Since its introduction in 1975, the venerable
Heathkit SB-220 has been continually upgraded and modified by its
owners. A search of the QST archives will reveal numerous excellent
technical articles specific to these modifications. For more
information, please refer to the following QST articles: Nov.
1990, p27; Dec 1990, p41; Feb 1979, p20; Apr 1979, p27; July 1979, p51;
Nov 1979, p57. In addition, CQ magazine Sept 1992, p 46.
The following images
show a few of the suggestions referenced in the above noted articles --
some in both a before and after state. |
Upgrade of the R3 (0.82 ohm,
2W) resistor on the diode board |
Before
 |
After
 |
Meter damage can be avoided by placing a pair of
diodes across the terminals on each meter.
 |
Ever wonder why you can't get the cabinet back on the
chassis? The wide part goes to the front of the amp
 |
Move more air over the tubes with an alternate style
of fan for more efficient cooling. Note the black paint on the cabinet
walls surrounding the tube bay -- this reduces the infrared radiation
heat being reflected back into the tubes from the bright aluminum
cabinet.
 |
Zener diode replacement circuit
 |
Switching relay Modified to
prevent filament transformer melt down in the event of a filament to
grid short |
Before
 |
After
 |
New parasitic suppressors |
 |
 |
To avoid transceiver-relay contact pitting and
eventual failure, add a 100 - 200 ohm, ½-W resistor.
 |
Nicely modified chassis
 |
Using the grounded grid approach
 |
Grounded grid close-up
 |
On the right is a home-brew version of the .
'Soft-Key' circuit.
This simple addition to the SB-220 allows it to be keyed by most
modern transceivers. |
 |