Heathkit SB-220

Modifications

Click on picture for a larger image
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

r1_old.JPG (7628 bytes)

After

r1_new.JPG (10597 bytes)

Meter damage can be avoided by placing a pair of diodes across the terminals on each meter. 

meter_damage.JPG (27213 bytes)

Ever wonder why you can't get the cabinet back on the chassis? The wide part goes to the front of the amp

cabinet.JPG (13203 bytes)

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.

more_air.JPG (50866 bytes)

Zener diode replacement circuit

zener.JPG (52439 bytes)

Switching relay

 Modified to prevent filament transformer melt down in the event of a filament to grid short

Before

antrelay_b4.JPG (59741 bytes)

After

antrelay_after.JPG (50102 bytes)

New parasitic suppressors

parasitic_suppres.JPG (53512 bytes)

parasitic_suppres_1.JPG (43089 bytes)

To avoid transceiver-relay contact pitting and eventual failure, add a 100 - 200 ohm, -W resistor.

antenna_relay.JPG (27430 bytes)

Nicely modified chassis

bottom_mods.JPG (70113 bytes)

Using the grounded grid approach

sb220_grd_grid.jpg (50893 bytes)

Grounded grid close-up

sb220_grd_grid1.jpg (35546 bytes)

 
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.

sb220_soft_key_mod.jpg (68764 bytes)

Last modified February 08, 2004 by Paul B. Peters, Show contact information
Copyright 2000 -2003 Paul B. Peters, VE7AVV. All rights reserved.