[R-390] If those Capacitors could talk

Sun Sep 12 14:03:03 EDT 2010

I'm using a Sencor cap anaylzer and I can't say I have found any that met 
the leakage spec at rated voltage.  I have a large plastic container I throw 
them in when replaced.  If you go to the trouble to remove one end for 
testing it's just about as easy to just go ahead and remove the thing and 
replace it.  If you are not removing one end and and testing you will be 
back.

For the leave it alone camp that says "yes it fails the leakage test at 
rated voltage but it's only run at 10% of that in the circuit" I 
respond...the leakage is a symptom of degredation that will only progress 
until it becomes leaky at it's working voltage.  The designers didn't plan 
for that value of parallel resistance in the bypass circuit.  It will 
eventually affect performance and load the power supply to the point of 
increased heat dissapation from other components.  Replacing them with 
quality modern caps means you can plan on not having to go back into the 
radio for a cap failure most likely in your lifetime.

Also yes...resistors should be checked.  It's easy enough to do most of the 
time with the tubes pulled out.  Try to put carbon resistors back in their 
place.  The modern metal film resistors do have inductance and can cause 
problems in some cases.

Also watch out for the old silver micas they fail as well. (silver 
migration)  Not nearly as often as the paper caps but they do fail.

Last but not least tighten all hardware.  You'd be surprised at the problems 
that can cause and have you chasing your tail.

Cecil
K5DL
----- Original Message ----- 
From: <wa4aos at aol.com>
To: <r-390 at mailman.qth.net>
Sent: Sunday, September 12, 2010 12:24 PM
Subject: [R-390] If those Capacitors could talk

>
>
>
> I have repaired/restored more R 390A and R 390 receivers than I care to 
> remember. From my experience I can honestly say most of those caps would 
> say, "Leave me alone, I'm just fine."
> While  some of the resistors are screaming, "change my diaper, I can't 
> handle anymore load ! ! !"
>
> In the beginning, I shot-gunned all of the 18 standard bypass caps in the 
> IF modules as well as 4 on the RF deck and the others on the audio module. 
> Most of the canned Electrolytic
> caps, C603 and C606, on the Audio modules have been physically and 
> Electrically leaky. That is to say, you will often see white crud coming 
> from the base and when tested for leakage at voltage, they test leaky.
> I see this so often that I buy batches of these from Sigma Technologies in 
> Germany. He advertises on ebay and makes a great replacement capacitor 
> set. I have used the ones from Hayseed electronics as well and they are 
> also good.
> I test capacitors for value with a digital tester and for load/leakage 
> with a Sprague TO6 tester which test the capacitor at or near it's 
> operational voltage.
>
> Here is what is interesting, out of the yellow or aluminum bypass foil 
> caps I have tested, I have found far less than 1% to be the least bit 
> leaky. all most all of these test and work excellent. And actually, 85% or 
> more of the black beauties also test fine.
> I know it can be argued that these capacitors are in some cases are near 
> 50 years old or older; that is a good argument. However, I so often hear 
> and see people looking for NOS, New Old Stock orange drops. God only knows 
> how old those might be or how many years they sat year round in a hot and 
> cold warehouse. I buy only NEW Orange Drops from Mouser and I believe they 
> are all made by Vishay now.
>
> I actually find far more resistors out of spec than capacitors. I have yet 
> to work on an IF or RF module that did not have 3 or more resistors 
> seriously out of spec. I believe more detail is paid to the capacitors, in 
> part, due to the fact changing out some of the resistors is more involved. 
> I guess ignoring them makes the problem go away???????
> Now for something funny.. Audio guys ABSOLUTELY LOVE Black Beauties and 
> think they sound wonderful. While  RF guys think Black Beauties are as bad 
> as having wired turds in their radio. Somewhere in between is the truth.
>
> On average, I spend 18 to 22 hours of bench time on an IF module, 
> sometimes more. And, I just love to see IF modules come in where someone 
> cut old capacitors out and twisted new ones to the old  leads and added a 
> cold solder joint on top, Please, have a little respect for these 
> receivers.
>
> I use lots of solder wick, a solder sucker, dental tools and various wire 
> cutters. Good lighting, magnifier, steady work bench and a comfortable 
> stool/seat for your big butt is just as important. One tool that I have 
> found to be very helpful are the miniature flush cutters with the flat 
> cutting side. These are/were often used to cut leads off close or flush to 
> the back of a PCB.
> Jameco Part no. 35482 is an example..
>
> Once most of the solder is removed from a lead, these flush cutters help 
> with getting on the lead and peeling it away from it's connection post. In 
> many cases, you can grap the lead, pull it and start twisting the lead CCW 
> or CW depending and it unwinds almost like tuna tins once did by using the 
> provided key. As far as dental tools, try to find good quality stainless 
> pieces, not the cheap off shore junk that seems to have permeated the 
> market. There are 5 or 6 places under the IF module where capacitors are 
> soldered to small phenolic post. These will easily crack with even gentle 
> care. I use those cheap Radio Shack solder heat sinks; the ones that look 
> like a flat clothes pin and try to not heat the post very much. Otherwise. 
> it will break and you will have to glue it back or find a replacement. I 
> found a source on ebay recently for some nice Russian made pieces that are 
> very close. The two standoffs near the front of the module will crack on 
> the phenolic. The 4 others on th
> e circuit board will easily crack into; take your time and EASY DOES IT. 
> If you are not a patient person this may not be a job for you.
>
> If I have to go deep to remove a out of tolerance resistor, I generally 
> remove and replace all of the bypass caps in this area. If I don't then I 
> replace the black beauties usually found on the older Motorola, Amelco and 
> Collins sets. I have decided to stop replacing capacitors for the sake of 
> doing so unless a customer request that service. I use to use my digital 
> camera and take pics to help remember where all the parts go but having 
> done it so often, I no longer need to, although, I do provide customers 
> with pictures of my work.
>
> Most of the performance gains I have observed on IF decks have been with 
> replacing resistors and tubes as well as cleaning all connectors with 
> DeOxit. With the exception of C603 and C606 as well as the caps 
> recommended by Chuck Ripple to improve audio, I think lots of time is 
> wasted on IF recap jobs.
> One note about resistors, just because they have not been soldered in 
> before don't ASSUME they are in spec. About 10% of the GOLD band resistors 
> I check are more than 15% out of spec.  I have a source in my area that 
> allows me to bring in an Ohm meter and cherry pick the one I want. I would 
> think any dealer trying to sell parts would not mind.
>
> BTW: Not to go on and on but recently, I was in the market for a 
> replacement electric screwdriver after by Black and Decker unit failed 
> again for the third time in a year.  I read all sorts of reviews and went 
> to the big box stores. When I got to Home Dpeot, the guy showed me a 
> wonderful tool made by Ryobi for $39.00. It is a 4volt Lithium-Ion unit 
> with bright work LED. Additionally it has a locking chuck that keeps the 
> hex head tools from falling. What I really like is the fact it has 
> variable torque just like the big electric screw drivers. It is not 
> variable speed but by adjusting the torque to 5 and driving screws into 
> the front panel of a 390 or (A) it simply spins once you get to the end of 
> travel. No stripping the head or worse, scratching the panel.
> I liked it so much, I went back and bought one for the other two 
> workbenches.
>
> 73,
> Glenn WA4AOS
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