Emtron DX-4 Linear Amplifier:
Restoration & Service Guide
Two GU84B (4CX2500A) Tetrodes in Parallel • 4,000 W Carrier / 5,000 W PEP • 3,100 V DC Plate Supply • Separate Standalone HV Power Supply (3 Variants) • Three-Interlock Serial Safety Chain • Auxiliary Transformer in RF Deck • 117 µF HV Filter (Single-Phase) • Two QSK Vacuum Relays • 9 Bands • 80 kg Total • Emona Electronics Sydney
The DX-4 plate supply voltage is 3,100 V DC, confirmed twice in the DX-4 manual: “The high voltage cable carries a DC voltage of about 3100V and currents up to 2.5 A” and “The plates are connected to the RF network and to the choke supplying the 3100VDC voltage to the plate.” 1 The two-unit architecture creates a unique additional hazard: the HV cable between the power supply and RF deck is energised at 3,100 V DC when the system is live. The DX-4 CAUTION states: “The high voltages present inside the DX-4 system are EXTREMELY DANGEROUS. Do not remove the top covers under any circumstances if any leads are still plugged into the rear of the amplifier and especially if the AC lead is plugged in the power outlet.”
- Disconnect all leads from both units before cover removal. This includes the HV cable, auxiliary cable, RF leads, PTT, and mains.
- Both units have independent microswitches. Safety requires both covers to be fitted before operation; removing either cover alone opens the interlock chain.
- The HV cable connector ends are energised at 3,100 V when the system is powered; treat the disconnected cable as live until both units are de-energised and discharged.
- Wait minimum 5 minutes; discharge both GU84B anodes to chassis via 10 kΩ/25 W insulated probe before any internal access.
- Use a 5,000 V-rated DMM; confirm <50 V DC at HV bus and at HV cable termination before any internal work.
- The power supply is naturally cooled; never obstruct airflow around the external supply enclosure.
1. The DX-4 Two-Unit Architecture — The Only Separate-PSU Emtron
The DX-4 is architecturally unique within the Emtron product range. All other Emtron amplifiers, including the DX-3 and DX-3SP, integrate the mains transformer and HV power supply within the main cabinet. The DX-4 separates these into two physically distinct units with matching form factors.
- RF Deck (35 kg) — Contains: two GU84B tubes, RF sub-chassis with 9-way connectors, pi-network (band switch, Tune/Load capacitors, coils), AMPC control board, display board, RF sensor, QSK module with two vacuum relays, turbine blower, two upper extraction fans, two temperature sensors, and crucially: an auxiliary transformer supplying all non-HV voltages (filament, bias, screen, logic, QSK). Ships in standard carton.
- Power Supply (45 kg) — Contains: mains transformer, HV rectifier and filter bank, soft-start module(s), and an independent cover safety microswitch. Provides ONLY the HV DC to the RF deck via a dedicated HV cable. Ships in a heavy-duty wooden crate. Available in three configurations (single-phase, European 3-phase, North American 3-phase). No cooling fan; naturally cooled only.
- HV Cable — Connects power supply HV output to RF deck HV input. Carries 3,100 V DC at up to 2.5 A. Heavy-duty insulated cable with lug terminations. Attached to the RF deck and connected to the power supply via a gland fitting on the rear panel.
- Auxiliary Power/Control Cable — Carries 240 VAC and low-voltage signals between the two units for interlock signalling and auxiliary power. Must be connected before any power-on.
- Three-Interlock Safety Chain (in series) — Power supply cover microswitch + RF deck cover microswitch + RF deck front panel ON/OFF switch are all in series. Opening any cover or switching OFF at the front panel breaks the chain. All three must be closed for the system to energise.
2. DX-4 Technical Specifications
2.1 RF Deck (Amplifier Unit)
| Output Power | 4,000 W carrier / 5,000 W PEP on most bands; 12 m: 2,500 W; 10 m: 2,000 W CW |
| Frequency Coverage | 9 amateur HF bands: 1.8–2.0, 3.5–4.0, 7.0–7.5, 10.0–10.3, 14.0–14.35, 18.0–18.5, 21.0–21.5, 24.0–24.5, 28.5–29.99 MHz (all WARC bands) |
| Final Tubes | Two GU84B (4CX2500A) in parallel; 2,500 W plate dissipation each; 5,000 W combined; 27 V / 3.7 A heater each |
| Plate Supply Voltage | 3,100 V DC (supplied via HV cable from external power supply) |
| Drive Power | 60–90 W nominal for full output |
| Input Impedance | 50 Ω; VSWR 1.5:1 or less |
| Output Impedance | 50 Ω; VSWR 2.6:1 or less |
| EBS Standing Current | ~1 A typical (per Technical Specifications Section 2.1); General Description says 0.75 A (see Footnote 2) |
| Auxiliary Transformer | Inside RF deck; provides filament, bias, screen, logic, QSK voltages; independent of external HV supply |
| Cooling (RF deck) | Turbine blower below chassis + two upper extraction fans; both two-speed; controlled by two temperature sensors |
| QSK Module | Standard; uses two vacuum relays; 3 ms switching time |
| Dimensions (RF deck) | 470 mm W × 230 mm H × 500 mm D (18.5”×9”×18.5”) |
| Weight (RF deck) | 35 kg (77 lb) |
2.2 Power Supply — Three Configuration Options
Parameter |
Single-Phase (standard) |
Three-Phase Europe (400 V) |
Three-Phase North America (230 V) |
|---|---|---|---|
| Input Voltage | 200, 220, 230, or 240 VAC (tap-selectable) | 400 V between phases (230 V/398 V European) | 230 V between phases |
| Fusing | 2× 40 A normal-acting | 3× 20 A normal-acting | 3× 30 A normal-acting |
| Soft-Start Module(s) | 1× standard 240 V soft-start | 3× HV 400 V soft-start modules | 3× standard 240 V soft-start modules |
| HV Filter Capacity | 117 µF (2 HV modules in parallel); 0.5 Ω sense resistor | 58 µF (1 HV module); 1 Ω sense resistor | 58 µF (1 HV module); 1 Ω sense resistor |
| Aux. Transformer (in RF deck) | Primary: 240 V single-phase tap | Primary: 400 V tap (connected between 2 phases) | Primary: 230 V tap (between phases) |
| Cooling | Natural convection only (no fan) — all three configurations | ||
| Dimensions | 470 mm W × 230 mm H × 500 mm D — all three configurations | ||
| Weight | 45 kg (99 lb) — all three configurations | ||
| Safety Interlock | Cover microswitch in series with RF deck cover microswitch and RF deck front panel switch — all three configurations | ||
| Notes | Most common; 40 A fuses higher than any other Emtron model | Side panel with soft-start system removable (4 screws) for access | Aux. transformer primary tap must be set to 230 V for North American 3-phase operation |
3. DX-4-Exclusive Characteristics — Six Unique Engineering Features
3.1 Output Power Reduction on 12 m and 10 m
The DX-4 manual (January 2007) is the only Emtron amplifier manual to document band-specific power output reductions. Technical Specifications Section 2.1 states: “OUTPUT POWER: Typical 4000 W carrier or up to 5000 W PEP except 12 m band (2500 W) and 10 m band (2000 W CW).” 3 This is a significant service consideration: the factory initial settings on the title page for 12 m and 10 m are calibrated to these lower power levels, not to 4,000 W. A DX-4 that produces less than 4,000 W on 12 m or 10 m is not necessarily faulty; it may be operating exactly as specified. Verify against the factory settings table before diagnosing a high-band power shortfall as a component failure.
3.2 Auxiliary Transformer in the RF Deck
The DX-4 RF deck contains a dedicated auxiliary transformer (visible in the internal view diagram, Figure 1 of the DX-4 manual, labelled “AUXILIARY TRANSFORMER”). The DX-4 manual (Section 8.2) confirms: “The auxiliary transformer inside the amplifier, supplies all the voltages other than the plate (high) voltage, as follows: [filament supply], [low voltage supply to the controller board], [QSK module supply], [bias supply], [screen grid supply], [high voltage supply].” Only the HV (3,100 V DC) comes from the external supply; everything else — filament, screen, bias, logic, QSK — comes from the auxiliary transformer inside the RF deck.4
3.3 Three-Interlock Serial Safety Chain Across Two Units
The DX-4 safety system is the most sophisticated interlock in the Emtron product line. The DX-4 manual Technical Specifications (Section 2.2 for all power supply variants) documents: “SAFETY INTERLOCK: micro-switch activated by the power supply cover; this switch is in series with the interlock switch in DX-4 (also activated by the DX-4 cover) and the DX-4 front panel ON/OFF switch.” Three separate switches must all be in the closed/engaged position for the system to operate. This is a critical service safety consideration: even if the RF deck cover is fitted and the front panel switch is ON, the system will not energise if the power supply cover is removed.
DX-4 INTERLOCK CHAIN (all in series)
─────────────────────────────────────────────────────────
MAINS AC ──► [PSU Cover Microswitch]
│
▼
[RF Deck Cover Microswitch]
│
▼
[RF Deck Front Panel ON/OFF]
│
▼
System Energises
Any single switch OPEN = System does NOT energise.
PSU Cover Microswitch: Inside power supply box, rear panel area.
RF Deck Microswitch: Inside RF deck, cover-actuated (same as all DX models).
Front Panel ON/OFF: User-accessible; dual-function (interlock + switch).
SERVICE NOTE:
If the system fails to power on, check ALL THREE switches.
A failed power supply cover microswitch is a frequent fault on
DX-4 systems where the PSU cover has been repeatedly removed.
Verify the PSU microswitch continuity BEFORE opening the RF deck.
Figure 1. DX-4 three-interlock serial safety chain. The power supply and RF deck cover microswitches are both in series with the front panel ON/OFF switch.
3.4 Two QSK Vacuum Relays
The DX-4 QSK module uses two vacuum relays rather than the single Jennings TJ1A-26S used in all single-tube Emtron models. The DX-4 manual (Section 8.6) states: “The QSK module uses two very fast, high power vacuum relays, and their control circuitry.” At 4,000 W, two relays in the QSK system provide higher RF power handling and redundancy in the switching path. When replacing QSK relays in the DX-4, both relays must be replaced as a matched pair; mixing an aged relay with a new relay in a series-switched path creates asymmetric contact resistance and potential hot-switching vulnerability.
3.5 Natural Cooling of the Power Supply
The external DX-4 power supply has no cooling fan and relies entirely on natural convection. The DX-4 manual Technical Specifications (Section 2.2) confirms: “COOLING: natural (no fan).” This imposes a critical installation requirement: the power supply must be positioned with adequate clearance on all sides for convective airflow. Positioning the power supply in a closed cabinet, rack without ventilation, or flush against a wall on the ventilation sides will cause thermal overload under sustained 4,000 W operation. The power supply should have at least 50 mm (2”) free space on all six sides.
3.6 The HV Cable — A Unique Service Item
The high-voltage cable connecting the power supply to the RF deck is a service component unique to the DX-4. The DX-4 manual describes its connection: “There is a high voltage cable, attached to the amplifier [RF deck], with heavy duty lugs to be connected to the High voltage power supply. The high voltage cable carries a DC voltage of about 3100V and currents up to 2.5 A.” The cable is permanently attached to the RF deck at the factory end and connects to the power supply via a gland and lugs at the supply end.
The HV cable between the power supply and RF deck carries 3,100 V DC at up to 2.5 A. This is lethal current at this voltage. The cable should be treated as energised whenever the system mains supply is connected. Before disconnecting the HV cable: switch OFF the front panel switch; unplug the mains from the power supply; wait 5 minutes; verify <50 V DC at the HV cable lug (using 5,000 V-rated DMM) before touching the cable or its terminations. After any HV cable service, inspect the full cable length for cracks, abrasion, or pinched insulation before reconnecting and applying power.
4. RF Deck — Two GU84B in Parallel, Circuit Details
The DX-4 RF module schematic (Section 7.0 of the DX-4 manual) shows the two GU84B tubes in parallel with the following documented circuit elements: 4× 1,000 pF/6 kV plate bypass capacitors, plate choke, two inductors (Lp) for each tube feed, RS1 (100 Ω), 2,200 pF coupling capacitor, 500 pF and 100 pF input capacitors, BALUN, 5× A106 clamping diodes in the screen/protection circuit, MOV devices at 140 V (4×) and 275 V (1×), 82 pF input capacitors per tube, EG2 (BLUE, screen supply feed), two 9-way connectors (one per tube), plus the BIAS, BIAS2, EBS, and COAX connections. The pi-network on the RF deck is the same 9-band architecture as all other 9-band Emtron models.
Component |
Service Notes |
|---|---|
HV CABLE (3,100 V / 2.5 A)
High-voltage interconnect between power supply and RF deck
Unique to DX-4; permanently attached at RF deck; lug-terminated at power supply; must be tested for insulation integrity annually; 5,000V-rated DMM required
|
Inspect the full cable length annually and after any mechanical disturbance. The cable insulation must be free of cracks, cuts, abrasion, or kinking. The lug terminations at the power supply end must make clean low-resistance metal-to-metal contact; a high-resistance HV connection causes localised heating and voltage drop under load. Measure the HV cable resistance with an insulation tester (1,000 V megohm range); insulation resistance should be >10 MΩ. Replace if any deterioration is detected. A replacement cable must be rated for at least 5,000 V DC insulation and 3 A continuous current. |
GU84B (×2; matched pair)
Two parallel tetrodes; 2,500 W plate dissipation each; same tube as DX-2 series
Replace as matched pair; 27 V / 3.7 A heater from auxiliary transformer; NOS requires gettering; bias must be rebalanced after any tube change
|
Replace as a matched pair for best current sharing. NOS GU84B requires 12–16 hours heater-only gettering before applying plate voltage. After any tube replacement, perform full BIAS, PRE-BIAS, and SCREEN adjustment sequence per DX-4 manual Section 17. The DX-4 uses the same GU84B as the DX-2 family; service procedures from those guides apply to individual tube handling. The DX-4’s two 9-way connectors (one per tube) must both be checked for heater contact integrity; a high-resistance heater connection on one tube degrades current sharing. |
AUXILIARY TRANSFORMER (RF deck)
Provides all non-HV voltages; filament, bias, screen, logic, QSK; primary tap must match supply variant
Single-phase tap: 240 V; European 3-phase: 400 V; North American 3-phase: 230 V; verify before any power-on; Category A for the transformer itself
|
The auxiliary transformer is the most critical single component in the RF deck other than the tubes. It powers everything except the HV; a failed auxiliary transformer leaves the system with HV present but no filaments, no screen voltage, no bias, and no QSK. Symptoms of auxiliary transformer failure: READY LED never illuminates; no filament glow through the upper viewing area; display board shows no plate current (tubes are cold). Before condemning the auxiliary transformer, verify the primary tap is set correctly for the power supply variant in use. The auxiliary transformer primary tap wire (brown, with yellow or white marking ring) must not be on the wrong tap. |
C-BYPASS (4× 1,000 pF / 6 kV; RF sub-chassis)
Plate bypass capacitors; same configuration as DX-2 RF module
Replace as set of 4 after any arc event; verify SRF above 50 MHz; Vishay/Vitramon type
|
Same type and value as used in all DX-series RF modules. At 3,100 V and 4,000 W output from two parallel tubes, these capacitors are under higher electrical stress than in single-tube models. Replace as a complete set of four after any flashover or arc event. Verify the 6 kV voltage rating provides adequate margin at 3,100 V DC with RF peak voltage superimposed. |
QSK RELAYS (×2; high-power vacuum)
Two high-power vacuum relays in QSK module; unique to DX-4 in Emtron range
Replace as matched pair; verify coil voltage before ordering; Jennings TJ1A-26S or equivalent high-power HF vacuum relay
|
The DX-4 QSK module uses two vacuum relays as confirmed in the manual. Replace both as a matched pair; mixing new and aged relays creates asymmetric contact resistance in the switching path. At 4,000 W, high contact resistance in either relay causes RF power loss and potential relay damage. Verify relay coil voltage from existing relay markings before ordering; the DX-4 manual does not specify whether these are TJ1A-26S or a higher-power model. Contact Dan at emtrondv.com for the correct relay specification. |
BAND SWITCH (9-position; all 9 active)
Ceramic 9-position switch; all 9 positions active; reduced power positions on 12m and 10m
Category A — unavailable; inspect all 9 positions; highest stress on 10m/12m even at reduced power
|
Category A — Unavailable. Inspect all 9 wafer positions for arc tracking. On the DX-4, the 10m and 12m positions see lower absolute power (2,000 W and 2,500 W respectively) than on lower bands, but the combination of 3,100 V plate supply and the tank circuit voltages at these frequencies still creates significant switch stress. Clean with DeoxIT D5; verify contact condition with magnifier. A donor DX-4 is the only parts source. |
PSU COVER MICROSWITCH (power supply unit)
Safety interlock in series with RF deck switch and front panel switch
Unique to DX-4; located inside power supply enclosure; must open when PSU cover is removed; verify continuity separately from RF deck microswitch
|
The power supply cover microswitch is a service item unique to the DX-4. It must open (break circuit continuity) when the power supply cover is removed. A failed-closed microswitch defeats the power supply interlock, allowing the HV to energise even when the power supply cover is removed — an extremely dangerous condition. Test the PSU microswitch separately from the RF deck microswitch: with power supply unplugged, verify the microswitch shows open-circuit on an ohmmeter when the cover is removed and closed-circuit when the cover is fitted. Replace a defective PSU microswitch before any operational use. |
5. AMPC Control Board — Horizontal Orientation
The DX-4 AMPC control board is horizontally mounted, the same as the DX-2SP and DX-3. The DX-3 manual’s adjustment instruction applies: counter-clockwise rotation increases protection sensitivity (tighter trip) for horizontally-mounted boards. The DX-4 manual Section 17 (Adjustments, January 2007) contains the full AMPC board adjustment procedure; all pot labels and positions are the same as the DX-3.
Pot / Label |
DX-4 Notes |
|---|---|
POT3 / BIAS |
Sets idle current for both GU84B tubes simultaneously. EBS jumper out during adjustment. Counter-clockwise reduces current (horizontal board). Target idle current per DX-4 manual Section 17.1.2. Two parallel tubes means the total combined Ip at bias should be approximately double a single-tube operating point. After any single tube replacement, rebalance bias to account for the new tube’s characteristics versus the retained tube. |
POT2 / SCREEN |
Screen voltage adjustment; measure at EG2 (blue wire) in READY/OPR. The auxiliary transformer in the RF deck supplies the screen voltage; a screen voltage error that persists after POT2 adjustment may indicate an auxiliary transformer primary tap error rather than a POT2 fault. Do not exceed 400 V GU84B G2 maximum. |
POT6 / IPTRIP |
SWR protection sensitivity. Counter-clockwise = more sensitive (horizontal board). Calibrate on 20m at nominal 4,000 W. On 12m and 10m, note that the rated output is 2,500 W and 2,000 W respectively; the IPTRIP threshold calibrated for 4,000 W on 20m should still be appropriate for protection on high bands. |
VR1 / RF F (forward power) |
Forward power sensor calibration; set on 20m at 4,000 W nominal into 50 Ω dummy load. Use Bird 43 with 5,000 W element or equivalent wattmeter as reference. Note: on 12m and 10m, the sensor will indicate lower power (2,500 W and 2,000 W respectively) — this is correct for the DX-4 on those bands and does not indicate a sensor miscalibration. |
POT7 / PRE-BIAS |
EBS pre-bias current; sets the small tube current when EBS is active (no modulation). Adjustment procedure identical to DX-3. Counter-clockwise direction on horizontal board. |
6. Power Supply Service — Single-Phase & Three-Phase Variants
6.1 Single-Phase Power Supply — Two HV Modules in Parallel
The single-phase DX-4 power supply uses two standard Emtron HV modules in parallel, producing a total filter capacity of 117 µF with a 0.5 Ω current sensing resistor. This is the highest HV filter capacity in the Emtron product line. The manual states: “HIGH VOLTAGE SUPPLY: 2 modules in parallel with a total capacity of 117 µF.” The 40 A fuses are the highest-rated fuses in the Emtron range — higher even than the DX-2SP and DX-3SP at 30 A.
Service of the single-phase HV modules follows the same procedures as documented for other Emtron models: replacement of filter electrolytics as a matched bank; verification of equalising resistors; inspection and replacement of HV rectifier diode strings. The two parallel modules must be disconnected and serviced individually to avoid cross-discharge interactions.
6.2 Three-Phase Power Supply — Internal Access
The three-phase power supply has a distinctive service feature documented in the DX-4 manual: “The side panel holding the soft start system can be removed for easy access. Remove the 4 screws, then lift the panel enough to clear the front and rear panel. Then the wires are long enough to allow the panel to be laid down on the bench with the components facing up.” This removable side panel is specific to the three-phase configuration, which uses three separate soft-start modules requiring individual access. The European and North American three-phase power supplies use one 58 µF HV module (same as the DX-3SP) rather than the two modules in parallel used in the single-phase version.
Component |
Service Notes |
|---|---|
C-HV (single-phase: 2×58 µF = 117 µF total)
Two parallel HV filter banks; highest filter capacity in Emtron range
Two independent HV modules in parallel; 0.5 Ω sense resistor unique to single-phase DX-4; replace both banks simultaneously
|
Replace both parallel HV modules as a matched set. In a two-module parallel configuration, the module with lower capacitance will share less ripple current and the one with higher capacitance will take more load; matched replacement ensures equal sharing. Use 105°C-rated electrolytics. Verify the 0.5 Ω sensing resistor (unique to single-phase DX-4; 3-phase uses 1 Ω) is intact after any HV fault event — this resistor is the current feedback path to the control board. |
FUSES (single-phase: 2× 40 A)
Rear panel fuses on power supply; highest current rating in Emtron range
40 A normal-acting; do not substitute lower rating; three-phase versions use 20 A or 30 A depending on variant
|
The 40 A fuses in the single-phase DX-4 power supply are the highest-rated mains fuses in the Emtron product line. Never substitute 30 A fuses (DX-2SP / DX-3SP rating); they will blow under sustained 4,000 W operation. Use only 40 A normal-acting ceramic fuses. The three-phase variants use 20 A (European) or 30 A (North American); consult the rear panel label to confirm the installed variant before replacing fuses. |
SOFT-START MODULE(S)
Single-phase: 1× standard 240 V module; 3-phase: 3× modules
3-phase European uses 400 V-rated modules; 3-phase North America uses standard 240 V modules; check variant before ordering replacement
|
The single-phase DX-4 uses one standard 240 V soft-start module, the same type used in the DX-2 family. The European three-phase variant uses three HV 400 V-rated soft-start modules (higher voltage components, not interchangeable with standard 240 V types). Identify the variant before ordering any soft-start replacement. The three-phase removable side panel (4 screws) makes soft-start module replacement significantly easier in those variants. |
PSU MICROSWITCH
Power supply cover interlock microswitch; in series with RF deck switch
Must open when PSU cover removed; verify before every operational use; replace if continuity not correct
|
See Section 4 notes above. The power supply cover microswitch is in a damp, naturally-cooled environment that may experience condensation; inspect for corrosion on the switch contacts after extended storage or operation in high-humidity environments. Verify correct operation by checking continuity with an ohmmeter after any PSU cover removal. Replace with an identical snap-action microswitch rated for the mains voltage and current carried through this switch position. |
7. Safety — The Serial Interlock Chain & Two-Unit Discharge
7.1 Complete Safe Shutdown & Discharge Procedure — Two-Unit System
STEP 1 ── OPR → STBY; POWER (RF deck front panel) → OFF.
│
STEP 2 ── UNPLUG MAINS FROM POWER SUPPLY.
(Both the PSU mains and any 3-phase cabling.)
This is the primary energy source; disconnect first.
│
STEP 3 ── UNPLUG ALL RF DECK REAR LEADS:
RF IN, RF OUT, PTT/Key, ALC, auxiliary control cable.
Do NOT yet disconnect HV cable.
│
STEP 4 ── Wait MINIMUM 5 MINUTES.
DX-4 single-phase has 117µF at 3,100V DC.
This is more stored energy than any other model in this series.
Allow full natural discharge time.
│
STEP 5 ── POWER SUPPLY DISCHARGE CHECK (5kV-rated DMM):
Measure HV bus in power supply to earth → < 50V DC.
Only then disconnect the HV cable.
│
STEP 6 ── RF DECK ANODE DISCHARGE — BOTH TUBES:
10kΩ/25W insulated probe; clip to chassis.
Contact GU84B #1 anode cap firmly. Hold 15 seconds.
Contact GU84B #2 anode cap firmly. Hold 15 seconds.
Discharge both individually.
│
STEP 7 ── HV BUS VERIFICATION (RF deck side):
Confirm HV bus on RF deck side < 50V DC.
│
STEP 8 ── SCREEN AND BIAS:
Measure EG2 (blue wire) to chassis → < 50V.
Measure –Eg1 to chassis → within –10V.
│
STEP 9 ── VERIFY BOTH TUBE ANODES: < 10V DC each.
│
STEP 10 ── TEMPERATURE SENSORS: verify both near-horizontal above tubes.
│
STEP 11 ── SAFE. Work may begin on RF deck.
For power supply service: re-verify PSU HV bus < 10V
before opening PSU cover.
Figure 2. DX-4 two-unit safe shutdown and discharge procedure. Mains must be disconnected from the power supply before any steps are taken; the HV cable should not be disconnected until the power supply HV bus is confirmed at <50 V.
7.2 Safety Capacitors — X2 and Y2
The DX-4 single-phase soft-start module schematic (Section 15.6) shows the same safety capacitor requirements as documented in all DX models. The three-phase European power supply uses 400 V-rated soft-start modules with correspondingly higher-rated X2 and Y2 capacitors.
Component |
Value & Class |
Position |
Replacement Rule |
|---|---|---|---|
| C13, C14 (Y2 pair) | 4.7 nF / 250 VAC / Class Y2 | Line-to-chassis | IEC 60384-14 Class Y2 mandatory. Standard ceramics prohibited. |
| X2 cap (470 nF) | 470 nF / 250 VAC / Class X2 | Line-to-neutral | Class X2 mandatory. Standard MKT film prohibited. |
| C2 / X2 (220 nF) | 220 nF / 250 VAC / Class X2 | Across mains switch | Class X2 at 250 VAC minimum. Three-phase 400 V modules require 400 V-rated X2 types. |
8. Cabinet, Front Panel & Assembly Hardware
Both the RF deck and the power supply have identical form factors: 470×230×500 mm in the same baked enamel texture finish. The RF deck cabinet construction is identical to other DX models (2 mm steel, 3 mm anodised aluminium front panel). The power supply is a steel box with matching finish. Both units use the same M4 cover screws; the power supply uses 6 screws (two each side) rather than the RF deck’s 8 screws.
The RF deck front panel carries all the standard DX controls (Band, Plate, Load) and displays (bargraph LEDs for Ip, Vp, forward/reflected power, Ig2+, Ig2−, and five single LED indicators). The power supply front panel is minimal — it may carry only labelling and indicator LEDs with no user controls. The interconnecting cables between the two units (HV cable and auxiliary cable) route through the rear panels; both rear panels should be inspected for cable strain at each gland point during any service.
9. Parts Sources & Reference Documents
- Emtron DX-4 Operating Manual (January 2007) — Hosted copy (myregisteredsite.com); also via the BAMA manual archive (bama.edebris.com/manuals/emtron/dx4) with thanks to the contributor Dave White G0OIL. Contains all schematics: RF module (7.0), HV supply (15.2), control board (15.3 + 15.9), soft-start (15.6), QSK (15.7). The primary service reference for all DX-4 work.
- emtrondv.com — Dan — emtrondv.com — Version 7 AMPC control board (DX-4 compatible); plate choke (stocked); QSK relay specification for dual-relay DX-4; free technical advice; refurbished Emtron amplifiers (Sydney pickup). Dan is the definitive resource for DX-4 auxiliary transformer tap settings, QSK relay identification, and power supply variant service.
- GU84B Tube Sources (NOS) — DL3JJ / QRO-Shop (qro-shop.com); RF Parts Co. (rfparts.com); Soviet-Tubes.com. Replace as a matched pair. NOS tubes require 12–16 hours heater-only gettering. Verify 27 V heater at tube socket pins before installation.
- Safety Capacitors (Y2 and X2) — Mouser (mouser.com), DigiKey (digikey.com). Standard 4.7 nF/Y2; 470 nF/X2; 220 nF/X2 for single-phase and North American 3-phase. European 3-phase requires 400 VAC-rated X2 types.
- High-Voltage Cable (custom) — For cable replacement, the DX-4 HV cable must be rated for minimum 5,000 V DC insulation at 3 A continuous current. Suitable cable from specialist HV cable suppliers; lug terminations must be crimped and heat-shrink insulated. Contact Dan at emtrondv.com for cable specification before sourcing.
References & Footnotes
- Emtron DX-4 Operating Manual (January 2007), Section 6.1 (rear panel description): “There is a high voltage cable, attached to the amplifier, with heavy duty lugs to be connected to the High voltage power supply. The high voltage cable carries a DC voltage of about 3100V and currents up to 2.5 A.” Also Section 8.1 (RF Section): “The plates are connected to the RF network and to the choke supplying the 3100VDC voltage to the plate of … the GU84 tubes.” DX-4 manual PDF; also hosted at BAMA archive. ↩
- Emtron DX-4 Operating Manual (January 2007). EBS current discrepancy: Section 1 General Description states “typically 0.75 A”; Section 2.1 Technical Specifications EBS entry states “about 1 A.” This is a genuine discrepancy within the DX-4 manual itself, not an error in this guide. The two parallel GU84B tubes at 4,000 W output likely have a higher combined idle current than either the DX-3’s 0.75 A or the DX-3SP’s 0.5 A; the Technical Specifications value of ~1 A is recommended as the service reference. ↩
- Emtron DX-4 Operating Manual (January 2007), Section 2.1 Technical Specifications, OUTPUT POWER entry: “Typical 4000 W carrier or up to 5000 W PEP except 12 m band (2500 W) and 10 m band (2000 W CW).” This is the only Emtron amplifier manual to document band-specific power output reductions. ↩
- Emtron DX-4 Operating Manual (January 2007), Section 8.2 Mains Transformer: “The auxiliary transformer inside the amplifier, supplies all the voltages other than the plate (high) voltage … Filament supply, Low voltage supply to the controller board, QSK module supply, Bias supply, Screen grid supply, High voltage supply [referring to auxiliary HV].” Also Section 2.2 Power Supply (single-phase): “The power supply box only contains the main transformer, with a single secondary winding: the high voltage. All other voltages required for filament, bias, screen, logic, QSK are derived from a small auxiliary transformer located in the amplifier.” ↩
- Emtron DX-4 Operating Manual (January 2007), Section 2.2 Power Supply (all variants), SAFETY INTERLOCK entry: “Micro-switch activated by the power supply cover; this switch is in series with the interlock switch in DX-4 (also activated by the DX-4 cover) and the DX-4 front panel ON/OFF switch.” Identical language in all three power supply variant sections (2.2, 2.3, 2.4). ↩
- Emtron DX-4 Operating Manual (January 2007), Section 8.6 Antenna Relay/QSK Module: “The QSK module uses two very fast, high power vacuum relays, and their control circuitry.” This is the only Emtron single-chassis model to document two QSK vacuum relays. ↩