[R-390] Something in the RF deck breaking into oscillation...

[Cecil Acuff]

Be careful in tightening everything.  I found, in the last night or so while 
working on my SJC project, that there was an insulated standoff on the back 
side of one of the screws that holds one side of a tube socket in place that 
was rotating while I was turning the screw top side.  Luckily I took a look 
underside and found that before I turned it more than 10 or 15 degrees. 
Could have twisted off whatever was soldered to that point creating more 
work.

Bottom line is look at what is attached on the bottom side of what you are 
twisting on or damage may occur.

Cecil...
----- Original Message ----- 
From: "Joe Foley" <redmenaced at yahoo.com>
To: <r-390 at mailman.qth.net>
Sent: Saturday, December 24, 2005 11:58 AM
Subject: RE: [R-390] Something in the RF deck breaking into oscillation...


> NUTS!  You beat me to it.
>
> Don't forget to tighten the tube socket
> bolts,......... ah, just go through and tighten
> everything!
>
> Joe
>
>
>
> --- Jim Temple <jetemp at insightbb.com> wrote:
>
>> I had this problem a couple of years ago.  It turned
>> out to be one of the
>> caps on the rf deck not being properly grounded.
>> After reworking all the
>> grounds, I found that one of the ceramic cap grounds
>> that is underneath one
>> of the cans, was corroded. Tightening the lug on the
>> can restored the
>> ground, and solved the problem.  So, the moral of
>> the story is that these
>> old r-390a's, especially the "massacre" ones, need
>> at least a one time
>> rework of most of the grounding points.
>>
>> Sincerely,
>>
>> Jim Temple
>> Louisville, KY.
>>
>>
>> -----Original Message-----
>> From: r-390-bounces at mailman.qth.net
>> [mailto:r-390-bounces at mailman.qth.net]
>> On Behalf Of Tim Shoppa
>> Sent: Saturday, December 24, 2005 9:06 AM
>> To: r-390 at mailman.qth.net
>> Subject: [R-390] Something in the RF deck breaking
>> into oscillation...
>>
>> On most bands my R-390A is breaking into oscillation
>> during the first hour
>> or so of warmup. After an hour, it's rare for it to
>> do this.
>>
>> An important clue is that as I rotate the "ANT TRIM"
>> I can start/stop/
>> modulate the oscillation. It sounds sort of like a
>> regenerative receiver as
>> it breaks into/out of oscillation.
>>
>> The gears/insulators in the "ANT TRIM" drivetrain
>> are clean.
>>
>> It's most likely to break into oscillation when "ANT
>> TRIM" is peaked but
>> sometimes it does it no matter what and other times
>> (especially after warmed
>> up) it never does it. It's only when it's going from
>> stable to oscillating
>> that the ANT TRIM has an effect.
>>
>> Is this a common problem with a known solution, or
>> am I gonna have to resort
>> to shotgunning everything around the 6DC6 RF Amp?
>> (I already subbed out the
>> 6DC6 with no apparent change in symptom.)  I do not
>> have extender cables/etc
>> for the RF deck so getting underneath is a rather
>> laborious process and I
>> don't want to do this more than once.
>>
>> Looking at the schematic for bypass caps that might
>> go bad and cause this I
>> see
>>
>>   C227 - .047 as the cathode bypass. Is this a brown
>> beauty? But
>>               being in the cathode bypass it seems
>> unlikely to be
>>               causing oscillation.
>>
>>   C229 - 5000mmF as the screen grid bypass. I guess
>> a ceramic
>>               disc?
>>
>> I remember seeing three brown beauties in a RF deck
>> - two as bypasses around
>> the crystal calibrator, and a third somewhere in a
>> mixer.  So maybe C227
>> isn't a brown beauty... but then what is it?
>>
>> Tim.
>>
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