SB220

[Lee Bahr]

It's been a long time since I worked on a SB220 but I remember having the
same problem with one many years ago.  THE METER SHUNT RESISTOR HAD DRIFTED
IN VALUE.  The correct voltage was on the rig but the meter did not show it.
It reflected a lower voltage.  Be careful as the voltages are leathel.
Measure the voltage across each filter cap and add them together.  If the
total is in the ballpark, the meter shunt has changed value.
As for the diode stack, I'd suspect they are OK.  Usually the diodes work or
don't work, nothing in between.  Todays diodes are much better then older
ones and are more balanced (uniform).  You might change them out just to
improve the balance across each diode.  (Bye the way, when you check the
voltage across each filter cap take note of voltage differences between each
of the filter caps.  They should be very close to being the same voltage.
If they are not, the filter caps are suspect.  If this is the case, change
out all the filter caps, not just one of them and maybe put in new silicon
rectifier diodes while you are at it.
But, my first guess is your first and maybe only problem is the meter shunt.
However, while you are in there check the voltage balance between caps.
With the age of the amp, you could have a secondary problem developing.  BE
CAREFUL.  The voltage is LEATHEL!
Lee, w0vt



> I have an SB220 in near mint condition.  My problem is that when testing
> according to the manual, the high voltage readings are a bit low, perhaps
400
> volts low.  I think it may be the silicon diode stack in the high voltage
power
> supply, but the manual and schematic only refers to them as silicon diodes
with
> a Heath in-house part number of 57-27.  Does anyone know a good modern
> substitute for these and a source for them?  Any assistance would be
appreciated.
> Alex Kaplan, WA4AK
> lawalexkaplan at aol.com
>
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