SB-220/SB/221 Amplifiers

[John C Owens]

I am in the process of repairing my SB220 and SB221 Amplifiers. One has
the R. Measures parasitic RF Chokes in the 3-500 Plate leads. I wanted to
check the current resistance values of the parallel carbon film
resistors, and the color rings are discolored to the point that they are
eligible. Does anyone know what the values are supposed to be?? These
probably came from Harbach.

I also have a question about the Zener Diode ZD1 which shows up on the
schematics on both units, has a location on the component board, but is
not installed on either amplifier. The assembly manual also shows this
diode location in the pictorials, but says it is not installed. Why is
this?? On both amplifiers, I have found bad metering resistors R6, R7,
R8, R9 and R3. One amplifier had 1 watt carbon film resistors, and one of
these was open. The other amplfier has had these replaced with 2 watt
carbon resistors, and the values of all of these had changed. One
amplifier has the back-back diodes across the Plate Current Meter, and
the other is lacking this mod. I will do it to the one lacking same, and
find that Radio Shack no longer sells the 1N4002 diodes I see recommended
in one article. They do sell the 1N4001 and 1N4003 which have different
PIV ratings. What should I use??
 Any other tips on what mods to do to this metering circuit since it
seems problematical?? Should this mod be done to both meters, or is it
just recommended for the Plate Current Meter?? Is a soft start circuit
advisable and if so, do you have a circuit diagram of one I can build??

I have reprints of the following articles re SB220/221 amplifier mods:

March 1988 73 Magazine article by VE5XZ whose modifications I have been
advised not to do.
Hints and Kinks article from August 1986.
Circuit Improvements for the Heath SB220 (Parts 1 and 2) by Rich
Measures, Nov/Dec 1990.
Upgrading your SB220, Feb 1979 QST by Ken Gleazer, W1KAY.
Tweaking you Linear the Right Way (source unknown) by AJ Masa, W5VSR.

Referral to any other good articles would be appreciated.

Any opinions on how to mod and care for these amplifiers would be
appreciated. They are terrific gear and should last an eternity if cared
for. Your help would be appreciated.

Thanks - John Owens (N7SEJ)

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