[R-390] A hard one ?

[Larry H]

Hi Pete,  Sometimes this is a tough one.  Here's some things you can try:1. disconnect both IF deck input cables.  If it stops, 455kc is getting into the RF deck.  Good metal tube shields on RF and mixers.2. turn IF gain all the way CW (low)3. Metal tube shields on all tubes carrying 455kc4. check L502 (detector filter) for short5. Weak bypass cap on cathode, screen, or plate circuits on V501 - V504 and V508.6. Wiring and component dressing on these tubes.7. Look for mods that could cause this.Good luck.
Regards, Larry

      From: Pete Williams <jupete at internode.on.net>
 To: r-390 at mailman.qth.net 
 Sent: Friday, September 9, 2016 12:45 AM
 Subject: [R-390] A hard one ?
   
​Why  do  I  get the  hard ones?  this is  a new  arrival.


IF  MODULE....It  goes into  oscillation..
1.. all  AGC  caps  changed
2... tubes  changed
3..  little or  no - AGC.    Can  also  see  + V on  AGC    (  about + 3
V)  depending on setting  of  RF  gain  and    local  audio..  AGC
terminals    correct R  to  chassis.
4.. Varying  local gain  and  RF  changes  character  of  oscillation as
heard  on  speaker.
 Testing  done  in  chassis  and    external on  extension  cable
5..  The  if  of  455 kHz  introduced  to  J513  from  sig, gen  and
output  level  varied      .Carrier  meter  works  but  has  gone
backwards  when AGC  goes  positive .
6...  tube potentials  to  be  checked  but  a  day of  audio
racket    a wonder  to  behold .


Do  I have a problem  ?

Cheers
Pete  Vk3IZ
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