[R-390] Image problem solved

Thu Oct 25 19:49:20 EDT 2007

Hi

Page 5-44 in the Y2K manual shows the RF input to the radio.

The unbalanced input goes straight to the grid of the RF amp via C212  
(39 pf) on the 2 to 4 MHz band. The input impedance at this point is  
around 10K ohms.

The balanced input goes into the tuned circuit L214 + caps. It's  
coupled to the second tuned circuit 215 and associated caps.  The  
input impedance on the L214 side is around 400 ohms or so.

When you put a low impedance antenna or generator in via the C212  
route you de-q the front end big time. That's can give you the  
"image" problem.

Bob

On Oct 25, 2007, at 12:10 PM, Tony Casorso wrote:

> I am looking at a PDF of the manual. Lets see how many ways I can  
> describe it. The schematic sheet I am looking at is spread over 6  
> PDF pages. On page 202 of the PDF document is the last of the 6. It  
> says "Figure 5-13 (schematic diagram) sheet 1 of 4. Then below that  
> it says 5-37,5-38 which look like section and page numbers to me.  
> This giant sheet is a schematic of the front end starting at the  
> antenna on the left edge.
> The PDF page with the antenna stuff is page 197. The border of the  
> schematic here shows zones A through E up the left edge and zones  
> 16 down to 13 on the top and bottom edges. The FC is in area D and  
> E, 16 and 15 where it says FC NO. 5 see note 3. The unmodified  
> circuit is shown in area B and C , 16 and 15.
>
> Hope that helps.
>
> Tony
>
>
> ----- Original Message ----- From: "Jon Schlegel"  
> <ews265 at rochester.rr.com>
> To: <r-390 at mailman.qth.net>
> Sent: Thursday, October 25, 2007 9:45 AM
> Subject: Re: [R-390] Image problem solved
>
>
>> Tony,
>>
>> Can you give a page/figure reference for the field change?  I'm  
>> looking at "TM 11-856A  Dept. of the Army, January 1956" and I'm  
>> not seeing the field change.  My copy may be too old.  If so,  
>> might you be able to direct me to a newer copy?
>>
>> Jon
>>
>>
>> At 06:36 AM 10/25/2007 -0600, you wrote:
>>> No, I am not feeding the two balanced inputs the same signal. I  
>>> tied the shield of the coax to one sine of the balanced input amd  
>>> the center of the coax to the other side. I just don't tie either  
>>> side to chassis at the same time. This seems to give the best  
>>> results.
>>>
>>> The field change connections look odd to me as well but I tried  
>>> it vecause it was shown that way on the schematic.It does seem to  
>>> work much better, but not as well as lifting the chassis ground  
>>> from one side of the balanced input.
>>>
>>> The schematic I am referring to is in TM-11-856a.
>>>
>>> Tony
>>>
>>>
>>> ----- Original Message ----- From: "Jon Schlegel"  
>>> <ews265 at rochester.rr.com>
>>> To: <r-390 at mailman.qth.net>
>>> Sent: Thursday, October 25, 2007 12:03 AM
>>> Subject: Re: [R-390] Image problem solved
>>>
>>>
>>>> Tony,
>>>> Please see inline.
>>>>
>>>> At 11:23 PM 10/24/2007 -0600, you 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.
>>>>
>>>> So it sounds like you are saying that you are feeding the two  
>>>> balanced twinax pins in common mode, i.e. the twinax pins are  
>>>> tied together and fed with the center conductor of the coax.   
>>>> Coax shield to radio case/ground. Is this correct?
>>>>
>>>>
>>>>>  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.
>>>>
>>>> It sounds like the intent of the field change is to ground one  
>>>> side of the balanced input and transfer the other side of the  
>>>> balanced input so that it would appear at the single ended  
>>>> connector referred to at least on my schematic as "J103 Whip  
>>>> Unbalanced".  Is this correct?
>>>>
>>>>
>>>>>  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.
>>>>
>>>> I'm not getting this part because it sounds like the balanced  
>>>> input is being fed common mode.  Maybe I'm misunderstanding what  
>>>> you mean swapped and not swapped.
>>>>
>>>> BTW - I'm still waiting for the eBay seller to ship my radio.  
>>>> GRRR &$@)&@!$%^.  If I had the darn thing to look at it probably  
>>>> wouldn't be confusing!
>>>>
>>>> Jon
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>> _____________________________________________________________
>>>>> R-390 mailing list
>>>>> Home: http://mailman.qth.net/mailman/listinfo/r-390
>>>>> Help: http://mailman.qth.net/faq.htm
>>>>> Post: mailto:R-390 at mailman.qth.net
>>>>> Unsubscribe: http://mailman.qth.net/mailman/options/r-390
>>>>
>>>>
>>>> _____________________________________________________________
>>>> R-390 mailing list
>>>> Home: http://mailman.qth.net/mailman/listinfo/r-390
>>>> Help: http://mailman.qth.net/faq.htm
>>>> Post: mailto:R-390 at mailman.qth.net
>>>> Unsubscribe: http://mailman.qth.net/mailman/options/r-390
>>
>>
>> _____________________________________________________________
>> R-390 mailing list
>> Home: http://mailman.qth.net/mailman/listinfo/r-390
>> Help: http://mailman.qth.net/faq.htm
>> Post: mailto:R-390 at mailman.qth.net
>> Unsubscribe: http://mailman.qth.net/mailman/options/r-390
>
> _____________________________________________________________
> R-390 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/r-390
> Help: http://mailman.qth.net/faq.htm
> Post: mailto:R-390 at mailman.qth.net
> Unsubscribe: http://mailman.qth.net/mailman/options/r-390
>