AN SB-220 COMPENDIUM

AN SB-220 COMPENDIUM

MODIFICATIONS & UPGRADES INSTRUCTIONS SCHEMATICS PHOTOS MATERIAL LISTS

SOURCES of SUPPLY PERFORMANCE MEASUREMENTS

3 Feb 2014

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 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 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 Section 1.5 Filter Capacitor Section 1.6 Rectifier ? Metering Board t Section 1.7 Inrush Current Limiter Section 1.8 AC EMI Line Filter Section 1.9 Hour Meter Filament time Section 1.10 Pilot Light Section 1.11 Adjustable ALC Section 1.12 Standby ? Operate & Fan switch Section 1.13 120V Bias Supply Upgrade

Harbach PS-220 kit or per AG6K kit Harbach FB-220 kit or per AG6K parts Harbach RM-220 kit or per AG6K parts Harbach SS-220 kit or per AG6 design VA7JW modifications VA7JW modifications VA7JW modifications VA7JW modifications VA7JW modifications VA7JW modifications

Section 1.14 QSK Circuits Section 1.15 High Speed Vacuum Relays

AG6K design as implemented by VA7JW 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 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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