[R-390] Re: Image Problem

[Jim M.]

How do we know that the broadcasters aren't actually radiating a harmonic?
What are the FCC requirements for harmonics on the BCST band?

-----Original Message-----
From: r-390-bounces at mailman.qth.net [mailto:r-390-bounces at mailman.qth.net]
On Behalf Of Bob Camp
Sent: Sunday, October 21, 2007 7:32 PM
To: Tony Casorso
Cc: R-390 List
Subject: Re: [R-390] Re: Image Problem

Hi

Roughly speaking, assuming that 1 micro volt is 0 db:

100 mv is 10^5 micro volts = +100 db

If your carrier meter is calibrated for 0 = 1 uv then a +40 reading  
would be 60 db second order rejection.

That's actually pretty good overload performance for that kind of input.

One mili volt would be 40 db rejection which is not quite as good.  
You do have a *lot* of signal there .....

You have to be very careful how you measure levels inside an R390.  
The impedances are very high. Unless you have a real good FET probe,  
it's very hard to measure anything without de-tuning or loading the  
circuits. That said a 5:1 reduction from the front end tuning doesn't  
sound like a real good performance from the front end.

Bob


On Oct 21, 2007, at 5:11 PM, Tony Casorso wrote:

> I'm losing track now :). I'm no longer positive where the sig gen  
> was. I tried it so many ways. I have checked the AGC before against  
> the curve in the manual and it is pretty close. You could pick off  
> -4.2 volts on the curve and have some idea what I was applying at  
> that time.
>
> Tony
>
> ----- Original Message ----- From: <SHELLY199 at aol.com>
> To: <r-390 at mailman.qth.net>
> Sent: Sunday, October 21, 2007 2:58 PM
> Subject: [R-390] Re: Image Problem
>
>
>>
>> Tony,
>>
>> When you measured the  voltages below what was the gen input voltage?
>>
>> Voltages are
>>
>> With  no AGC applied:
>> Plate 201v
>> screen 178v
>> cathode 2.1v
>> grid  -.5v
>>
>> With AGC applied:
>> Plate 215v
>> screen 201v
>> cathode  .22v
>> grid -4.2v
>>
>>
>> The agc, with the Langford mod, should be  about:
>>
>> 10 Uv rf input = -4.36 Vdc
>> 100Uv rf input = 6.66
>> 1000 Uv rf  input = 8.53
>>
>> I don't know if this will help, but you are talking some  very big  
>> rf input
>> voltages (20mv).
>>
>> I also had a problem with the Ist rf  amp not responding to the  
>> AGC voltage
>> once.   Tube tested good and the  agc voltage was there.  Was  
>> fixed with
>> another rf amp tube.  That took  a lot of work.
>>
>> By the way, if you use In914 or In4148 diodes in that  Langford  
>> mod I've
>> found that one of the diodes will fail.  I use  1n5060.  I think  
>> it's breakdown
>> when the AGC switch is turned from slow to  medium. This has  
>> happened in at
>> least 5 IF decks.
>>
>> Rich WD2Q
>>
>>
>>
>>
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