[R-390] Image problem solved - Not.

[Bob Camp]

Hi

Second order distortion should follow a normal power law. More or  
less, you should get a larger drop in the "spur" than in the signal  
generating the spur. I'd have to dig out my old course notes to be  
sure, but a 2:1 ratio in db sounds right for second order.

Bob


On Oct 25, 2007, at 10:03 PM, Tony Casorso wrote:

> 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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>>
>>
>> -- 
>> --------------------------------------------------------------------- 
>> -
>> 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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