AN SB-220 COMPENDIUM
MODIFICATIONS & UPGRADES
INSTRUCTIONS
SCHEMATICS
PHOTOS
MATERIAL LISTS
SOURCES of SUPPLY
PERFORMANCE MEASUREMENTS
by
John White VA7JW
Original: 17 November 2011 ©
Rev 1: 13 June 2014
Rev 2: 3 Feb 2014
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AN SB-220 COMPENDIUM
INTRODUCTION
This Compendium is a collection of upgrades, modifications and performance measurements
applicable to the Heathkit SB-220 RF Power Amplifier. These may be of interest to owners and
users of this venerable and popular amplifier.
The authors SB-220 amplifier was purchased from the original owner in 1988. The amplifier was
complete, unaltered, fully functional and in very nice condition. Through the years 1992 to 1998
extensive work was undertaken to upgrade operational features, replace aging parts and tame
parasitic oscillation tendencies. All changes have been implemented by the author and have been
in service for over 10 years. No failures or performance issues have arisen.
Part 1 – UPGRADES and MODIFICATIONS
This part of the document provides a number of upgrades and modifications commonly published,
plus some by the author not seen elsewhere.
CREDITS
Allen Harbach WA4DRU of Harbach Electronics http://www.harbachelectronics.com/ offers
kits which come with complete installation instructions. The kits as listed are current whereas
the kits used in this amplifier were purchased over 10 years ago. There are likely some
differences and probable improvements but functionality is expected to be the same.
Richard Measures AG6K http://www.somis.org/ provides a wealth of information on most
aspects of RF power amplifier operations, modifications and upgrades. AG6K provides
information rather than kits in general although many parts are available on his web site.
A special mention of a very nice publication called “Heathkit Upgrades for the Linear Amplifier
Model SB-220” by Anton Suarez antonsuarez@free.fr came to my attention after this work was
essentially completed.
There are common modifications in both independent works, but there are also unique
modifications which expands the scope of the possible re-work that can be done. The manual
has been posted on eBay, for about $10 from time to time. Search under “HEATHKIT SB-220
Amplifier Upgrades”. The seller ID is “tito_de_taboada”
Part 2 – PERFORMANCE MEASUREMENTS
This part of the document provides performance data as measured on this amplifier. RF carrier
power output, PEP measurements, as well as spectral harmonic and two tone tests and a
unique noise loading test are presented.
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INDEX
Part 1 Upgrades and Modifications
Items 1.3 through 1.12 are “stand-alone” modifications; they can be done independently of each
other.
Items 1.13 and 1.14 describe an integrated QSK feature and require additional skills.
Section 1.1 Skills and Safety Notes
Section 1.2 A Quick Overview
Section 1.3 Parts Notes
Section 1.4 Parasitic Suppression t Harbach PS-220 kit or per AG6K kit
Section 1.5 Filter Capacitor Harbach FB-220 kit or per AG6K parts
Section 1.6 Rectifier – Metering Board t Harbach RM-220 kit or per AG6K parts
Section 1.7 Inrush Current Limiter Harbach SS-220 kit or per AG6 design
Section 1.8 AC EMI Line Filter VA7JW modifications
Section 1.9 Hour Meter Filament time VA7JW modifications
Section 1.10 Pilot Light VA7JW modifications
Section 1.11 Adjustable ALC - VA7JW modifications
Section 1.12 Standby – Operate & Fan switch VA7JW modifications
Section 1.13 120V Bias Supply Upgrade VA7JW modifications
Section 1.14 QSK Circuits AG6K design as implemented by VA7JW
Section 1.15 High Speed Vacuum Relays AG6K design as implemented by VA7JW
Appendix I QSK Keyer Schematic
Appendix II Current Source Detail
Appendix III Current Source / Keyer Construction Detail
Appendix IV Kit Listings
Appendix V Bill of Materials for QSK System
Part 2 Performance Measurements
Section 2.1 Instrumentation
Section 2.2 Carrier RF Power Output
Section 2.3 PEP RF Power Output
Section 2.4 Harmonic Distortion
Section 2.5 Two Tone Test
Section 2.6 Noise Loading Test
Section 2.7 SB-220 Specifications
Section 2.8 A Little History
Appendix VI Power Tutorial
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PART 1 - UPGRADES and MODIFICATIONS
1.1 SKILLS and SAFETY NOTES
The modifications listed herein will variously require removal of covers, side panels, the faceplate
and disassembly of mechanical parts. There will be reworking of electrical circuits, drilling of
holes for new mounting screws in the chassis, and re-dressing of wiring. Some modifications are
more complicated than others. In all instances, you should be experienced and comfortable with
bench work, have tools for drilling, cutting, filing, soldering etc, possess good workmanship skills,
and be able to read schematic diagrams. Note that the QSK section requires some scratch
building experience. Use a DVM to check wiring for shorts, opens, continuity, correct resistances,
and voltages under powered up conditions. Do NOT try to directly measure the 3.5 kV High
Voltage with your DVM. The author does not provide step x step instructions for the mod’s and
rebuilds herein. Portions of the original Heathkit schematic are presented in black with the
modifications indicated in RED.
FIRST and ALWAYS - UNPLUG the 120/240 VAC LINE
Then remove other cables
LAST - Remove the Green Wire Safety Ground from the Chassis Ground bolt.
The amplifier is heavy. Removal of case can be simplified by cutting a 5” long block off a wooden
2 x 6, place and holding it on the rear panel at the back of the power transformer, tip the amp up
on end, on the block, until it is upright. The amp will balance on the block because of the heavy
power transformer and the connectors on the back will be clear of the bench surface.
Remove the four feet, but before removing the last foot, prevent the cabinet from sliding down off
the chassis on to the bench – it may be difficult to lift the cabinet back up and off as the cabinet
tends to snag on the chassis. Place a 2” x 2” wooden block on the bench under the edge of the
case. Remove the last foot and lift the cabinet straight up off the chassis. The chassis can then
be tilted back down to the horizontal for work. Reverse the process for re-installing.
Before proceeding with any modifications, be advised to remove the 3-500Z’s from their sockets
using a clean cloth to keep finger oils and other contaminates from being deposited on the glass
bulb. Then wrap each tube in a protective cloth (towel) and put somewhere out of harms way.
The glass bulbs need to be squeaky clean on reinstallation.
Do not leave clippings, drill debris or any other conductive material loose inside the amp. Take
care that there is no possibility of short circuits developing between components or circuit boards
to chassis or to each other. Ensure that the integrity of wiring insulation and coaxial cable
dielectrics are maintained.
There is a High Voltage (HV) safety shorting “switch” in the tube compartment. Do not power up
amp with RF compartment screen off as the HV will be shorted to ground. If you defeat this switch
when servicing, understand the peril to which you expose yourself should you contact the HV.
LEATHAL AC and DC VOLTAGES EXIST
USE EXTREME CARE
Disclaimer
Author only provides best of knowledge information only.
Author makes no warranties or guarantees regarding these modifications.
Author is not liable for costs, loss, damage or injuries due to undertaking such work.
This work should only be done by those who are competent to do so.
High voltages exist and troubleshooting is dangerous.
Check your work before powering up.
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1.2 A QUICK OVERVIEW
Figure 1 gives a quick insight as to the nature and extent of the under-chassis changes described
in this document.
Figure 1.1
UNDER CHASSIS VIEW of UPGRADES and MODIFICATIONS
1.3 PARTS NOTES
A list of materials is provided mostly using Digikey http://ca.digikey.com/ part numbers. All parts
are stock items at the time of writing. Digikey is recommended due to the extensive and diverse
stock available and quick, typically overnight shipping by courier.
If parts are not available from Digikey, alternate sources of supply are provided.
Parts used were selected over 10 years ago. Many are still the same with some being slightly
different. Form, Fit and Function for current parts are specified to the same requirements.
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Part
Digikey p/n
Approx Cost
5.1V Zener 1N3996A, (obsolete)
10 watt, 5%, DO-4 case, cathode to stud
None listed
-
NTE-5177AK *
$14+
0.82 ohm / 2 W resistor
P0.82W-2BK-ND
0.39
Rectifier Diode, 1N5408G (3A /1000V), qty 8
1N5408GOS-ND
0.60 each
Plate L and Discreet R’s
Output L
Nichrome R
1.4 THE PARASITIC SUPPRESSOR KIT
It is a common belief that the SB-220 is vulnerable to uncontrolled high power VHF parasitic
oscillations that may occur when the amp is keyed into transmit. Instantaneous destruction of
certain components in the cathode circuits follows. The author has experienced this failure mode
and application of available VHF Parasitic kits has remedied this problem. Whether it is an
oscillation or not is not altogether clear and there is differing opinion as to the actual cause.
AG6K supports the VHF position whereas W8JI has differing thoughts on the subject,
http://www.w8ji.com/Amplifiers.htm . Many other interesting topics are discussed.
The cause will not be debated here but the effect of the failure is not subtle – a loud bang,
possibly some smoke, and an amplifier that may have too high an idle current or none at all. The
components prone to failure are ZD-1, a 5.1V stud mount zener and R3, 0.82 ohm / 2 W resistor
on the rectifier board. They usually blow open circuit but may also short. The stud mount zener is
expensive and is most often replaced with a series string of rectifier diodes (Section 1.6 and 1.14,
pg 19).
* NTE supplies replacement parts to distributors for obsolete components. Newark Canada,
http://canada.newark.com/ can supply the NTE part under their own part number 29C4650.
The Harbach PS-220 kit is shown below. The plate circuit discrete resistor / coil suppressors are
shown going to each plate cap from the plate choke and to the TUNE cap is another L and R
suppressor although the R in this case it is a Nichrome (resistance) wire.
Figure 1.2
PS-220 Parasitic Suppressor Kit – Plate Circuits
Additional suppression components are installed on the tube sockets under the chassis as shown
in Figure 1.3. These consist of various R’s and C’s in the grid and cathode circuits. Note that
W8JI and others recommend DIRECT grounding of the grid to chassis using short, wide, multiple
copper straps to chassis rather than installing R’s and C’s.
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0.01uF / 10 ohm 2 places
27 ohm / 820 pF two places
820 pF
820 pF
Figure 1.3
PS-220 Parasitic Suppression – Grid- Cathode Circuits
Both kits from Harbach and AG6K have been installed in other SB-220 amplifiers. They are
somewhat different from each other but both are supplied with excellent instructions and both
have worked reliably.
1.5 HV FILTER CAPACITOR KIT
The SB-220 amplifiers were sold between 1970 and 1978. The follow-on SB-221 was sold
between 1978 and 1983. If you are still using the original filter caps it is time for a change before
they fail. These caps have a “ripple” current flowing through them as a result of rectification which
results in internal heating due to their ESR (Equivalent Series Resistance). External heating is
also prevalent in this closed space. Heat is very detrimental to the longevity of components in
general and particularly electrolytic caps. They slowly loose capacitance with increasing ESR and
may eventually rupture due to heating, possibly explosively. Expected life time of caps of this
vintage is in the order of ~ 25 years.
Harbach supplies a complete kit of properly sized capacitors and voltage equalizing - bleeder
resistors. AG6K also offers replacement capacitors and resistors as individual components. Be
certain the capacitors ordered for the SB-220 amp will fit the Heathkit capacitor “block”.
This mod is requires delving into the power supply – band switch compartment. Good idea to do
both capacitors and rectifiers, next section, at the same time if never replaced.
1.6 RECTIFIER – METERING BOARD KIT
Harbach offers the RM-220 kit, an improved PWB metering board with protective diodes for the
meters, modern low leakage rectifier diodes, new HV resistors and a diode bias string to replace
the 5.1V Zener diode. AG6K also offers components.
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Figure 1.4
RM-220 Rectifier and Metering Board Upgrade
AN SB-220 COMPENDIUM
1.7 INRUSH CURRENT LIMITER
The SS-220 kit offered by Harbach limits the inrush current to the filaments of the 3-500Z’s. The
tubes are rated ~ 15 amps at 5.0 volts each (R = 0.33 ohms) when hot. Total current is twice that
as there are two tubes. The cold resistance is lower than the hot resistance. On power-up, there
is some concern that a high inrush current may shorten the life of the tubes or damage them.
AG6K reports that the Heathkit filament transformer T2 has an inherent current liming capability
which limits inrush current and so the usefulness of this modification is debatable. Regardless,
the limiter was installed to reduce the stress on components throughout the power system
circuits. AG6K also suggests a circuit for reducing inrush current.
Figure 1.5
SS-220 Inrush Current Limiter
The SS-220 kit is mounted close by the input 120 / 240 VAC terminal board. The board is shown
strapped for 240 VAC operation with terminal 2 and 3 connected. The two red insulated leads
are across the 120 VAC portion of the circuit go to the Hour Meter module, Section 1.9.
Ensure the PWB is well insulated from chassis; use tape or insulating paper. Mounting hole
drilling of the chassis required to mount the PWB.
1.8 AC EMI LINE FILTER
Overview
There is no effective RF AC line filtering on the SB-220. RF fields and currents inside the chassis
can couple to the AC wiring and can be conducted out on to the AC power circuits where the RF
can radiate or be conducted elsewhere to cause RF problems within the shack or the household.
Figure 1.6
Curtis RF Line Filter
It would seem prudent to install readily available RF filters in the amplifier to ensure that the AC
power systems are eliminated as a source of RFI. A Curtis brand filter rated at 20A / 220 VAC,
model number F1700AA20 was used at the time.
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Part
Digikey p/n
Approx Cost
Curtis Filter F1700AA
364-1086-ND
$42
Corcom Filter 20VSB1
CCM9069-ND
$24
Part
Digikey p/n
Approx Cost
Panasonic ERZV14D431
P7267-ND
Qty 3 @ $1.04 each
As an additional note, all other leads external to the amplifier ought to be of high quality shielded
wire; flexible coax such as RG-58 works well. Bypassing them at the amp apron is a good idea.
Refer to section 1.9 following.
Common Mode attenuation is > 30 dB at 1 MHz rising to 40 dB at 30 MHz, and somewhat higher
in between these end points. Differential Mode attenuation is 60 dB falling to 55 dB respectively.
A better choice would be a (Tyco) TE Connectivity Corcom filter, p/n 20VSB1. Attenuation
performance is better than Curtis and about half the price.
Also installed are Metal Oxide Varistors (MOV’s). These devices are surge rated, transient
voltage protection devices. They will clamp voltage transients to relatively safe values should
they appear on the AC line from external sources. The 240 VAC household branch circuit is not
protected / filtered at the wall socket as are the 120 VAC circuits in this shack. The intent is to
clamp high voltage AC spikes from over-voltaging components within the amplifier. The
packaging is in the form of a ceramic disk and they look like a disc ceramic capacitor, just thicker.
The original MOV’s used were type ZNR14K391 where ZNR is an industry recognized prefix for
MOV’s. The part number deciphers as 14 = diameter of the MOV in mm, and 391 = clamping
voltage. Closest equivalent today is the Panasonic ERZV14D431 which is of the same diameter
and the clamp voltage is 431volts rather than 390 volts.
Schematic
Figure 1.7
Complete Circuit Diagram for MOV, RF Line Filter and Current Limiter
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Part
Part number
Approx Cost
Curtis 701FR001048150D100230A
DigiKey 267-1004-ND.
$32
Ferrite Beads, Amidon Assoc. FB-73-801
RPE 73-801
Qty 8 for $10
Installation Photo of Inrush Current Limiter & EMI Filter
The Inrush Current Limiter is on the left and the EMI filter is on the right. Further to the right are
the original circuit breakers. The MOV’s are on the back of the filter, one just visible as a black
object. The three MOV’s are mounted on the filter terminals.
Figure 1.8
Inrush Current Limiter & RF Line Filter Installation
Original AC Circuit breakers on right hand side
1.9 HOUR METER
Overview
This device records the hours accumulated on the tube filaments, that is, cumulative time when
the amp is powered ON. It is not RF hours which are much less.
Figure 1.9
Curtis LCD Hour Meter
The Hour meter is a Curtis Instruments LCD Hour Meter, Model 701, 3 wire. Hexagonal case
code “F”. Nominal voltage operation range from 100 to 230 VAC.
Order Ferrite beads direct from Amidon Associates. https://www.amidoncorp.com/
Product is also available from RP Electronics in Burnaby. http://www.rpelectronics.com/
Beads plus 0.01 uF disk caps to ground make for a reasonable RF filter. The link in Section 1.13
regarding capacitors is a great place to shop for many different capacitor types (mica, disk, poly,
electrolytics etc) and values. All new, reasonably priced and some hard to find anywhere else.
The beads were not used to remedy malfunctions due to RF, but were installed as a cautionary
measure.
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Schematic
AN SB-220 COMPENDIUM
Figure 1.10
Hour Meter Wiring and Bypassing
The wiring is run down through the chassis fan cutout and along underneath the chassis to the
input voltage strapping terminal board. Bundle wires with existing filament and power wires for
neatness.
Installation Photo
The meter is mounted on rear of amplifier. The screen has to be cut away to fit the body through.
Figure 1.11
Hour Meter Mounted on Rear Panel
1.10 ON / OFF PILOT LIGHT
A green LED lamp is installed on the lower, left hand side of the front panel. The lamp is ON
when AC power is ON. It is not a “Transmit On” lamp.
Lamp voltage is taken from the filament AC supply, same as the meter pilot lights. The LED is
series connected with a 200 ohm resistor with a 1N4005 diode across the LED to ensure that the
LED does not breakdown on AC reverse polarity. The resistor limits the LED current. This circuit
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Part
Digikey p/n
Approx Cost
Dial Light Green LED / Black Body
350-1908-ND
$5.27
is connected to the “cold” (RF blocked) side of the filament choke. Junk box parts were used to
make this LED lamp but if you buy a lamp assembly, a series resistor is usually incorporated in to
that assembly. The product listed below operates from 6 VDC which is fine even though the
filament supply is approximately 5 VAC. The lower voltage may result in a slightly dimmer LED.
The AC reverse diode should still be installed across the LED assembly to protect it. Ensure that
the pilot light case is long enough to mount through the thickness of the front panel + the chassis,
about 5 mm is required,
Schematic
Figure 1.12
LED Pilot Light
DigiKey has a line of Dial Light product under part numbers 350-19XX-ND that appear to have a
sufficiently long threaded body (~ 12mm) to pass through the thickness of the faceplate + chassis
(~ 5 mm). Look at others in that series for style and color to suit.
Installation Photo
Pilot light is mounted on center line of power ON-OFF switch and CW/TUNE SSB switch, and half
way between TUNE and LOAD controls
Figure 1.13
Pilot Light Placement
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