[R-390] R-390a Image problem?

Jon Schlegel ews265 at rochester.rr.com
Sat Oct 20 12:55:26 EDT 2007 

Tony,

Some further thoughts.  Again, I'm not really familiar with the 390 
yet so these are just some general ideas.

Could the screen or suppressor/cathode of the RF amp be lifting off 
of RF ground (bad C229, C227?).  This could cause some 
instability.  Another possibility along these lines is a problem with 
the parasitic suppressor, E212, or something downstream from 
it.  E212's presence in the design is tip off that the stage could be 
*hot*.  If the stage is oscillating at some frequency that's not 
immediately obvious (VHF range?), a non-linearity could be created 
that might generate the 2nd harmonic of 1650 kHz that you are 
seeing.  You could try to *sniff* the existence of an oscillation 
using other receiver(s) you may have on hand.  Use a short wire probe 
as the search receiver's antenna, put it next to the RF tube and 
start searching for any signs of life.

Regards,
Jon



At 09:42 PM 10/19/2007 -0600, you wrote:
>Checking the injection levels is a good idea. I'll try that tomorrow.
>
>Tony
>
>
>>From: Bob Camp <ham at cq.nu>
>>To: Tony Casorso <canthony15 at msn.com>
>>CC: <R-390 at mailman.qth.net>
>>Subject: Re: [R-390] R-390a Image problem?
>>Date: Fri, 19 Oct 2007 23:06:32 -0400
>>
>>Hi
>>
>>Stage voltages are a good idea if you have a VTVM. With an RF 
>>probe  you can also check the injection levels at the mixers. Low 
>>drive to a  mixer can cause all sorts of odd things. Your 17 MHz 
>>crystal  oscillator may be a bit "sick" for instance ....
>>
>>Another stupid thing to check - did somebody put a couple of 
>>diodes  up in the antenna relay to "overload protect" the radio?
>>
>>Bob
>>
>>
>>On Oct 19, 2007, at 10:48 PM, Tony Casorso wrote:
>>
>>>I see what you mean about AGC.  I checked it anyway. -11 volts on a
>>>strong local station. I swapped tubes with no improvement. I 
>>>wonder  if the stage bias could be wrong? I guess I should measure 
>>>the  voltages just to be sure.
>>>
>>>When I said 1st RF stage, I guess I forgot this was an R-390a. 
>>>Part  of the cost reduction removed one of the RF stages didn't 
>>>it? There  is only one.
>>>
>>>The antenna is a 100 foot longwire.
>>>
>>>Tony
>>>
>>>
>>>----- Original Message ----- From: "Bob Camp" <ham at cq.nu>
>>>To: "Tony Casorso" <canthony15 at msn.com>
>>>Cc: <R-390 at mailman.qth.net>
>>>Sent: Friday, October 19, 2007 8:36 PM
>>>Subject: Re: [R-390] R-390a Image problem?
>>>
>>>
>>>>Hi
>>>>
>>>>I'd swap it for another tube just in case.
>>>>
>>>>With no input to the radio, there should be pretty much no AGC 
>>>>to  the front end. It may be the problem, but only indirectly.
>>>>
>>>>If the radio is fine with no front end tube, you probably 
>>>>have  one  very large antenna.....
>>>>
>>>>Bob
>>>>
>>>>
>>>>On Oct 19, 2007, at 10:17 PM, Tony Casorso wrote:
>>>>
>>>>>Aha! The 1st RF tube is correct (6DC6) but when I remove 
>>>>>the  tube  and put a small cap from pin1 to pin 5 (grid to 
>>>>>plate),  the  receiver works fine and NO image. I'm going to 
>>>>>check the AGC  to  that stage.
>>>>>
>>>>>Tony
>>>>>----- Original Message ----- From: "Bob Camp" <ham at cq.nu>
>>>>>To: "Tony Casorso" <canthony15 at msn.com>
>>>>>Cc: <R-390 at mailman.qth.net>
>>>>>Sent: Friday, October 19, 2007 7:13 PM
>>>>>Subject: Re: [R-390] R-390a Image problem?
>>>>>
>>>>>
>>>>>>Hi
>>>>>>
>>>>>>I'd check the grounds as Jon suggested. The R390 can pick 
>>>>>>up   "stuff" from the power line pretty easily. The AC line 
>>>>>>filter   works both ways with a loose ground ...
>>>>>>
>>>>>>Next up would be to start swapping front end tubes. You 
>>>>>>may  find  that you simply have a defective tube. You may also 
>>>>>>find  that  somebody "improved" the design by putting in a tube 
>>>>>>that  didn't  belong there.
>>>>>>
>>>>>>Bob
>>>>>>
>>>>>>On Oct 19, 2007, at 8:58 PM, Tony Casorso wrote:
>>>>>>
>>>>>>>Hi Bob, I'm using the balanced input. I bought a twinax to BNC
>>>>>>>adapter so one side of the balanced input is grounded.
>>>>>>>
>>>>>>>Same antenna doesn't bother the R-392.
>>>>>>>
>>>>>>>Tony
>>>>>>>
>>>>>>>
>>>>>>>----- Original Message ----- From: "Bob Camp" <ham at cq.nu>
>>>>>>>To: "Tony Casorso" <canthony15 at msn.com>
>>>>>>>Cc: <R-390 at mailman.qth.net>
>>>>>>>Sent: Friday, October 19, 2007 6:52 PM
>>>>>>>Subject: Re: [R-390] R-390a Image problem?
>>>>>>>
>>>>>>>
>>>>>>>>Hi
>>>>>>>>
>>>>>>>>Which antenna input are you running into? The situation you
>>>>>>>>describe would be pretty typical if you run a long antenna  into
>>>>>>>>the "unbalanced" input.
>>>>>>>>
>>>>>>>>Bob
>>>>>>>>
>>>>>>>>
>>>>>>>>On Oct 19, 2007, at 8:31 PM, Tony Casorso wrote:
>>>>>>>>
>>>>>>>>>Hi everyone. This is my first post to the list although 
>>>>>>>>>I've   been following it for a long time now. A few months 
>>>>>>>>>back I   bought a  67 EAC R-390a. The only problem I had was 
>>>>>>>>>an open   Z-503 AGC  coil. I replaced that and did an 
>>>>>>>>>alignment and  the  radio seemed  to be working real well. 
>>>>>>>>>Sensitivity was  very  good on all bands.
>>>>>>>>>Recently I've noticed broadcast  stations  coming through in the
>>>>>>>>>3Mhz band. A local station  on 1650KHz  is about 40db at 
>>>>>>>>>3300KHz  on the R-390a. I don't  hear it on my  R-392. Even 
>>>>>>>>>moving the  2-4MHz RF slug rack by  hand, I still  hear the 
>>>>>>>>>station (although  it does get a bit  quieter). There  are 
>>>>>>>>>several other stations  coming through  as well on 
>>>>>>>>>other  frequencies. I didn't notice  anything odd  when I 
>>>>>>>>>aligned it,  everything seemed to peak like it should.
>>>>>>>>>Looking around at other frequencies with my signal 
>>>>>>>>>generator,  I started noticing  all kinds of harmonic 
>>>>>>>>>responses,  but  that is probably (at  least to some degree) 
>>>>>>>>>just my signal   generator (a Boonton  102C). I'm pretty 
>>>>>>>>>sure something is  wrong  here but I'm
>>>>>>>>>scratching my head trying to think of a  good  explanation. 
>>>>>>>>>If  the BC station harmonic was that bad,  I'd hear  it in 
>>>>>>>>>my other  radios I would think.
>>>>>>>>>
>>>>>>>>>Ideas anyone? How do I track this one down?
>>>>>>>>>
>>>>>>>>>Thanks,
>>>>>>>>>
>>>>>>>>>Tony
>>>>>>>>>_____________________________________________________________
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>
>
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