[R-390] Blowing B+ fuse

[Cecil Acuff]

Never mind my drivel.....I should have read on down the list.  You guys have 
got it covered as usual....

Sorry for the extra bytes...

Cecil....
----- Original Message ----- 
From: "Cecil Acuff" <chacuff at cableone.net>
To: "Barry" <n4buq at aol.com>; <r-390 at mailman.qth.net>; "Tim Shoppa" 
<shoppa_r390a at trailing-edge.com>
Sent: Tuesday, July 05, 2005 1:12 PM
Subject: Re: [R-390] Blowing B+ fuse


> Slo-Blo's are usually used when there is an inductive load and/or to 
> survive the much higher in-rush current that can cause nuisance fuse 
> blowing with standard fast acting fuses.  Has the power supply been solid 
> stated or are the tubes still present.  If solid stated it may still be 
> in-rush current along with filter leakage current that is opening it up. I 
> would think the turn on current surge would be a good bit greater than 125 
> ma. in the B+ if solid stated.  Much more gradual with the tubes....
>
> Another thing...how did the fuse blow?  Was it just opened up at one end 
> of the element or was the inside of the glass plated with discolored 
> metals indicating a sudden spike in current draw.?  (should also blow the 
> 1/4 A if that's happening though.)
>
> Just some thoughts...
>
> Cecil...
> ----- Original Message ----- 
> From: "Barry" <n4buq at aol.com>
> To: <r-390 at mailman.qth.net>; "Tim Shoppa" <shoppa_r390a at trailing-edge.com>
> Sent: Tuesday, July 05, 2005 8:13 AM
> Subject: Re: [R-390] Blowing B+ fuse
>
>
>>> > I finally got a couple of 1/8A B+ fuses (SloBlo)
>>>
>>> I think that the B+ fuses are supposed to be non-sloblo (fastblow).
>>> The idea is that a short/overload on the B+ should break the circuit
>>> immediately.
>>>
>>
>> I figured they were all SloBlo, but maybe not.  Any other comments here?
>>
>>> Incidentally both my R-390A's came to me with 2 or 3A fuses in the B+
>>> positions.  May as well have been a penny...
>>>
>>
>> The reason I've been running with the 1/4A is: 1) It's what I had on 
>> hand,
>> and 2) The older radios didn't have this fuse at all.  I figured if
>> something catastrophic occurred, the 1/4A *might* protect something, but 
>> a
>> "penny" would have been the same as the older radios so I wasn't too
>> worried.
>>
>>> > [Blowing fuse = bad electrolytic filters]
>>>
>>> Probably.  Do the electrolytic cans get noticeably hot (hotter than
>>> the chassis around them at least?)
>>>
>>
>> If I recall, the caps do get a bit warm (at least one of them).  I'll do 
>> a
>> current check with the cans and the new caps and see what difference it
>> makes.
>>
>>> It's entirely possible for the electrolytics to still have enough
>>> capacitance to effectively filter but have so much leakage that
>>> they blow the fuse.
>>>
>>> Also check for burnt resistors on the decks... indication that
>>> a tube is drawing way too much current.
>>>
>>
>> I didn't notice any obviously burned resistors when I recapped the decks. 
>> I
>> could have some high values, but none looked obvious.
>>
>>> The army manual has a good procedure for finding B+ shorts with an
>>> ohmmeter but it doesn't always show up too-much-current-when-real-power-
>>> applied faults.
>>>
>>> Tim.
>>
>> I need to do a resistance/voltage check on all tube pins before I call 
>> this
>> one complete.  Hopefully, I can get things going with the 1/8A fuse with 
>> the
>> electrolytic changes, but if not, I have other issues.
>>
>> Thanks,
>>
>> Barry - N4BUQ
>>
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>
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