[R-390] Mica caps and problems

[Perry Sandeen]

 Hi Matt and list,
I have a Fluke 6080A and a HP 8656B.
I prefer the Fluke as it has dial tuning.  Its big downside is that it weighs 40 pounds and is only slightly smaller that Rosie O'Donnell.
Where the a good modern signal generator is really necessary for the A is the 170 uV injection to the IF.  As Roger R has noted in the Y2KR3 manual, it is very easy to get too much gain.  If one hasen't done the 10 turn pot or David Weis (sp) mod that setting  can be a nightmare.

Regards,
Perrier

    On Saturday, January 19, 2019, 10:11:30 AM PST, mparkinson <mparkinson1 at socal.rr.com> wrote:  
 
 Hey
Perry what generators are you talking about HP, Tek
I have used several in my years of repairing R 390 series radios and others.
I find it interesting to hear and see other opinions.
What models are  you talking about.
Matt Parkinson

-----Original Message----- 
From: Perry Sandeen via R-390
Sent: Friday, January 18, 2019 5:00 PM
To: r-390 at mailman.qth.net
Subject: [R-390] Mica caps and problems

Yo Dudes!
1. There are free on line calculators that will do both series and parallel 
LC circuit calculations.

2. Considering the age of our receivers one can expect that some micas may 
be failing or are gradually changing values.
3.  One should consider the prospect that a wholesale replacement may be 
prudent or becoming necessary in the coming years.  Yes, a wholesale cap 
change is a PITA but is far less than having to find and change defective 
caps on a regular irregular basis.
4.  I purchased quite a few micas from Dan's small parts.  He had several 
package deals that made them quite affordable.  Either with Dan's or just 
radios there's a minimum plus shipping.  It's just the cost of doing 
business.
Many subtle and maddeningly difficult problems to solve have been found to 
leaking bypass caps.  Buying a complete set for the *A* from just radios 
could turn out to be a prudent move.
Many people on this list see to believe that the small caps and resistors in 
our B/A's are going to last forever.  It wasn't so when they were made and 
it will never be so.

One item that was never in the original specs was an extended lifetime 
performance specification.  This is because all involved realized that 
technology would progress and at some future point thy would be replaced.

Now as they get older, just like us geezers, they are going to require more 
maintenance.
Just because your receiver is *working* doesn't mean its working well or to 
the optimal OEM specs.  With many good documented mods it can become so much 
better.  The late Chuck Felton made a living proving this.
Also some sensitivity measurements made with the venerable HP 606 arn't 
valid as it leaks.  Now sensitivity isn't the only important receiver 
parameter but it happens to be the easiest to do.  If one wants real and 
repeatable information one needs one of the better HP or Tek signal 
generators from ebay.  They cost from $400 on up. (And yes I have two so I 
can verify their output values are accurate.)
Regards,
Perrier










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