1. The DX-2A in the Emtron Product Sequence — The 9-Band Upgrade

The Emtron DX-2 was produced in two distinct configurations. The standard DX-2, documented in the Emtron DX-2 operating manual, covers seven bands (160–15 m) using a 9-position ceramic band switch with two positions inactive. The manual explicitly notes: “The last two switch positions are not used in the U.S. version of DX-2. If a special licence is produced, the amplifier can be delivered with all 9 bands operational.” The DX-2A is that full-coverage version — the same GU84B-based 1,500-watt amplifier with the tank circuit configured and tuned for all nine amateur HF bands including 12 metres (24 MHz) and 10 metres (28 MHz), with all nine band-switch positions active.¹

The RF module circuit shared by the DX-2A and DX-1D is documented in the Emtron DX-1D operating manual schematic (Circuit Diagram 15.8) and the DX-1B FCC filing (document Q8VDX1B). The schematic caption reads: “4CX800 (DX-1b) or GU74B” and “GU84B (DX-2A)” — both tube types in the same circuit. This means the DX-2A’s RF deck circuit is the only Emtron single-tube 9-band configuration with the GU84B documented in publicly available schematics, making the DX-1D/DX-1B RF module schematic the primary circuit reference for DX-2A RF deck service.²

2. DX-2A vs DX-2 vs DX-2SP — Position in the Production Sequence

3. RF Deck — 9-Band Coverage & DX-1D Circuit Architecture

3.1 The DX-1D RF Module Schematic as the DX-2A Primary Reference

The DX-2A RF deck is the most important service topic unique to this model, because it is the single element that distinguishes the DX-2A from the DX-2 and is documented by cross-reference in the DX-1D schematic rather than in a dedicated DX-2A manual. The RF module schematic (Circuit Diagram 15.8 in the DX-1D manual / FCC filing Q8VDX1B document 332565) shows both tube types labelled on the same circuit: the GU74B for DX-1b and DX-1d service, and the GU84B for DX-2A service.

The DX-1D circuit architecture uses two interchangeable tank coils — a wound coil on a ceramic bobbin for 40–160 m and a silver-plated large-diameter copper tube coil for 10–30 m. The 9-way RF sub-chassis connector, the plate bypass capacitor network (4× 1,000 pF/6 kV), the A106 clamping diodes, the input BALUN, and the RF sensor connection are all shown in this schematic. For the DX-2A, the GU84B tube is populated in the sub-chassis in place of the GU74B, but the surrounding circuitry — the high-voltage bypass network, protection diodes, plate choke, and coaxial routing — is unchanged.

3.2 9-Band Coverage — Bands Added Over DX-2

4. GU84B Tetrode at 9-Band Coverage — High-Band Service Notes

The GU84B tetrode service requirements are identical to those documented in the DX-2 guide: 27 V / 3.7 A heater, NOS gettering (12–16 hours heater-only conditioning), bias target 370–380 mA via POT3 (clockwise reduces current on vertical board), and two-temperature-sensor management above the tube anode.

The DX-2A’s extension to 10m and 12m introduces one additional GU84B consideration: at 28.5 MHz, the tube’s inter-electrode capacitances (input capacitance ~102.5 pF) become a more significant design factor than at HF frequencies below 15 MHz. The input matching circuit must resonate the G1 capacitance correctly at 28.5 MHz; if the DX-2A shows higher input SWR specifically on 10m compared to lower bands, this typically indicates that the input matching network requires adjustment or has degraded components rather than a tube problem. The GU84B is rated for full-power operation up to 250 MHz, so 28.5 MHz itself is well within the tube’s design envelope.

5. AMPC Control Board — Same as DX-2, Vertically Mounted

The DX-2A AMPC control board is identical to the DX-2: vertically mounted, AMPC v.3.x family, Version 7 compatible. All adjustment procedures, known failures (C7, U5, TIPL760A, M4-12H relays), bias target (370–380 mA), screen voltage adjustment (POT2), and IPTRIP procedure (POT6 via TP2) documented in the DX-2 guide apply without modification to the DX-2A.

The critical pot direction note applies equally: for the DX-2A’s vertically mounted AMPC board, clockwise rotation increases protection sensitivity (same as DX-1 and DX-2 models, opposite to DX-2SP and DX-3 which use horizontally mounted boards). This direction is explicitly confirmed in the Emtron DX-3 manual adjustment section: “Rotate clockwise for DX-1, DX-2 and anti-clockwise for DX-2SP and DX-3.” The DX-2A, sharing the DX-2’s vertical board orientation, follows the DX-1/DX-2 clockwise direction.

6. RF Sensor Calibration at 28.5 MHz

The DX-2A’s extension to 10m requires that the RF sensor module (small metal box at the amplifier RF output) produces accurate forward and reflected power readings at 28.5 MHz. The directional coupler in the RF sensor has a coupling factor that may differ between low HF (1.8 MHz) and 10m (28.5 MHz) due to the frequency dependence of its coupling elements. If the Vp and SWR bargraph readings are consistently inaccurate on 10m compared to a calibrated external wattmeter, the RF sensor requires calibration using the DX-2 manual’s RF Sensor Adjustment procedure (Section 15.1.3) with a Bird 43 or equivalent wattmeter as the 28.5 MHz reference.

7. High Voltage Power Supply & Soft Start

The DX-2A HV power supply and soft-start module are architecturally identical to the DX-2: the same 2,500 V DC plate supply, the same filter capacitor bank, the same TDA1085C soft-start module with its 5-second ramp, and the same X2/Y2 safety capacitor requirements. All HV power supply service procedures from the DX-2 guide apply without modification to the DX-2A. The same 200 VAC minimum mains requirement applies.

8. Safety: Interlock, HV Discharge & AC Line

8.1 Cover Safety Microswitch

The cover safety microswitch is identical in function to the DX-2. Verify it opens (breaks mains continuity) when the cover is removed at every service. A failed-closed microswitch must be replaced before returning the amplifier to service.

8.2 Safe Discharge Procedure

  STEP 1 ── STBY switch to STBY; POWER to OFF.
               │
  STEP 2 ── DISCONNECT ALL REAR-PANEL LEADS.
            Mains, antenna cables, PTT/key, ALC, accessories.
               │
  STEP 3 ── Wait minimum 5 minutes.
            GU84B thermal mass; allow extra cooling time.
               │
  STEP 4 ── ANODE DISCHARGE:
            10kΩ/25W insulated probe clipped to chassis.
            Apply to GU84B anode contact. Hold 10 seconds.
               │
  STEP 5 ── HV MEASUREMENT:
            4000V-rated DMM: HV filter cap positive to chassis.
            CONFIRM < 50V DC.
               │
  STEP 6 ── SCREEN CHECK: EG2 (blue wire) to chassis.
            GU84B screen supply is higher than GU74B models.
            Confirm < 50V.
               │
  STEP 7 ── BIAS CHECK: negative bias to chassis.
            Confirm < 10V absolute value.
               │
  STEP 8 ── TWO-SENSOR REPOSITIONING CHECK:
            Verify both temperature sensors above GU84B are
            positioned near-horizontal before next power-on.
            HIGH VOLTAGE EXISTS BETWEEN SENSORS AND TUBE ANODE.
               │
  STEP 9 ── Re-verify HV bank: < 10V.
               │
  STEP 10 ── Now safe to work internally.

Figure 1. DX-2A safe discharge procedure — identical to DX-2, including dual temperature sensor repositioning reminder.

8.3 Safety Capacitors — X2 and Y2 Requirements

The soft-start module safety capacitors are identical to the DX-2: Class Y2 (4.7 nF/250 VAC; line-to-chassis) and Class X2 (470 nF/250 VAC and 220 nF/250 VAC; line-to-line). These certifications are mandatory. Standard ceramic disc capacitors must never be used in these positions.

9. Display Board — Ig2+/Ig2− at All 9 Bands

The DX-2A Display Board is identical to the DX-2: LM3914 bargraph ICs providing Ip (plate current), Vp (plate voltage), forward power, reflected power, and both Ig2+ and Ig2− (screen current) channels. On the DX-2A, the Ig2+/Ig2− channels should be monitored on the newly-added 10m and 12m bands as part of initial setup: elevated negative screen current on 10m can indicate inadequate drive or mistuning at that frequency, while positive screen current above the limit suggests the tube is being driven beyond Class AB1 on the high band.

10. Cabinet & Assembly Hardware

The DX-2A cabinet is identical to the DX-2: 2 mm steel chassis with dark yellow chromate coating, 3 mm anodised aluminium front panel, and baked enamel texture finish. All cabinet service procedures from the DX-2 guide apply. The DX-2A front panel will be labelled to reflect its 9-band coverage; the band switch knob legend should show positions for all 9 bands including 12m and 10m (positions unlabelled or absent on the standard 7-band DX-2 front panel).

11. Parts Sources & Reference Documents

• emtrondv.com — Dan (former Emtron technician) — emtrondv.com — Version 7 control board (compatible with DX-2A); plate choke 1" OD / 6" long (stocked); free technical advice; refurbished DX amplifiers (pickup Sydney). The definitive human service resource for any DX-2A query, particularly RF deck configuration and tank circuit specifications.
• Emtron DX-1D Operating Manual (April 2003) — RF Module Schematic — manualslib.com — Circuit Diagram 15.8 shows the RF module annotated for both GU74B (DX-1b/DX-1d) AND GU84B (DX-2A). This is the primary schematic reference for the DX-2A RF deck. Required reading for any RF deck service.
• Emtron DX-1B FCC Filing (Circuit Diagrams) — fccid.io/Q8VDX1B — Complete schematic set including RF module (shared with DX-2A), AMPC control board, soft-start, QSK. Download and keep as the DX-2A RF deck circuit reference.
• Emtron DX-2 Operating Manual — manualslib.com — All shared-module procedures (AMPC adjustments, soft-start, display board, HV supply, QSK) apply to DX-2A. The DX-2 manual is the procedural reference for all modular service.
• GU84B Datasheet (OK1RR) — ok1rr.com/tubes/gu84b.pdf — 2,500 W plate dissipation; 27 V / 3.7 A heater; 2,200 V max anode; 400 V max G2; 3-minute warm-up; 96 m³/h cooling; 250 MHz max frequency.
• GU84B Tube Sources (NOS) — DL3JJ / QRO-Shop (qro-shop.com); RF Parts Co. (rfparts.com); Soviet-Tubes.com. Mandatory NOS gettering (12–16 hours heater-only). Verify 27 V heater supply at tube socket before installation.
• Mouser / DigiKey — mouser.com / digikey.com — Class Y2 and X2 safety capacitors; LMC555; TIPL760A equivalents; LM3914; 1,000 pF/6 kV disc ceramics; M4-12H PCB relays; 275 V MOV devices.
• Jennings Technology (QSK vacuum relay) — jenningsrelays.com; RF Parts Co. — Jennings TJ1A-26S used in all Emtron QSK modules.