[ARC5] Powering an ARC-5 Receiver After Long Storage

[Geoff]

----- Original Message ----- 
From: "J. Forster" <jfor at quikus.com>
To: <kgordon2006 at frontier.com>
Cc: "ARC5" <arc5 at mailman.qth.net>
Sent: Wednesday, May 21, 2014 10:56 AM
Subject: Re: [ARC5] Powering an ARC-5 Receiver After Long Storage


>
>
>
>> If whomever is doing this hopes to "reform" the bad capacitors, I am 
>> sorry
>> to have to tell you, that will not work.
>>
>> First of all, "reforming" only works with electrolytic capacitors. The 
>> bad
>> capacitors in our favorite receivers are, except for 2 or 3, NOT
>> electrolytics.
>
> Yes, and yes. I know that.
>
>> Secondly, the reason they fail is that they are filled with beeswax, 
>> which
>> is very slightly corrosive, and the cans are brass.
>
> The point of asking about the nominal current is precisely to detect such
> leakage.
>
>> After 70 years, the beeswax reacts with the brass to form verdigris, that
>> green crap, which, essentially, shorts the capacitors out.
>>
>> The only solutions are replacement or restuffing.
>>
>> In my experience, the few electrolytic caps in the receivers are not
>> always bad, and when they are, nothing much bad happens.
>
> If a bad electrolytic shorts out a cathode resistor, the tube bias is
> altered, possibly blowing an output transformer. This happens in other
> sets.
>
> -John

**  I dont recall ever seeing a completely shorted LV, low value cathode 
bypass. Very leaky yes but still way too high a DC resistance to affect the 
typical low value resistor in parallel.

The output transformers fail due to a leaky coupling cap which creates 
positive bias causing excessive dissipation. Very often that tube will then 
be gassy and should be replaced even if the transformer survived.
Another failure point is those radios that have the tone control from plate 
to ground and its coupling cap is leaky enough to cause excess current thru 
the transformer.

Carl



> ==================
>>
>> Ken W7EKB
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