[R-390] R390A- Intermittent sensitivity variation

[David Wise]

If "buzzing" doesn't reveal bandswitch trouble, look to the mica caps.

I could be remembering wrong, but I think there are fixed mica caps that are in service over the 16-32MHz octave and not used elsewhere.  One could be shifting in value or intermittently partially opening or leaking, which would mistune the affected coil or reduce the Q, respectively.  A little electrical "jostling", in the form of transients that occur during switching, can temporarily restore operation.  This happened to my receiver, although it was in the IF not RF.  Unfortunately, you have to lift one end of each suspect to check this.  Even though most of these caps see no DC voltage in operation, applying voltage, say with a TelOhMike-type cap analyzer, can drive this kind of problem into the open.

Dave Wise 

>-----Original Message-----
>From: r-390-bounces at mailman.qth.net 
>[mailto:r-390-bounces at mailman.qth.net] On Behalf Of Roger Ruszkowski
>Sent: Sunday, May 06, 2012 5:29 PM
>To: R-390 at mailman.qth.net
>Subject: Re: [R-390] R390A- Intermittent sensitivity variation
>
>
>Eric,
>
>I will move back over to the other computer and send you a 
>couple of long posts.
>
>The up shot is you have a dirty switch contact in the RF deck 
>band width switch.
>
[SNIP]
>
>Roger AI4NI
>
>
>
>
>
>Hi Group,
>                 I have mentioned in previous posts (Greetings 
>from Sydney, 
>ustralia) that I have noticed variation in calibration signal 
>levels with my 
>963 Imperial R390A.  After a longer period of investigation, I 
>note that the 
>roblem is actually confined to the 16 -32 MHz octave. On any