[R-390] Caps and more caps

Tue Jan 11 20:35:17 EST 2005

Hi

I must be shopping for my R-390A's at the wrong store.

Of corse I don't seem to see very many modules from later builds. 
Certainly the later modules I have seen have plastic insulation parts 
rather than paper insulation capacitors.

I have found both black and brown coated tubular paper capacitors in 
R-390A's. I agree that cracking is pretty common in the black version 
and less common in the brown ones. As far as I can tell the contractors 
either got a deal on black ones or brown ones. The modules rarely have 
a mix of black and brown parts.

What got me started on all this capacitor stuff was a pair of IF decks 
both with black caps. Both decks had at least some of the capacitors 
cracked. In both cases all of the paper caps leaked a lot. I carefully 
removed each of them in a fashion that I could reasonably solder them 
back in. Needless to say this is a bit more work than just yanking them 
out. All that work was wasted in those cases.

Now I normally check one capacitor in a module by lifting the ground 
lead. If it is leaky then I step through the rest of the caps on that 
module clipping each one out and testing each of them. I have yet to 
find very many of the paper caps (brown or black) that did not leak 
quite a bit when biased at > 60 volts and observed for long enough to 
stabilize the current.

The plastic insulation capacitors I replace them with test just fine on 
the same test. Interestingly they don't even leak at a level close to 
their specification. That's what I would expect from a good capacitor 
of this type, leakage well below the specified maximum.

The only exception I make are for the filament and thermostat bypass 
capacitors. As far as I can see they could leak an awful lot and not 
have much affect on anything. I generally don't fiddle with them unless 
they are cracked or otherwise visually defective.

One thing that I started out doing on the paper caps and then stopped 
doing was to check the capacitance. I found a number of parts that 
leaked but never found one that was off value. I don't know if this is 
true across the board or not. I assume it's valid though.

Electrolytics on the other hand certainly leak, change value and change 
ESR. I never have checked ESR on the paper parts ... I check the 
electrolytics for both leakage and value. With an electrolytic I don't 
worry about the leakage until it's about 5X the specification. I don't 
see how an extra few ma of current in a power supply bypass cap is 
going to bother the radio much at all.

By no means am I suggesting we throw away good parts. Life is to short 
to spend it bulk swapping one good part for another. I simply have not 
seen enough of the paper caps show up good yet to make it worth saving 
any of them. If four out of five are bad who knows how long the fifth 
one will hold out .... I agree that in one case out of 4 that makes for 
a good part / good part swap but that's the choice I make.

	Take Care!

		Bob Camp
		KB8TQ

On Jan 10, 2005, at 9:19 PM, Dallas Lankford wrote:

> I have never seen a black capacitor in an R-390A.  You must be 
> confused.
> The bad black capacitors were in the SP-600's (except for some of the 
> very
> last production, which had disc ceramics).  If any of those black 
> beauties
> were in an R-390A, you wouldn't have to test them to figure out they 
> were
> bad.  You could see the cracks.  I seem to recall that there were other
> black capacitors from the 50's that were O.K., but don't quote me on 
> that.
>
> Have fun,
>
> Dallas
>
>
> ----- Original Message -----
> From: "Bob Camp" <ham at cq.nu>
> To: "Dallas Lankford" <dallas at bayou.com>
> Sent: Monday, January 10, 2005 8:13 PM
> Subject: Re: [R-390] Servicing Advice on Signal Generator URM-25D
>
>
>> Hi
>>
>> Thanks for the information, it's not what I expected to hear.
>>
>> Last time the question came up there where no replies that indicated
>> any tested good black beauties in any of the R390's. Since I have 
>> never
>> seen any either that was what I was basing my recommendations on.
>>
>> Take Care
>>
>> Bob
>>
>>
>> On Jan 10, 2005, at 8:46 PM, Dallas Lankford wrote:
>>
>>> Every one I have ever checked from an R-390A was good.  Every black
>>> one I
>>> ever saw from an SP-600 was bad.  But if you want to replace R-390A
>>> capacitors anyway, I am sure you will.
>>>
>>> Dallas Lankford
>>>
>>>
>>> ----- Original Message -----
>>> From: "Bob Camp" <ham at cq.nu>
>>> To: "R-390 HF Receiver List" <r-390 at mailman.qth.net>
>>> Sent: Monday, January 10, 2005 7:43 PM
>>> Subject: Re: [R-390] Servicing Advice on Signal Generator URM-25D
>>>
>>>
>>>> Hi
>>>>
>>>> I asked this back a few years ago and maybe it's time to ask it 
>>>> again.
>>>>
>>>> Has anybody seen any of the black or brown body / epoxy coated paper
>>>> and foil capacitors made in the 1950's and 1960's show up good?
>>>>
>>>> As originally asked the question was specific to the R-390. Since 
>>>> then
>>>> I have seen a bunch of posts on a number of reflectors about these
>>>> parts. They all seem to indicate these parts are a problem.
>>>>
>>>> At least in my experience, testing pulls from a couple dozen radios
>>>> they show up leaky (as in 5X out of their original specification)
>>>> roughly 80% of the time. They show up at least 2X the specification
>>>>> 90% of the time. Certainly there are places where you will not 
>>>>> notice
>>>> leakage even if it is 5X the specification. Most of these seem to 
>>>> get
>>>> used in high impedance circuits where you will notice the effect
>>>> though.
>>>>
>>>> Take Care!
>>>>
>>>> Bob Camp
>>>> KB8TQ
>>>>
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>>
>>
>>
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