[R-390] Blowing B+ fuse

[Cecil Acuff]

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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