[R-390] R-390a Image problem?
Tony Casorso
canthony15 at msn.com
Sat Oct 20 13:13:30 EDT 2007
I just finished checking the injection level at the first mixer. It seems
more than adequate at 1 volt rms but I don't really know what is supposed to
be. First mixer cathode is at about 8.5 volts (manual says 9.7).
I have a spectrum analyzer over in the corner. I might be able to use it to
look for oscillations in the RF stage. Maybe I'll have time later today.
Tony
----- Original Message -----
From: "Jon Schlegel" <ews265 at rochester.rr.com>
To: <r-390 at mailman.qth.net>
Sent: Saturday, October 20, 2007 10:55 AM
Subject: Re: [R-390] R-390a Image problem?
> 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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