[R-390] R-390a Front End Selectivity

[Bob Camp]

Hi

What matters is the response into the normal load on the coils. The  
only way to properly check the response of the grid network is to pick  
off of the plate. If you use a low impedance on the plate (say 10K  
ohms) you will swamp the plate coil's response and only "see" the  
front end.

Bob

On Jan 19, 2008, at 7:27 PM, Tony Casorso wrote:

> Hi Roy. I have done all of the usual performance measurements on my  
> receiver and it seems to be fine with the exception that the Ferrari  
> is being hammered by a strong local AM station while the tractor is  
> immune.
>
> The secondary of the antenna coil is tied to the RF amp grid on one  
> side and to the AGC on the other side. The point where it is tied to  
> the AGC is at RF ground through a 5000pf cap. If this ground were  
> missing, the antenna coil Q would probably be affected. This is why  
> I mentioned the AGC feedpoint as being intact. This is the only spot  
> I have identified in the circuit that would affect multiple bands.
>
> I'm not sure what you are getting at with the Ferrari reference. Are  
> you suggesting that the R-390a is being overwhelmed because it is a  
> superior receiver (much more sensitive)? I am probably missing the  
> point here.
>
> The R-392 performance also checks out. Not as sensitive and not  
> nearly as quiet as the 390a but it is in good working order.
>
> Tony
>
> --------------------------------------------------
> From: <roy.morgan at nist.gov>
> Sent: Saturday, January 19, 2008 4:52 PM
> To: "Tony Casorso" <canthony15 at msn.com>
> Cc: <R-390 at mailman.qth.net>
> Subject: Re: [R-390] R-390a Front End Selectivity
>
>> Quoting Tony Casorso <canthony15 at msn.com>:...
>>> You all probably remember the thread I started about my problems  
>>> with a
>>> strong local AM station (groan)....today I swept the front end of  
>>> the R-390a
>>> and compared it to my R-392. ... The sharper response with the  
>>> lower peak is
>> the R-392. The wider, taller response is the R-390a.
>>
>> Tony,
>>
>> May I paraphrase?
>>
>> I have two vehicles, and the intake ports are not the same at all.   
>> One is a
>> tractor  and has an intake port about three inches in diameter. The  
>> other one
>> is a Ferrari with eight ports, each only an inch and quarter in  
>> diameter.
>>
>> Why the difference?
>>
>> Well... I suggest it might be good to try the two radios each as a  
>> whole.
>>
>>> As I mentioned last time, I
>>> can't find anything wrong with the R-390a.
>>
>> There may be nothing at all wrong with the R-390A OR the R-392.
>>
>>> ... I am attributing the higher response of the
>>> R-390a to the turns ratio of the front end RF transformers.
>>
>> A number of factors affect the frequency response of the first  
>> stage in a radio:
>> among them would be: The Q of all the tuned circuits, the coupling  
>> of the
>> input circuit to the secondary, The load on the secondary, the  
>> value of the
>> driving impedance (antenna or signal generator).
>>
>>> I have checked for a proper RF ground at the AGC feed point.
>>
>> Its not clear to me what you mean here: The AGC feed point might  
>> have very
>> little to do with the RF ground used for RF input, or for the  
>> detector (the
>> scope probe in your case.)  when you say "...I am monitoring the  
>> grid of the
>> first RF stage with the power off using a 10x scope probe. The  
>> input signal is
>> about 160mv peak to peak for this picture."  Where is the signal  
>> taken from
>> thats shown in the vertical deflection of your picture?  Is that  
>> from the 1st
>> grid?  (I suspect so because with power off, you won't get any  
>> amplification
>> from the tube.)
>>
>>> I just can't believe that this is the way it is supposed to look.
>>
>> I suggest you test the whole receiver for band pass/ out of band  
>> signal
>> rejection and the like.
>>
>> Roy
>>
>>
>> Roy Morgan
>> 13033 Downey Mill Road
>> Lovettsville VA 20180
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