[R-390] Help getting unstuck!

[Craig C. Heaton]

Dan/All,

Ok, here goes nothing, I'll do my best, and remember this is not my forte'.
I bought the Sprague just for this type of fun so I'll condense the manual.

First there are two groups of caps; group ONE are electrolytic caps, then
group TWO are all the others (ceramics, micas, paper, film, air, etc.). The
Sprague TO-6A runs three basic test: checks the capacitance(both groups),
leakage current of electrolytics, and insulation resistance of the others.

I like to remove the cap and connect its leads direct to the tester, don't
have to figure in the capacitance of the leads that way. Run the capacitance
value test first, just about every Brown Beauty in the Motorola failed here.
The tester could not determine their value.

If it is an electrolytic, the tester can apply the same dc voltage (up to
600V) the capacitor is rated at. Looking at the manual there is a nice table
showing what the leakage current (uA) of a new electrolytic MIGHT be (uF vs
DC working voltage). In general lower leakage is better.

Group TWO:all the others: The TO-6A tests them as two groups 50V to 200V,
200V and higher. Here, the high the insulation value the better the cap (in
general). Connect them to the post, push a few buttons, and read the meter.
Most of the Brown Beauties had very low insulation resistance or none (nil).

It was sort of fun connecting the old caps, testing them, and then comparing
them to a new electrolytic or orange drop. Big difference, but since most of
the old caps were 1956-57 vintage, someone got there money's worth. Nothing
last forever.

The object of this exercise is to end up with a receiver that I might not
have to put on the repair bench for several years.

As to my use of nil in the first post, I've never tested an oil filled cap
before and didn't have an exact replacement to see how it would test. I
don't think C551, the 2uF oil-filled cap when new would have zero insulation
resistance. Back to paragraph one, this is not my forte'.
Just having fun and will keep it that way.

Craig,



-----Original Message-----
From: Dan Merz [mailto:djmerz at 3-cities.com]
Sent: Saturday, April 02, 2005 10:33 PM
To: 'Bob Camp'; 'R-390A List'
Cc: 'Craig C. Heaton'
Subject: RE: [R-390] Help getting unstuck!


Bob/Craig,   I assume the simple answer to my question would have been that
insulation resistance refers to the measurement of dc resistance across the
terminals of the capacitor,  or inversely the dc leakage current - what
confused me was that with "nil" insulation resistance,  why would one even
consider not replacing a capacitor such as the 2 ufd oil-filled cap
mentioned - that's essentially a short. Maybe Craig meant nil leakage,
which would make more sense considering he was able to measure the
capacitance.  Or maybe he was measuring resistance on a scale where "nil"
was still a pretty high resistance.   I also wondered if the insulation
resistance Craig was talking about had to do with the "snot" on the leads
that might be bridging the leads,  in which case I might learn something by
asking what he meant.  So I'm still in the dark on that,  hope this helps
clarify what I was after.  Dan.

-----Original Message-----
From: Bob Camp [mailto:ham at cq.nu]
Sent: Saturday, April 02, 2005 6:24 PM
To: Dan Merz; R-390A List
Cc: 'Craig C. Heaton'
Subject: Re: [R-390] Help getting unstuck!

Hi

An ideal capacitor would pass no current at DC once it had been charged. In
fact it passes no DC current at all, it just stores it up.
That's another issue though.

Back to insulation resistance ....

So we put a DC voltage on a real capacitor and *suprise* it has leakage
current. It passes some current at DC even though an ideal capacitor should
not. This is simply because we can't make an ideal part.

In order to decide just how good a real capacitor is we need to have a
measure of how much it leaks. The leakage behaves like a resistor the easy
way to describe it is as a resistor across a an ideal capacitor.
The bigger this resistor the better the capacitor.

So far so good ...

Unfortunately there are two ways to describe a resistor. One is in ohms
(zero is bad in this case) the other is in 1/ohms (conductance or Siemens).
In the case of conductance zero is good. You have to be careful to be sure
which your machine is calibrated in ...

Just to make things even more complex ....

The specification on capacitors is normally written in terms of megaohm
microfarads. The bigger the capacitor the more it leaks. The more it leaks
the lower the insulation resistance.

The capacitors in question all should have insulation resistances in the
hundreds of megaohms if they are working right. Anything below about 50
megaohms is likely to be a defective part. This of course only applies to
the parts that are not electrolytic.

The electrolytic parts often are rated in leakage current. In an R-390
anything below a few miliamps is fine. A miliamp at 300 volts gives you a
third of a watt in heat in the capacitor. At ten mils you are up to three
watts and the capacitor can get a bit warm.

The problem with leakage measurements is that they do not totally represent
the situation with the capacitors. On the black beauty caps the leakage does
not entirely behave like a resistance. It goes up and down in bursts. The
fancy term for this is popcorn noise. When you get a burst of noise (or
current) the voltage on the capacitor drops fairly quickly in a typical
R-390 circuit. This gives you an intermittent snap crackle pop noise in the
background on an otherwise quiet signal. Since atmospheric noise can do the
same thing it may take a while to figure out what's going on.

None of this is to suggest that you should replace otherwise good
capacitors. The issue is that bad capacitors can, but don't always do cause
real problems in a radio.

About the only other point to make is that you do not want to replace the
paper or plastic insulated capacitors with electrolytic capacitors.
The leakage levels on all of the electrolytics are *much* higher than the
leakage of the plastic or paper parts.

Hope that helps some.

	Take Care!

		Bob Camp
		KB8TQ




On Apr 2, 2005, at 7:13 PM, Dan Merz wrote:

> Hi Craig,  you use the term "insulation resistance" or "IR".  What
> does this refer to or what are you measuring?  It's probably obvious
> to most of the guys but I don't know what you're referring to so
> pardon my lack of familiarity with the term with respect to the
> capacitors you're checking.
> thanks,  Dan.
>
> -----Original Message-----
> From: r-390-bounces at mailman.qth.net
> [mailto:r-390-bounces at mailman.qth.net]
> On Behalf Of Craig C. Heaton
> Sent: Saturday, April 02, 2005 1:55 PM
> To: R-390A
> Subject: RE: [R-390] Help getting unstuck!
>
> Hello to All,
>
> The worst of the caps were the Brown Beauties! Using a Sprague TO-6A,
> couldn't get any of these to provide a reading as to value. As far as
> insulation resistance, most were close to nil. Some of these had
> something that looked like dried brown snot on them. Good stuff to
> auction as Rare vintage Collins equipment on that E place.
>
> The Vitamin Qs were for the most part fine business, good values and
> IR.
> Since I had them out for inspection, it was easy to replace these
> while the iron was still hot.
>
> C603, all three 30uF sections read about 60uF and somewhat leaky.
> C606, both
> 47uF sections read about 75uF and leaky. Not bad for 1956 vintage,
> they served their country well. C103, a Pyramid from 1957 was a dead
short.
> Stuffed that can with a new 50uF 50Vdc electrolitic cap. Last, C551
> that oil bath 2uF can, it read 2.25uF and no IR. I cut that bad boy
> open with a dremel tool and put a NTE-MLR205K630 (Mylar 2uF) inside.
> Never tested an oil paper cap before, maybe didn't need to replace
> C551, but didn't like the zero insulation resistance.
>
> Was this Motorola a Saint Julians Creek special? I don't know, its a
> Order NO 14-PM-56-A1-51, if that adds to the conversation. All the
> sub-chassis have the same Order NO and are Motorola.
>
> I've been trying a little WD-40 applied with a tooth-pick and a hair
> drier to free the slug. Gots lots of time to free that pesky critter
> up.
> Could
> leave it alone and do the best I can with the others. But I do like
> the idea of a root canal. Need some spare parts before surgery!!
> Either a good
> T501
> or a slug will do the trick.
>
> Tnx,
>    Craig
>
> -----Original Message-----
> From: Cecil Acuff [mailto:chacuff at cableone.net]
> Sent: Friday, April 01, 2005 2:38 PM
> To: Craig C. Heaton; R-390A
> Subject: Re: [R-390] Help getting unstuck!
>
>
> Craig I don't have any magic answers on the stuck slug but I am
> curious about the caps.  Black Beauties?  Brown Beauties?  Split,
> leaky or out of spec in some way?  Was it a Saint Julians Creek
> special?
>
> Others experiences are important info in this area.
>
> Common wisdom ranges from "Leave em alone it's not necessary" to
> "Change em all their evil"....and even "Only change em if they came
> from the SJC".
> (not so common a wisdom I guess)
>
> You might try removing the can cover on the stuck slug and gently
> heating the form with a blow dryer or the slug with the tip of a
> soldering iron then trying to break it loose.  In another product with
> the same problem I have actually drilled out the center of the slug
> progressively....like a root canal and eventually reduced it to pieces
> to fully remove it and replaced it with a similar slug....I wouldn't
> think it would be too critical.
>
> My couple pennies worth.
>
> Cecil...
> ----- Original Message -----
> From: "Craig C. Heaton" <wd8kdg at worldnet.att.net>
> To: "R-390A" <r-390 at mailman.qth.net>
> Sent: Friday, April 01, 2005 12:35 PM
> Subject: [R-390] Help getting unstuck!
>
>
>> Good Morning to All,
>>
>> Just finished putting a Motorola R-390A back together. Had to replace
>> many evil out-of-spec and leaky capacitors. The good news is no magic
>> smoke escaped while plugging the receiver into a source of electrons.
>> Thought it might be a good idea to realign the receiver, it was
>> receiving slightly before my attempts at restoration.
>>
>> Now the sticky question. Can't seem to get the top slug in T501 to
>> move.
>> All
>> of the other slugs will move. This transformer has a cover with a
>> hole punched in the top center. Looks like a little rust between the
>> slug and coil form. I doubt I'm to first to tackle this sort of
challenge.
>> Anyone been here before? What sort of fixes for a stuck tuning slug?
>> Of course, if it never moves again or I break it, does anyone have a
>> spare?
>>
>> Seem to remember hearing in the Navy, "If its tight, its right". This
>> one is real tight, need to make it wrong.
>>
>> Tnx,
>>    Craig
>>
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