>Howdy,
>I currently own two SB220s, one of which I bought in 1982. When I
>purchased it, it had a standy switch, step start in the power supply,
>filament step start, and an open .82 ohm resistor.
IMO, a kaput R1 [0.82 ohms] can be caused by:
1. Broken spot welds between the 3-500z anode cooler and the anode stem.
The cooler breaks loose on one side, tips sideways, and shorts against
the grounded grid--i.e., B+ shorts to chassis ground. The sole return
path to HV neg. is through R1, so it blows out.
2. A leaky glass to metal seal. Air slowly enters the tube.
Eventually, an anode to grid gas arc may occur. R1 blows. Gas easily be
detected with the lights off by a blue glow in the envelope. Gas can
also be detected with a high pot after R1 is blown--despite the
technoblatherings of naysayers who say gas can spontaneously vanish
whilst the 3-500z is NOT being operated. ......... If no gas is present
and R1 is blown, see #3.
3. An internittent, push-push, VHF parasitic oscillation. During such
an event, high grid currents typically flow, sometimes melting the copper
wire in the SB-220's 1A grid/ground RFCs.
> My repairs
>consisted of removing all the step start garbage, replacing the
>resistor , removing the 200 pf caps and rf chokes from the grids and
>directly grounding them, and getting rid of the standby switch ( I
>even filled the hole in the front panal and touched up the paint).
>
>Four or five years later the fan got noisy ...
A certainty if the bearings are not oiled.
>...snip...
>
>Now, after 15 years of operation it is on my workbench with the
>covers off so I can repair the on/off switch...it has been turned on
>and off so many times the switch finally gave up. ...
However, the 'step-start garbage' reduced the start-up current burden on
the switch that turned to garbage. .
>...I plan to use the
>cw/tune - ssb switch as a replacement...
If you would rather not, Bob, I have around a half-dozen higher-current
(43a-peak) rated rocker switches that would allow you to switch on the
amplifier reliably without resorting to "step-start gargage".
>
>Since the covers are off, I cant help but see the diode board. All
>those diodes
>in a string bother me for some reason, and I do have some diode modules
>from
>"Siicon Alley".
If something is working okay, why fix it?
Would you be interested in selling one of your diode modules for
$20--provided it is at least 3 months old? Deduct $2 if you want one of
the high peak-current rated rocker switches.
-Rich-
R. L. Measures, 805-386-3734, AG6K
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