[R-390] Help getting unstuck!

[Bob Camp]

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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