
when tuned to 1.65MHz by 8db. This says to me that the Q is much better.

I wasn't going to keep this up here on the list but people keep asking me 
questions... :).

Tony


----- Original Message ----- 
From: "Mark Huss" <mhuss1 at bellatlantic.net>
To: "Tony Casorso" <canthony15 at msn.com>
Cc: <r-390 at mailman.qth.net>
Sent: Thursday, October 25, 2007 7:40 PM
Subject: Re: [R-390] Image problem solved - Not.


> For what it is worth, Tony, I asked around about your problem with people 
> who have had extensive experience with troubleshooting R-390A's. All four 
> said the same thing. AGC on the First RF Stage is driving it too hard, and 
> into distortion. I just gave the symptoms, not that you had worked on the 
> AGC Transformer. That may not be it, but it is the first place they would 
> start.
>
> Tony Casorso wrote:
>> Sorry everybody,
>>
>> I lied. I didn't mean to but I did. Tonight I tried to repeat the 
>> experiment and it doesn't work.
>> Lifting the ground from one side of the balanced input does absolutely 
>> nothing to help the situation.
>>
>> I'll bet that I had a bad connection to one side of the balanced 
>> connector since I had to jerry rig it to connect my BNC from the antenna 
>> to the balanced input without ground. I hear no harmonic at all when only 
>> one pin of the balanced input is connected to the antenna. I also hear 
>> only a faint hint of a harmonic when I use the unbalanced input.
>>
>> So, I still have the problem (if it's really a problem). It's not an 
>> image problem really. I'm sorry I called it that. It has nothing to do 
>> with the IF.
>>
>> I'm going to go away now. I'll make sure everything is repeatable before 
>> I bring it up again.
>>
>> I feel like the Rockies must feel after the disaster in Fenway last 
>> night. Well, maybe not quite that bad.
>>
>> Tony
>>
>>
>>
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>
> -- 
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> Some people are like a Slinky .. not really good for anything,
> but you still can’t help but smile when you shove them down the stairs.
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