1. History & Specifications

The Heathkit SB-220 was introduced in 1969 as the companion amplifier to the SB-Line transceiver family (SB-101, SB-102, KWM-2/2A). It was sold as a build-it-yourself kit through Heath Company of Benton Harbor, Michigan, and remained in production through the mid-1970s.¹ It covers the amateur HF bands 80–10 metres (excluding 30, 17, and 12 m WARC bands) and is type-accepted for SSB, CW, and RTTY service.

A closely related model, the SB-221, was produced from approximately 1974 onward as a factory-wired version with minor circuit updates. Most restoration procedures in this guide apply equally to both models. References to factory service bulletins SB-220-1 through SB-220-8 are noted where relevant.²

2. Pre-Restoration Assessment

2.1 Initial Inspection Checklist

Before ordering parts or energising the amplifier, perform a cold visual inspection:

• Inspect the tube envelopes for cracks, dark deposits, or getter flash changes (milky/silver getter indicates a failed vacuum)
• Check the eight electrolytic filter capacitors for bulging ends, electrolyte leakage, or heat-deformed plastic holders
• Examine the plate transformer for signs of insulation breakdown, burn marks, or hot-spot discolouration
• Inspect the rectifier diode stack and associated wiring for arc damage, discoloured PCB laminate, or melted insulation
• Check the band switch contacts for corrosion, arcing pits, or loose contacts — a known failure point on units operated at high duty cycle
• Inspect the tuning capacitor (C55) for arcing damage to the plate coil bracket (SB-220-5 bulletin issue)
• Verify the interlock switch actuates correctly when the top cage cover is removed

2.2 Required Test Equipment

• High-voltage digital multimeter (Fluke 117 or equivalent) — minimum 4,000 V DC range
• RF wattmeter covering 1.5–30 MHz, 1,500 W range (or Bird 43 with appropriate slug)
• 50Ω dummy load, 1.5 kW minimum
• Variable low-voltage bench supply (for relay / fan testing)
• Digital capacitance meter capable of 1,000 µF and working voltage verification

3. Internal Circuits — Component Restoration

3.1 Power Supply Section

The SB-220 uses a full-wave voltage doubler configuration. The plate transformer delivers approximately 1,450 VAC RMS to the rectifier stack; two banks of four series-connected electrolytic capacitors (eight total) smooth the doubled DC to approximately 2,800–3,000 V at idle. Eight equaliser/bleeder resistors (30kΩ, 3 W each in the original design) sit across each capacitor to equalise the voltage distribution across the series string and provide a discharge path when power is removed.³

3.2 RF Deck — Tank Circuit

The SB-220 pi-network output tank circuit uses a switched roller inductor (L1) and a variable plate capacitor (C55, air variable). Input tuned circuits are provided for each band via a small PCB located under the main chassis.

3.3 Metering & Control Circuits

The SB-220 provides plate current, grid current, and plate voltage metering via a front-panel meter movement switched between functions. The plate voltage metering circuit uses three 4.7MΩ resistors (R6, R7, R8) as a voltage divider — these are rated 350 V working voltage but see nearly three times that in service. They are a near-universal failure point on aged examples.⁴

A replacement metering/rectifier board (Harbach RM-220) is available that incorporates correctly rated metering resistors and additional protective components as a drop-in upgrade.

3.4 Relay & Keying Circuit

The SB-220 uses a single 3PDT relay to switch the antenna between transceiver and amplifier and to switch the amplifier’s RF path. The original design was keyed via a 120 VDC circuit derived from the amplifier’s own supply — incompatible with all modern transceivers which provide low-level (0.5–12 V) PTT keying outputs.⁵

4. Safety: High Voltage, Bleeder Resistors & Interlocks

4.1 The Role of Bleeder Resistors

The eight equaliser resistors across the SB-220’s filter capacitor bank serve a dual function: voltage equalisation during operation and capacitor discharge (bleed-down) after power-off. When the AC mains are switched off, the stored charge in the capacitor bank bleeds through these resistors to chassis ground over a period of seconds to minutes depending on their resistance value.⁶

A key safety characteristic of large electrolytic capacitors is dielectric absorption: even after an apparently complete discharge, energy stored in the dielectric can re-appear at the terminals over a period of seconds to minutes. This is why a permanently connected bleeder is preferred over a manual discharge procedure alone — and why voltage should always be confirmed with a meter before touching internal components, not assumed from elapsed time.⁷

4.2 The SB-220 Interlock — A Specific Hazard

The SB-220 includes a shorting-type HV interlock: when the perforated top cover is removed, a micro-switch shorts the HV positive rail directly to chassis ground. This is intended as a safety measure, but it creates its own hazard specific to this design.⁸

When the interlock short is applied, the discharge current from the capacitor bank must flow through R3 (0.82Ω) — the only HV-negative path to chassis in the circuit. At even 100 V across the filter bank, the peak discharge current through R3 can reach approximately 100 A for a brief instant (I ≈ V/R ≈ 100/0.82). At full charge, the pulse is catastrophically destructive to R3 and potentially to the grid-current meter movement.

4.3 Safe Working Discharge Procedure

The following procedure should be followed on every occasion before internal access:

  STEP 1 ─── Switch amplifier to STANDBY, then switch AC POWER off
                │
  STEP 2 ─── Unplug the AC mains cord from the wall outlet
                │
  STEP 3 ─── Wait 5 minutes minimum (bleeder resistors discharge via ~100kΩ path)
                │
  STEP 4 ─── Set multimeter to 4000V DC range
             Connect meter (-) to chassis, probe (+) to HV positive terminal
             on filter capacitor bank. Verify reading < 50V before proceeding.
                │
  STEP 5 ─── (Belt and suspenders) Make discharge stick:
             10kΩ / 25W resistor on insulated probe handle, lead to chassis.
             Touch probe to each capacitor positive terminal for 10 seconds.
                │
  STEP 6 ─── Re-measure with meter. Confirm < 10V.
                │
  STEP 7 ─── Now safe to remove cover and work internally.
             ──────────────────────────────────────────────────
             RE-VERIFY before each work session, even if you
             only stepped away briefly. HV can return via
             dielectric absorption in large electrolytics.

Figure 1. SB-220 safe discharge procedure before internal access.

4.4 Mains Safety & AC Line Bypass Capacitors

The SB-220 original design includes small ceramic capacitors across the AC mains for RFI suppression. These are Class X or Y rated safety capacitors. If the original components have been replaced at any time with standard ceramics (not line-rated), they present a fire and shock hazard: a standard ceramic capacitor across the AC line can fail short, putting mains voltage on the chassis.⁹

Inspect all mains bypass capacitors. Any capacitor connected across AC line conductors or from line to chassis must be a certified Class X2 (line-to-line) or Class Y2 (line-to-earth) safety capacitor. Harbach Electronics stocks the correct line-rated ceramic bypass capacitors for the SB-220.

5. Cabinet Restoration

5.1 Paint — The Heathkit SB-Series Colour

Heathkit used a distinctive grey-green wrinkle or semi-gloss enamel on SB-Series cabinets. Frustratingly, Heath used at least five different shades of this colour across the production run, with no consistent assignment of shade to model or date — it was entirely possible to order an SB-101, SB-401, SB-200, SB-600, and SB-220 simultaneously and receive units with five different cabinet colours.¹⁰

For a single-unit restoration, the best approach is a colour match to the existing cabinet if the paint is in reasonable condition. For a complete repaint, the following sources provide SB-Series colour-matched paint:

• MyPerfectColor — myperfectcolor.com — acrylic enamel spray or brush, custom-matched from original Heathkit metal swatch.
• Sherwin-Williams formula (Glen Zook K9STH) — k9sth.com — computer-matched formula for the preferred light grey-green shade used on the SB-104 and many other SB-Line units; any paint store can mix from this formula.
• N5OLA Restoration Supplies — willeverett.net/n5ola.html — SB-Line matched acrylic enamel in spray can (11 oz) or touch-up bottle.

5.2 Cabinet Preparation

• Remove all knobs, hardware, feet, and panel overlays before stripping the cabinet.
• Degrease with acetone or MEK, then sand with 220-grit to provide tooth for the new coat.
• Apply a self-etching primer on bare steel before top coat.
• Apply two light coats of colour-matched acrylic enamel, allowing full cure between coats.
• Do not attempt to spot-touch aged cabinets — the aged paint will have faded and a perfect match is not achievable; full recoat is the correct approach.

5.3 Front Panel Overlay

The SB-220 front panel is silk-screened over a green anodised aluminium panel. If the original silk-screening is worn or damaged, replacement vinyl overlay kits are available from eBay seller KD0ILM in both the original green and alternative black colourway. These overlays adhere over the existing panel and faithfully reproduce the original artwork.

5.4 Hardware, Knobs & Feet

5.5 Internal Wiring & Cables

The SB-220 was wired at the factory with PVC-insulated wire. Aged PVC insulation becomes brittle and prone to cracking on flexure. During restoration, inspect all wiring especially in the high-temperature zones around the power transformer and tube plate connections.

• HV wiring: Replace any cracked or discoloured high-voltage wiring with 20 AWG or heavier HV-rated wire (rated ≥5 kV). Harbach Electronics stocks 20 AWG high-voltage cable in appropriate colours. Never use standard hookup wire in the HV circuit.
• Plate transformer leads: These carry full transformer primary and secondary voltages and heat-cycle with every use. Inspect carefully; replace with silicone or Teflon-insulated wire if any stiffness or cracking is present.
• Relay coax jumpers: Inspect the short RG-8 or RG-58 coax segments used in the T/R switching path for centre-conductor damage or shield migration at the connectors. Re-terminate or replace if in doubt.
• Ground bonding: Verify that all chassis ground connections and copper grounding straps are secure and oxide-free. Poor chassis grounds contribute to RFI and meter reading errors.

6. Recommended Modern Upgrades Summary

7. Parts Sources & Reference Documents

The following suppliers and resources are specifically useful for SB-220 restoration:

• Harbach Electronics (KB1SEL) — harbachelectronics.com — the primary dedicated SB-220/221 restoration parts supplier. Stocks the SK-220, SS-221, FB-220, RM-220, parasitic suppressors, glitch resistors, fuse holders, grid chokes, high-voltage cable, and the complete Master Kit ($599) covering most restoration items.
• RF Parts Company — rfparts.com — 3-500Z replacement tubes, cabinet feet, SO-239 connectors, Eimac and generic tube stock.
• CTR Engineering — ctrengineeringinc.com — U220FC universal filter/rectifier replacement board for SB-220/221 with 10Ω glitch resistor, clamp diode, and metering components.
• W6ZE / OCARC Heathkit of the Month #49 — w6ze.org — excellent overview article by R. Eckweiler with circuit description and operational notes.
• AG6K Circuit Improvements for the SB-220 (Richard Measures) — qsl.net/wa5bxo/ag6k/sb220ci.html — the definitive technical reference for SB-220 circuit deficiencies and corrections. Covers metering resistors, interlock hazard, relay bypass capacitor fix, and capacitor heating.
• Heathkit Factory Service Bulletins SB-220-1 through SB-220-8 — puck.nether.net (Heath reflector archive) — official Heathkit modifications and corrections including zener replacement (No. 2), fan motor (No. 4), capacitor arcing (No. 5), front panel fitment (No. 7).
• KV5V Notes on Rebuilding an SB-220 (Brad Rehm) — hotera.org — detailed rebuilding notes with schematics.