>
>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.
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