[R-390] need help with R-390A symptom

[Ed Wirtz]

I agree with Dan. Without actually experiencing the problem. from what you said I would suspect something that is heat related. The only thing that heats that fast in your radio is probably the tubes themselves unless there is also a leaky filter cap. In addition, it sounds like it's probably in the audio section, since you said that AGC appears to be normal, which means that the RF stages are working as well.  Is the hum low pitched and constant or is it more like a bad audio ground? If it's low pitched I would suspect a bad filter cap. If it's higher pitched I would look for a bad tube or connection that is intermittent. Wiggle the tubes around in the sockets. Quite often that will identify a bad connection in a tube socket which happens more that you think. 

Have fun. I love these old radios!!

Ed, KØBKA
  ----- Original Message ----- 
  From: Dan Merz 
  To: 'Bob Young' ; 'r-390' 
  Sent: Thursday, January 08, 2009 12:46 PM
  Subject: Re: [R-390] need help with R-390A symptom


  Bob, as I understand it,   hum and higher volume are the "abnormal" condition and no hum and lower volume are the "normal" condition.  My first thought was that one of the tubes is hanging with a grid voltage that is too positive or one of the tubes is bad.  A poor grid return,   bad connection, high resistance might cause this.  I believe switching to standby also takes high voltages off most of the tubes so this could be the effect that is resetting things and not something related to the antenna connections per se.    The cure might be as simple as establishing a better connection somewhere,  perhaps on one of the tube pins,  but maybe you've already explored that.    Dan.



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  From: r-390-bounces at mailman.qth.net [mailto:r-390-bounces at mailman.qth.net] On Behalf Of Bob Young
  Sent: Wednesday, January 07, 2009 11:33 PM
  To: r-390
  Subject: [R-390] need help with R-390A symptom


  After my radio been on for perhaps 10 minutes, the volume will instantly increase along with some hum to where you can still hear it with the volume on 0, this happens suddenly almost like a bad connection just got straightened out excpet for the hum. Something as simple as switch from one antenna to another will make the radio go back to normal. Also turning it from on to standby and back again always straightens it out. The AGC seems to work fine. I had suspected maybe a partially burned antenna radio relay at first but don't think it could be that. It will do this off and on for a while and sometimes after several hours it seems run normally for a while. Any ideas on where I should begin to look?

  Bob Young
  Analog, MA
  KB1OKL


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