Alpha 87A Linear Amplifier:
Restoration & Service Guide

3CX800A7 Tubes • PIN Diode T/R Switching • Microprocessor Control • Fault Code Reference • Cabinet & Safety

📝 VK6ADA Technical Papers 📅 March 2026 ⚡ 1,500 W Continuous — 3CX800A7 💻 Microprocessor Controlled Autotune ⚠ High Voltage Equipment

Abstract. The Alpha 87A is a microprocessor-controlled, self-tuning HF linear amplifier produced by Alpha Radio Products (now Alpha RF Systems) from October 1991 through approximately 2003. It uses a pair of Eimac 3CX800A7 ceramic triodes in a grounded-grid configuration, operating from approximately 3,000–3,200 V DC, to deliver 1,500 W continuous output on all amateur HF bands from 1.8 to 29.7 MHz. The 87A is significantly more complex than a manually-tuned amplifier: it incorporates PIN diode T/R switching (in place of mechanical relays), motor-driven auto-tuning capacitors, a comprehensive microprocessor fault monitoring system, and an RS-232 serial interface. This guide covers restoration and service of the 87A including the power supply, PIN diode T/R system, tube deck, control boards, firmware, safety systems, and cabinet. The Alpha 87A fault code reference table is included as a diagnostic aid.

☠ Critical Safety Warning — Read Before Proceeding

The Alpha 87A operates at approximately 3,000–3,200 V DC on the plate supply. This voltage is present across the HV filter capacitors, plate transformer secondary, rectifier stack, plate choke, and all associated plate wiring. This voltage level is immediately lethal on contact. Alpha designed the 87A to meet international safety standards and incorporated a HV crowbar interlock that actively discharges the HV filter capacitors when the top cover is removed — but this must not be the only safeguard relied upon during service. See Section 5 for the complete safe-access procedure.

Disconnect AC mains completely before opening the amplifier for any service.
The HV crowbar interlock activates when the top cover is removed; however, always verify HV is absent with a meter before touching any internal component.
The 87A manual specifically states that the operator should additionally short the HV to chassis ground as a final precaution, using an insulated screwdriver at the plate or plate choke, before proceeding.
Defeating the interlocks for live testing must only be done by technically qualified personnel with full awareness of the voltage levels present. See Section 5.2.

1. History, Variants & Specifications

The Alpha 87A was the culmination of Alpha Radio Products’ development of self-tuning HF amplifiers. First production units shipped in October 1991; the final variant was the limited-edition 87A Omega, the last of which completed the production run in the early 2000s. The 87A Omega featured silver-plated coils and connectors in addition to the standard 87A specification.¹

Alpha Radio Products became Alpha Power, then RF Concepts, and is currently known as Alpha RF Systems, still actively supporting legacy 87A amplifiers with parts and service from their facility in Ohio. Owner-requested schematics are provided at no charge. The 87A community is also served by an active groups.io reflector with extensive shared repair experience.

A key difference between the 87A and earlier Alpha designs (76A, 78) and the contemporary Heathkit SB-220 is the replacement of mechanical T/R relays with PIN diode switching. This enables true full-break-in (QSK) CW operation with no relay noise and no relay wear. It is also the source of the most common service issues encountered on older units.²

⚠ Complexity Note: The Alpha 87A is substantially more complex than any manually-tuned amplifier. If this is your first experience working inside a high-power tube amplifier, the 87A is not the recommended starting point. N8RR (WVDXA) specifically advises: “If you don’t have any experience working on high power tube type amplifiers, the 87A is probably not the best candidate to start learning on.” That said, many common faults — Molex connector oxidation, PIN diode replacement — are well-documented and within the capability of a careful, technically-minded operator.

Output Power	1,500 W PEP, CW, or continuous carrier; no time limit
Frequency Coverage	1.8–29.7 MHz all amateur HF bands (10 m factory-locked; unlock available with licence copy)
Final Tubes	Two Eimac 3CX800A7 ceramic triodes, grounded grid
Drive Power Required	Typically 50–55 W for full rated output
Input VSWR	<1.5:1 within amateur bands; <2:1 elsewhere
Plate Voltage	~3,000–3,200 V DC (idle)
T/R System	PIN diode switching — no mechanical relays; handles hot-switching up to 150 W exciter power
Band Change & Tuning	Fully automatic microprocessor control; average band change <1 second
Cooling	Internal squirrel-cage blower, forced air; optional auxiliary fan for RTTY/contest duty
Interfaces	RS-232 serial port (25-pin); AlphaMax firmware upgradeable
AC Mains	120 VAC or 240 VAC (strapped at transformer); separate transformer carton
Weight	Amplifier: ~42 lb (19 kg); Transformer: ~43 lb (19.5 kg)
Production	October 1991 – ~2003; Omega edition was final variant
Current Support	Alpha RF Systems, Ohio — alpharfsystems.com

2. Pre-Service Assessment

2.1 Before Opening the Amplifier

The 87A’s comprehensive fault monitoring system is your first diagnostic tool. Before opening the amplifier, record every fault code displayed. Many faults are soft (the amplifier auto-recovers) or hard (requires power-off reset); fault code 99 indicates a locked state from six consecutive identical hard faults and requires factory assistance to clear. Refer to the fault code table in Section 6.

Power on and record the fault code from the segment LED display.
Note whether the fault is hard (AC power removed on fault) or soft (amp continues operating with fault logged).
Attempt power cycling; a Fault 24 (system voltage) on first power-up after long storage often clears spontaneously as the filter capacitors charge.
Check the RS-232 port: connecting a terminal emulator at 9,600 baud and using the FAULTS command gives a complete fault history.
Check the AlphaMax firmware version with the VER command; units built before 1999 may benefit from the firmware upgrade (see Section 3.4).

2.2 Required Test Equipment

High-voltage digital multimeter, minimum 4,000 V DC range
Standard digital multimeter for low-voltage measurements and diode testing
RS-232 serial cable and terminal emulator (HyperTerminal, PuTTY, etc.) at 9,600 baud for command-line diagnostics
RF wattmeter, 1.5–30 MHz, 2,000 W range (Bird 43 or equivalent)
50Ω dummy load rated 1,500 W continuous
Oscilloscope (useful for PIN diode bias voltage waveform analysis)
DeoxIT D5 contact cleaner and DeoxIT Gold — essential tools for Molex connector service

2.3 Disassembly Notes

The top cover of the 87A is held by approximately 30–34 screws depending on production date. An additional access cover below the tube compartment requires four more screws. Organised storage of hardware is essential — screws, nuts, flat washers, and lock washers are used in combination throughout. A divided parts tray or labelled bags for each sub-assembly are strongly recommended. This is a noted point from virtually all 87A service reports: the screw count makes reassembly an exercise in patience.³

3. Internal Circuits — Component Service

3.1 The Number One Issue: Molex Connector Oxidation

The single most common cause of Alpha 87A faults across all fault code types is oxidation of the Molex wiring harness connector pins. The amplifier’s various boards and modules are interconnected via a Molex plug-and-socket wiring harness. Over years of service, the tin-plated Molex contacts oxidise, increasing contact resistance. The microprocessor monitors many parameters to tight tolerances; even a small increase in connector resistance on a sensing line is enough to trigger fault codes.⁴

Fault 20 (high filament current) is the most documented example: Alpha confirmed over a ten-year period that virtually every Fault 20 was connector-related, not a tube failure. Faults 1, 8, 18, and others have also been traced to Molex connector issues before any component failures were found.

⚠ Before Any Component Replacement: Clean and reseat every Molex connector in the amplifier with DeoxIT D5. This single action resolves a majority of 87A faults and must be performed before concluding that any component has failed. Apply DeoxIT to the female socket pins, insert and withdraw the plug 5–10 times to work the cleaner into the contacts, then treat with DeoxIT Gold as a long-term inhibitor. Place paper towels behind each connector during spraying to prevent overspray reaching other components.

Wiring Harness — Molex Connector Service

Connector / Location	Service Notes & Fault Association
J3 (FILAMENT SENSE) Tube Deck filament sense connector Molex multi-pin; inside tube deck enclosure	The primary cause of Fault 18 (low filament current) and Fault 20 (high filament current). Oxidation on J3 adds series resistance to the filament sense line, directly raising the sensed voltage and triggering Fault 20. Clean with DeoxIT and re-seat. If fault recurs, consider permanently bypassing the filament sense connection by directly grounding the line in the tube deck chassis, or shimming the plug halves for increased contact pressure.
ALL HARNESS CONNECTORS All Molex plugs throughout amplifier Tin-plated contacts; age-related oxidation	Clean every connector during any service visit, not just the connector implicated in the current fault. Oxidation progresses uniformly across all connectors; cleaning only the implicated one typically results in a different connector causing the next fault within months. Alpha connectors are keyed to prevent incorrect re-insertion; note the plugged centre pins on specific connectors before disassembly. Never force a plug into the wrong receptacle.
J1, J2 (CPU BOARD) CPU board diagnostic connectors Used for parameter measurement during live testing	With the cover defeated for live testing (see Section 5.2), many monitored bias voltages can be measured at J1 and J2 on the CPU board. Exercise extreme caution not to short between pins when making measurements; a short at these points can damage the CPU board. Use insulated probes and probe one pin at a time only.

3.2 Power Supply Section

The 87A power supply uses a conventional rectifier and filter bank. The HV filter capacitors are located on a dedicated board accessible after cover removal. Fault 24 (system voltage fault) is specifically associated with the HV capacitor bank: if one capacitor in the series string has significantly lower capacitance or higher leakage than the others, its voltage will deviate, triggering the fault. Individual capacitor voltage measurement across the bank is the correct diagnostic procedure.

Power Supply — Commonly Replaced Components

Component & Original Value	Replacement / Notes
C-HV (bank, 7×) HV filter electrolytic capacitors Seven series-connected electrolytics on HV board; voltage >3,000 V total stack	Individual capacitor voltage measurement is the correct diagnostic for Fault 24. Measure across each capacitor in the bank with HV present (use correct meter range). An unequal distribution indicates a degraded unit. Replace the entire bank when one fails — do not replace individual capacitors in a mixed-age stack, as the new unit will be stressed by uneven voltage sharing. Modern 105°C rated high-capacitance electrolytics are the correct replacements.
R-BLEED (per cap) Equaliser / bleeder resistors across each HV cap High value, HV-rated resistors	Inspect for discolouration or open circuit. Verify each resistor is within 5% of its rated value. A failed-open bleeder leaves that capacitor with no discharge path after power-off — a lethal retained-charge hazard. Replace any suspect resistors with metal-film types rated for the full working voltage per position.
D-HV (rectifier stack) HV rectifier diode(s) on HV board HV silicon diodes	Inspect for evidence of prior arc or heat damage. The HV board is accessible after cover removal. W8TN documented replacement of a power diode on the HV board as part of the Fault 1 / PIN diode repair procedure. Replacement diodes available from Alpha RF Systems; confirm PIV rating before substituting. A MOV (Metal Oxide Varistor) across the AC line protects rectifier diodes from line transients.
D-ZENER (bias string) Zener diode string in bias supply (200 V add) Used to increase Rbias for improved PIN diode back-biasing (downlevel modification)	The “downlevel” modification for early 87A units (see Section 3.3) includes adding 200 V to the Rbias supply via an additional zener string. If the amplifier is an early production unit and has not received this modification, the full PIN diode upgrade package should be performed. Alpha RF Systems can advise on current-production zener string specifications. Insulate the modified string with heat-shrink tubing.
T1 (PLATE TRANSFORMER) Main plate transformer (separate carton) Custom Alpha unit; ~43 lb; primary strapped 120/240 VAC	The transformer ships and may be stored in a separate carton. Verify the primary voltage strap is correctly set for your mains voltage before first power-up. Inspect the primary strap connections; a mis-strapped transformer on 240 V mains with a 120 V strap will immediately destroy the rectifiers and likely the filter capacitors. Inspect transformer leads for insulation integrity; rewind services available for failed transformers from specialty shops.
F-MAINS AC mains fuses Specified in manual; available Mouser / DigiKey	Alpha RF Systems confirms that the 87A fuses are available from Mouser or DigiKey. Verify the correct current and time-delay ratings before substituting. Never replace a fuse with a higher-rated type to cure a tripping fuse; this masks the underlying fault and risks fire.

3.3 PIN Diode T/R System — The Primary Service Area

The T/R (transmit/receive) switching system is the most failure-prone and most complex portion of the 87A for service purposes. The system uses three distinct PIN diode bias sources and two types of PIN diodes. Understanding their roles is essential before attempting any PIN diode diagnosis or replacement.⁵

The two diode types used are:

Receive PIN diodes — original P/N KS1001 (axial lead, tan body with black markings). Alpha now recommends replacing all KS1001 diodes with standard 1N4007 silicon diodes, which are widely available and electrically compatible.
Transmit PIN diodes — original P/N MA4P4006D (power stud type, mounted on the ABX-X220 board). These are larger, high-power devices that handle the full transmit RF current. A healthy TX PIN measures approximately infinite in reverse bias and approximately 800kΩ or higher in forward bias; a leaky unit typically measures around 220kΩ forward and 46MΩ reverse.

🔌 PIN Diode Bias Sources: The 87A uses three bias voltages to control its PIN diodes. A −109 V source (Rbias) forcefully reverse-biases the receive PINs during transmit, preventing amplifier RF output from reaching the transceiver input. A ±34 V Tbias source forward-biases the TX PINs (transmit mode) or reverse-biases them (receive mode). A −109 V source reverse-biases the input PIN diode during receive. The CPU monitors all three sources and faults if any are out of range. Measuring these voltages at J1/J2 on the CPU board is the primary live-test diagnostic for PIN diode circuit faults.

PIN Diode T/R System — Commonly Replaced Components

Component & Original P/N	Fault Association, Diagnosis & Replacement
D-RX (KS1001 / 1N4007) Receive PIN diodes Original: KS1001; current replacement: 1N4007	Associated with Fault 1 (PIN back-bias voltage below minimum on keydown). A shorted receive PIN allows the Rbias voltage to be pulled towards ground, triggering Fault 1. Diagnosis: measure Rbias voltage (−100 to −109 V nominal). If low, disconnect the coax from the safety choke board; if Rbias recovers to normal, the fault is in the RF section. Systematically disconnect one TX PIN diode at a time to isolate the leaky device. The K4RO troubleshooting document provides a step-by-step procedure.
D-TX (MA4P4006D) Transmit PIN diodes (power stud type) Original: MA4P4006D on ABX-X220 board	Associated with Fault 8 (TX PIN back-bias fault). A leaky TX PIN diode pulls the bias rail towards ground. Diagnosis: measure the voltage at the pair of resistors connected to the TX PINs; a reading near 0 V (instead of the normal −99 to −109 V) confirms a leaky TX PIN. Disconnect one PIN at a time to identify the culprit. Apply a small amount of heat-sink compound to the diode stud before reinstallation. The K4RO document provides detailed photo-illustrated procedures. Accessing the T/R module requires removing approximately 30 screws and unsoldering the RF input cable centre conductor from the board — a time-consuming but achievable procedure.
SGA (SPARK GAP ARRESTOR) Spark gap transient protection (downlevel mod) Added to T/R board during downlevel update	Early production 87A units (“downlevel” amplifiers) used square ceramic PIN diodes rather than the current cylindrical KS1001 types. When updated to the newer PIN diodes — which are more electrically fragile — a spark gap arrestor (SGA) should also be installed on the T/R board for transient protection. Alpha provides this as part of the downlevel modification care package. If the amplifier has been upgraded to newer PIN diodes without the SGA, it is more vulnerable to transient damage.
RFC (SAFETY CHOKE BOARD) Static drain choke on safety choke board RF choke from amp output to ground (receive static drain)	This RF choke drains static charge from the antenna during receive, protecting the PIN diodes from static buildup. Inspect the blue capacitor above the safety choke board — measure for a short to ground (near-zero ohms indicates something is shorting in the RF section). Check D1 (1N5404) on the safety choke board in diode-check mode. A faulty safety choke board can contribute to Fault 8 symptoms.

3.4 Tube Deck

The 3CX800A7 is a ceramic power triode with a thoriated-tungsten filament. It is rated at 800 W plate dissipation per tube; the pair in the 87A operate well within this limit at normal output power. The tubes are mounted inside a shielded sub-chassis known as the tube deck, which includes the filament supply connections and the grid circuit. Accessing the tube deck requires removal of the top cover and the separate tube deck access cover.

Tube Deck — Commonly Replaced Components

Component & Original Value	Replacement / Notes
V1, V2 Final amplifier tubes Eimac 3CX800A7 ceramic triode pair	The Eimac 3CX800A7 is the correct tube; Alpha designed the 87A around it. New Eimac production has been sporadic; current pricing is approximately $1,000 USD per tube. Taylor Tubes (Chinese manufacture via RF Parts Co.) have been reported to function correctly in the 87A at a significantly lower cost per pair, though long-term reliability relative to original Eimac production is uncertain. Inspect tubes for getter condition (white/milky = vacuum failure), cracked ceramic base, or visible grid structure damage. The 3CX800A7 runs a 13.6 VAC filament, directly supplied — do not confuse this with the 5 V filament of older types. The 87A has filament current monitoring; new tubes should be burned in before relying on power output measurements.
TUBE SOCKETS 3CX800A7 ceramic tube sockets Ceramic ring socket, correct contact force	Inspect socket contact springs for correct tension. A loose contact at the 3CX800A7 pin causes intermittent filament connections and erratic fault codes. The WVDXA repair blog documented a cold solder joint on a tube socket resolved a persistent Fault 20 after multiple rounds of connector cleaning had provided only temporary relief. Inspect all socket solder joints with magnification.
RFC-PLATE Plate RF choke HV-rated wound choke	Inspect for cracked or burnt enamel on the winding. This choke carries both the full HV plate current and must provide high RF impedance across 1.8–30 MHz. A damaged plate choke can cause intermittent arcing or a Fault 5 (high plate current). The 87A manual specifies the plate choke location for manual HV discharge using an insulated screwdriver.
BLOWER (squirrel-cage) Tube deck cooling blower 230 VAC blower motor + cage assembly (Alpha PN); internal mount	The blower motor is a known wear item on older 87A units. Reduced airflow raises tube operating temperature, shortening 3CX800A7 life and increasing the probability of arc faults. Alpha RF Systems stocks the correct 230 VAC blower assembly (motor + squirrel cage). The blower replacement procedure requires partial disassembly of the T/R module and is documented by Alpha. An optional auxiliary external fan is recommended for high duty-cycle operation (RTTY, FT8 contesting).

3.5 RF Tank Circuit & Auto-Tune System

The 87A’s output pi-network uses motorised variable capacitors (Tune and Load) driven by small DC motors under microprocessor control. Capacitor position is read back via Hall-effect position sensors. The per-band tuning data is stored in non-volatile memory. A band change initiates an automatic retune cycle taking approximately one second.

Tank Circuit & Auto-Tune — Service Items

Component / System	Service Notes
C-TUNE / C-LOAD Motorised auto-tune variable capacitors Motor-driven, Hall-effect position feedback	Fault codes 15/16 (Tune/Load capacitor not initialising) and 17/18 (capacitor not moving, or zero sensor fault) indicate motor or position sensor issues. Verify motor power supply voltages. Check that the capacitor rotates freely by hand (amplifier unpowered, HV discharged). Inspect for arcing damage on the capacitor plates if excessive SWR operation has occurred. Soft Fault 9/10 (capacitor at rotational limit) during operation usually indicates a tuning data issue or antenna mismatch rather than a hardware failure.
TUNE MOTORS (2×) DC stepper / DC motors for Tune and Load Small DC motors, internal mount	If the motor runs but the capacitor does not move, inspect the coupler between motor shaft and capacitor shaft. If the motor does not run at all, check the motor driver on the control board and the motor supply voltage. Motor replacements available from Alpha RF Systems.
BAND SWITCH (ceramic wafers) Input network band switch wafers Ceramic rotary wafers (multi-band)	Ceramic band switch wafers are available from Alpha RF Systems and are shared across multiple Alpha models (87A, 89, 99, 8100, 8410, 9500). Inspect for arcing damage or carbon tracking. Clean with DeoxIT D5 and isopropyl alcohol. Ceramic wafers are more durable than the phenolic types used in older Heathkit amps but can still be damaged by repeated parasitic oscillation or sustained SWR fault operation.
RF WATTMETER BOARDS Input and output wattmeter PCBs Directional coupler type, forward/reverse	The 87A uses wattmeter boards in the signal path for monitoring gain, detecting SWR, and computing ALC. Inspect for damaged directional coupler toroids and check all connector terminations on these boards. The wattmeter boards are part of the fault detection chain for Fault 14 (excessive reflected power). Island Amplifier USA stocks replacement wattmeter boards for legacy Alpha amplifiers.
RELAY (I/O RELAYS) Input/output RF path relays Heavy-duty, not PIN diodes; for bypass switching	The 87A uses PIN diodes for fast T/R switching, but mechanical relays for the input and output bypass path (connecting the transceiver directly to the antenna when the amplifier is in bypass). Note N8RR’s caution: if the relays fail to close in OPERATE mode, the OPERATE LED can still illuminate and internal T/R switching still functions, but the main RF path remains in bypass — the amplifier makes no power. Inspect relay contacts for pitting.

3.6 Control System & Firmware

The 87A CPU board is the nerve centre of the amplifier. It monitors over a dozen parameters in real-time and can shut down AC power within milliseconds of detecting an out-of-range condition. The RS-232 serial interface allows connection to a PC for diagnostics, remote control, and firmware updates.

The AlphaMax firmware upgrade provides enhanced features on older units. Alpha RF Systems notes that AlphaMax for amplifiers built before 1999 is no longer available in their web store; contact Alpha directly for current availability. The AlphaRemote software package provides PC-based remote control functionality via the RS-232 port.

4. Fault Code Reference

The 87A displays fault codes on a seven-segment LED display. Faults are classified as soft (amplifier continues operating, fault logged) or hard (AC power removed on fault, manual reset required). Six consecutive identical hard faults without an intervening successful power-up results in Fault 99, which locks the amplifier and requires factory assistance to clear.⁶

💻 RS-232 Diagnostic Commands: Connect a terminal emulator at 9,600 baud to the 87A RS-232 port. Key commands: FAULTS lists all logged faults with counts; VER reports firmware version; FREQ reports current frequency; TUNE / LOAD report capacitor positions. Use these before opening the amplifier to characterise the fault history without physical disassembly.

Alpha 87A Fault Code Summary

Code	Type	Description & Most Common Cause
1	Hard	PIN back-bias voltage below minimum on keydown. Most common cause: failed or leaky receive PIN diode (KS1001 / 1N4007 replacement). Also caused by broken Rbias monitor wire or Molex connector on bias supply.
2	Hard	Grid overcurrent. Excessive grid current detected; possible tube fault, overdrive, or antenna mismatch. Check antenna SWR and drive level before assuming tube failure.
3	Hard	Plate overcurrent (hard). Plate current exceeded safety threshold — immediate AC cutoff. Possible tube arc, severe SWR, or failure of the HV supply regulation.
5	Soft	Plate overcurrent (soft). Plate current elevated but below hard-fault threshold. Check drive level, antenna SWR, and tuning. May precede a Fault 3 if the load is not corrected.
8	Hard	TX PIN back-bias fault. Leaky transmit PIN diode (MA4P4006D) on ABX-X220 board. Measure voltage at resistors connected to TX PINs. Detailed K4RO replacement procedure available at k4ro.net.
9	Soft	Tune capacitor at rotational limit. Usually indicates a tuning data mismatch (e.g. wrong frequency set) or antenna SWR beyond the tuner’s matching range. Re-tune on a dummy load.
10	Soft	Load capacitor at rotational limit. Same causes as Fault 9. Check both Tune and Load capacitor positions via RS-232 TUNE / LOAD commands.
11	Hard	Gain too low. Output power significantly below expected for drive level. Indicates degraded tubes, failed tank circuit component, or T/R path problem in bypass. Check tubes and wattmeter boards.
12	Soft	Overtemperature. Blower airflow restricted or blower failure. Check blower motor and cage for obstruction; verify airflow across tube anodes. Inspect rear ventilation clearance (minimum 4 inches free space required).
13	Hard	HV relay failure. Input or output bypass relay failed to operate. Inspect relay coil and contacts. Note: OPERATE LED may still illuminate; verify RF is actually being routed through the amplifier.
14	Hard	Excessive load reflected power or RF voltage. Triggered by antenna SWR >4:1 at 1,500 W, or excessive power at moderate SWR. Check antenna system; do not overdrive the amplifier. Fault 14 is the most commonly encountered operational fault from antenna system issues.
15/16	Soft	Tune/Load capacitor not initialising. Motor or Hall-effect sensor fault. Verify motor power supply; check capacitor rotation by hand (power off, HV discharged).
18	Soft	Low filament current. Almost always caused by Molex connector J3 oxidation in tube deck. Clean and reseat J3 connector as first step. Rarely indicates an actual filament failure.
20	Soft	High filament current (>3.5 A / hard threshold: 4.0 A). Alpha confirmed over a 10-year period this has never been caused by a bad tube. Root cause is always Molex connector J3 oxidation adding resistance to the filament sense line. Clean all connectors before any other diagnosis.
24	Hard	System voltage fault. May clear on second power-up after long storage (capacitors charging). If persistent, measure voltage across each HV filter capacitor individually. Unequal distribution indicates a degraded capacitor; replace the entire bank.
99	Locked	Amplifier locked from 6 consecutive identical hard faults. Cannot be cleared by the operator. Contact Alpha RF Systems for service. This fault state was designed to prevent repeated damage from an unresolved hardware fault.

5. Safety: High Voltage, Crowbar & Interlocks

☠ Lethal Voltage Present

The Alpha 87A operates at approximately 3,000–3,200 V DC. This is among the highest plate voltages encountered in amateur amplifiers. The energy stored in the HV filter capacitor bank at full charge is sufficient to cause immediate cardiac arrest. No amount of experience or familiarity with the equipment eliminates this hazard. Every internal access requires verified discharge of the HV capacitors.

5.1 The HV Crowbar Interlock

The Alpha 87A incorporates a HV crowbar interlock — when the top cover is removed, a switch activates a crowbar circuit that shorts the high voltage to chassis ground, forcing rapid discharge of the filter capacitors. This is a more active safety mechanism than the simple shorting interlocks found in Heathkit amplifiers.

There is also a microswitch on the AC input that disconnects AC power when the cover is removed. Together these two interlocks provide a high level of protection — but they must not be the only safeguard:

After cover removal and crowbar activation, always verify HV is absent with a meter before touching internal components.
The 87A manual and experienced users both recommend additionally shorting the HV to chassis ground at the plate or plate choke with an insulated screwdriver as a belt-and-suspenders measure.
Dielectric absorption in the filter capacitors means charge can partially recover after an apparently complete discharge; the permanently-connected bleeder resistors are the defence against this, but their integrity must be assumed verified, not guaranteed.

5.2 Defeating the Interlocks for Live Testing

Some diagnostic procedures — PIN diode bias voltage measurement, motor control testing — require the amplifier to be powered up with the cover removed. This requires defeating both safety interlocks and must only be done by technically qualified personnel.

The documented technique for the crowbar interlock is to insert a plastic pen cap over the HV shorting screw to block the crowbar switch from closing. The AC microswitch can similarly be mechanically blocked open. With interlocks defeated and HV present at full voltage, extreme care must be taken at every moment:

One-hand-only rule applies at all times — other hand kept away from the chassis.
Use insulated probes; probe one point at a time.
Never lean over the open amplifier.
Do not work alone when HV is present.
Restore all interlocks immediately after live testing is complete.

5.3 Safe Discharge Procedure (Cover-Off, Power-Off)

  STEP 1 ─── Press STANDBY; then press OFF (front panel)
                │
  STEP 2 ─── Unplug the AC mains cord from the wall outlet
                │
  STEP 3 ─── Remove top cover
             Crowbar interlock activates; HV shorted to chassis by crowbar circuit.
                │
  STEP 4 ─── Wait 2 minutes (crowbar has already shorted HV to ground,
             but bleeder resistors continue the discharge)
                │
  STEP 5 ─── Set multimeter to 4000V DC range.
             Measure across HV filter capacitor bank: confirm < 50V.
                │
  STEP 6 ─── Additionally: use insulated screwdriver to short plate choke
             or plate to chassis ground as a final manual precaution
             (as described in the Alpha 87A operating manual).
                │
  STEP 7 ─── Re-measure: confirm < 10V on all HV nodes.
                │
  STEP 8 ─── Now safe to service internally.
             ──────────────────────────────────────────────────────
             NOTE: Dielectric absorption can cause residual charge
             recovery. Re-verify before each work session.
             NOTE: If interlocks are defeated for live testing,
             none of the above safety steps apply — HV is fully
             present and must be treated accordingly at all times.

Figure 1. Alpha 87A safe HV discharge procedure for cover-off, power-off servicing.

5.4 AC Line Safety Capacitors

Any capacitors connected across the AC mains or from AC line to chassis in the 87A must be certified Class X2 (line-to-line) or Class Y2 (line-to-chassis) safety-rated components. Standard ceramic disc capacitors are not suitable for this service; a failed standard capacitor across the AC line can put mains voltage directly on the chassis, causing a fire and lethal shock hazard. Inspect all AC input filter components and replace any non-line-rated capacitors with correctly-rated types from Mouser or DigiKey.

6. Cabinet Restoration

6.1 Chassis Construction

Unlike the painted steel cabinets of the Heathkit SB-Line, the Alpha 87A uses an aluminium chassis. The top cover and side panels are aluminium with a powder-coated or anodised finish in a distinctive dark grey/charcoal colour. The front panel is a thick aluminium extrusion with silk-screened or printed legends. The aluminium construction provides excellent RF shielding and superior thermal management compared to steel, but presents different restoration challenges.

6.2 Cabinet Screws

The top cover uses 6-32 × 1/4" Phillips truss-head black screws — a distinctive fastener type. Alpha RF Systems stocks the correct cover screws and also offers a complete Amplifier Case Screw Kit covering the 87A, 89, 91B, 99, 8100, 8410, and 9500 models. Note that original 87A covers used 31 screws; contact Alpha if ordering the screw kit for an original-style 87A cover.⁷

6.3 Cabinet Paint & Finish

The 87A aluminium panels use a consistent dark grey powder-coat finish across all production years. Unlike the Heathkit SB-Series with its multiple colour variants, the Alpha finish is relatively uniform. Minor surface scratches in anodised or powder-coated aluminium can be addressed with a matching grey powder-coat spray from an automotive or industrial paint supplier. Deep scratches or dents in the aluminium panels require professional panel repair or replacement; replacement front panels and cosmetic components are available from Island Amplifier USA, which specialises in legacy Alpha amplifier parts and service.

Cabinet Hardware & External Components

Item & Description	Source & Notes
Top cover screws 6-32 × 1/4" Phillips truss-head black	alpharfsystems.com — complete Amplifier Case Screw Kit covers 87A and most other Alpha models. Note original 87A (31-screw cover) vs later production difference; specify when ordering.
ON/OFF rocker switch DPDT ON-(ON) momentary rocker; Alpha 86, 87A & 89	alpharfsystems.com — correct DPDT momentary rocker switch shared across Alpha 86, 87A, and 89 models. Verify momentary action — a standard maintained-contact rocker is not a correct substitute for the ON/OFF function of this amplifier.
Front panel assembly Thick aluminium extrusion with silk-screened legends	Island Amplifier USA stocks reproduction front panels, meters, and indicators for legacy Alpha amplifiers including the 87A. Contact for availability of specific components. This is the primary source for cosmetic restoration parts when Alpha RF Systems does not have the item in stock.
LED indicators and meters Front panel segment LED display & bar-graph LEDs	The seven-segment fault display and front panel LED indicators are standard electronic components. If individual LEDs fail, identify the LED type (colour, package, forward voltage) and replace with equivalent modern types. Ensure the replacement LED forward voltage matches the original to maintain correct current through the driver circuit.
RF connectors (SO-239 / Type N) SO-239 4-hole panel mount (factory)	Inspect all RF connectors for deformation, pitting, or damaged centre pins. Replace with silver-plated SO-239 4-hole panel mount types from Harbach Electronics or RF Parts Co. The 9500 (successor to the 87A) uses BIRD quick-change SO-239 connectors that allow type-N adapters without re-wiring; this upgrade is not factory-supported on the 87A but is technically feasible.
RS-232 serial cable (25-pin) 25-pin D-sub to PC serial port	A 25-pin to 9-pin adapter and a USB-to-RS-232 adapter allows connection of the 87A serial port to any modern PC for AlphaRemote and diagnostic access. Verify that the USB-to-serial adapter uses a chip with genuine RS-232 level shifting (FTDI or Prolific chipsets are most compatible); some cheaper adapters use only 3.3 V logic levels which may not reliably communicate with the 87A.
Internal wiring harness PVC-insulated Molex-terminated multi-conductor harness	Inspect all wiring for cracked insulation, especially in zones near the transformer and tube deck. HV wiring must be rated ≥5 kV; use correct HV cable for any replacements. The Molex connector pins themselves can be re-terminated using the correct Molex crimp tool (Molex part 11-01-0197 or equivalent) if a specific connector position is damaged. This avoids replacing an entire harness when only one pin is faulty.

7. Parts Sources & Reference Documents

Alpha RF Systems (factory support) — alpharfsystems.com/87a-parts — 143+ listed parts for the 87A including blower motor, switches, screws, and ceramic band switch wafers. Note: online store intermittently closed during facility moves; email or call for parts availability. Schematics provided free to owners on request.
Island Amplifier USA — islandamplifier.com — specialised in Alpha amplifier repair and cosmetic restoration parts including reproduction front panels, meters, and indicators for 87A and other legacy Alpha models.
K4RO TX PIN Diode Replacement Document (Kirk K4RO) — k4ro.net — the definitive step-by-step guide for Fault 8 / transmit PIN diode replacement, with photo documentation. Updated July 2023.
WVDXA Blog — N8RR Repair Notes (Charlie, N8RR) — wvdxa.blogspot.com — comprehensive repair narrative covering PIN diode system theory, Molex connector service, and live testing with interlocks defeated. Essential background reading before any 87A internal service.
WVDXA Blog — N8RR Fault 20 / Filament — wvdxa.blogspot.com (W8OM repair) — detailed analysis of Fault 18/20 filament current faults, connector oxidation mechanism, and J3 bypass modification.
W8TN Repair Blog (W8TN) — w8tn.blogspot.com — first-person account of Fault 1 and Fault 20 repairs with photo documentation; particularly useful for understanding the T/R module access procedure.
N9ADG Fault 1 / Downlevel Modification — n9adg.com — documents the downlevel modification process including SGA installation, zener string addition, and the distinction between original square ceramic PIN diodes and KS1001 replacements.
Alpha87A groups.io reflector — groups.io/g/Alpha87A — the active community of 87A owners; extensive archived discussion on all fault codes, repair procedures, and parts substitution.
Alpha 87A Operating Manual (2016 edition) — alpharfsystems.com (PDF) — complete operation and installation manual; includes fault code table, RS-232 command reference, and installation procedure. Free download from Alpha RF Systems.
RF Parts Company — rfparts.com — 3CX800A7 replacement tubes (Eimac and Taylor brands), SO-239 connectors.

References & Footnotes

QSL.net / NA1A. “Alpha 87A Omega.” qsl.net/na1a. Omega variant description; production history notes. ↩
N8RR (WVDXA). “Repairing an Alpha 87A for a Friend.” WVDXA Blog, January 2017. wvdxa.blogspot.com . PIN diode T/R switching theory; complexity assessment; Molex connector service. ↩
W8TN. “Repairing the Alpha 87a Amplifier.” W8TN Blog, July 2013. w8tn.blogspot.com . Disassembly procedure; screw count; T/R module access. ↩
N8RR (WVDXA). “Alpha 87A Repair for W8OM.” WVDXA Blog, February 2017. wvdxa.blogspot.com . Molex connector oxidation mechanism; Fault 20 analysis; J3 bypass options. ↩
K4RO (Kirk). “Alpha 87A — Transmit PIN Diode Troubleshooting and Replacement.” Version 2, July 2023. k4ro.net . Fault 8 diagnosis; TX PIN diode types and measurements; step-by-step replacement with photos. ↩
Alpha Radio Products. Alpha 87A Operating Manual, 2016 edition. Pages 36–40 (Fault Code Tables). alpharfsystems.com . Soft fault, hard fault, and Fault 99 lock-out definitions. ↩
Alpha RF Systems. “Amplifier Case Screw Kit.” Product listing. alpharfsystems.com . 6-32 × 1/4" Phillips truss-head black screws; original 87A uses 31 screws. ↩

✍ Mike Peace VK6ADA / r-390a.net Administrator vk6ada.com.au — Vintage Radio Restoration Technical Series

Alpha 87A Linear Amplifier:Restoration & Service Guide