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[AMPS] advice on and old SB-220

To: <amps@contesting.com>
Subject: [AMPS] advice on and old SB-220
From: 2@vc.net (measures)
Date: Fri, 10 Nov 2000 12:30:45 -0800
>
>Hello all,
>
>I acquired an old SB-220 that has not been energized in at least 6 years and
>not really used for more than 10. Cosmetically it is almost perfect.
>Electrically it is supposed to be perfect, "the last time he used it".  My
>questions to the group are:
>
>1.  What would be the best process to bring this unit back into daily
>service?    

Inspect the filter cap vents. If you find any white deposits, the entire 
set of 8 should be replaced.  

> I have a 10Amp 120V variac. The amp is wired to 240V, but has a
>120V cordset on it.  Should I energize it at 120V with it wired for 240V to
>ease it into service?
>
Starting on the CW position is enough. 

>2.  Should I automatically, before energizing it, replace all of the
>Electrolytic caps? I would rather not spend the extra money.
>
>3.  Should I increase the capacitance values in the filter caps, if
>available, 

Sure.  Heath cut  corners.  

> and if you guys think the caps need be replaced?
>
>4.  The tubes test good, but could they be gassy from sitting all of these
>years? I am not very familiar with 3-500Z's, will letting the filaments
>heat, and just sit there for a few hours, cooking, de-gas the tubes if they
>are gassy?

In my experiences, gassy tubes do not miraculously de-gas.  
>
>5. Any other suggestions would be greatly appreciated on or off the list!

>From my reading of the archives of this list I surmise the following :
>The step start circuit for the filaments is overkill.

yes.  It uses a special current limiting transformer.  However, the hv 
transformer draws so much current on startup that it will eventually 
destroy the on/off switch unless a stepstart circuit is added.  /see 
diagram on Web site/  

>The meter circuits should have a few resisters changed instead on Carbon
>comp.because of value change over the years.
>The bleeder resisters should be changed to 100KOhm 3W Film resisters, to
>lower the heat dissipation.
>There may be a Parasitic oscillation problem. (although I noticed that the
>original builder failed to solder one of the suppressors in, it just has a
>mechanical connection!)
>If desired a QSK mod is available.
>Don't use the ALC circuit.
>
RR

cheers, Eric

-  Rich..., 805.386.3734, www.vcnet.com/measures.  
end


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