Emtron DX-2a with the 4CX1600B:
Service Manual Reality & First-Generation Service Notes

1. There Is No Separate Service Manual — and That Is Good News

Emtron, a division of Emona Electronics in Sydney, documented its amplifiers the way a small engineering shop does: one book per model, with the service material at the back. The DX-2 Operating Manual of January 2003 has sixteen sections. Section 15, Service, covers the control-board adjustments (pre-bias, bias, SWR protection, EBS threshold, screen voltage, screen current limit and plate-current trip), the display-board calibrations and the RF-sensor trimmer procedure. Section 16, Diagrams and Schematics, contains twelve drawings.²

If an owner’s “Ops Manual” ends at the glossary, he has an abridged or early edition. The full edition is the service manual. It is also free.

1.1 Where to Download the Full Manual

2. One Name, Two Amplifiers — The DX-2a Generations

The name “DX-2a” appears in the record attached to two quite different tube decks. Owners who bought in 1998–1999 describe a DX-2a built around the Svetlana 4CX1600B. The first US delivery reported on the Amps reflector in January 1999 was a 50 Hz export unit running a 4CX1600B in class AB1, and the writer chose the “a” version specifically for its analogue meters.⁵ An eHam reviewer says the same thing a decade later: he has the DX-2a “because I like the meters rather than LEDs,” and his NOS spare 4CX1600B was still in its box after ten years.⁶

The January 2003 manual, by contrast, describes a GU84B (4CX2500) and a “moving LED” display. Emtron’s April 2003 DX-1d manual labels the shared RF-module schematic “GU84B (DX-2A),” which is why the earlier DX-2 guide and its DX-2A companion on this site treat the DX-2A as the 2003-era GU84B amplifier with all nine bands enabled.⁷ Both readings are supported by primary sources. The most consistent explanation is that Emtron used the “a” suffix for the analogue-metered variant and carried the name across the change of tube, exactly as it carried the DX-2 name itself.

Emtron confirmed the tube change in its own words. In an April 2005 posting on the Amps reflector, Emona stated that it had encountered faulty 4CX1600B tubes for which new replacements were not available, and that it had developed a replacement module for amplifiers built with that tube.⁸ An Alaskan owner who ran both versions hard on digital modes reported that the GU84B DX-2 fixed most of the weaknesses of his 4CX1600B unit with heavier-duty components, with one exception: the band switch still arced at its top mounting post.⁹

3. What in the 2003 Manual Applies to a 4CX1600B DX-2a

Emtron’s manual states that all its amplifiers are modular and that most modules are common across the DX range.² That modularity is what makes the 2003 manual useful to a first-generation owner — with discipline about which pages to trust. The 1999 reflector report describes the Generation 1 HV supply as a series-string full-wave bridge of 6 A diodes with B− fault diodes to ground and B+ glitch resistors on a compact module,⁵ which is exactly what drawing 16.4 shows. Dan Voican states that his Version 7 control board is compatible with every Emtron built after 1996.¹⁰ The two areas that do not transfer are the tube deck and the metering.

4. The Svetlana 4CX1600B — Data & Service Parameters

The 4CX1600B is a ceramic-metal external-anode tetrode from Svetlana, St Petersburg, originally a military transmitter tube. Its distinguishing feature is a machined anode with annular cavities that trap secondary electrons, which is why it holds screen current down when the instantaneous plate voltage swings below the screen potential and why it runs efficiently at a modest 2.4 kV.¹¹ Svetlana specified it for passive-grid class AB1 service: a 50 Ω swamping resistor at the grid, about 25–35 W of drive, and essentially zero grid current. The two Communications Quarterly application articles reproduced in Svetlana’s technical bulletin series are the best public datasheet for the tube.^11,12

5. Bias, Screen Voltage & the Numbers Trap

5.1 Document the Amplifier As Found — Before Touching Any Pot

The factory set-points for a Generation 1 DX-2a are not published. They are, however, almost certainly still in the amplifier. Before any adjustment, record the following with the cover off, the discharge procedure complete, the cover interlock temporarily bridged only for the measurement, and one hand behind your back:

1. Plate voltage at idle, from the panel meter and from a HV probe.
2. Screen voltage at the EG2 pin (blue wire) with the amplifier in OPR and no drive.
3. Standing current at TP2 with the EBS link OFF, PTT on, no drive (1 V = 1 A).
4. Pre-bias current at TP2 with the EBS link ON, PTT on, no drive.
5. Heater voltage at the SK3A contacts.
6. Photographs of every pot position on the AMPC board and the board’s silk-screen version.

Those six numbers are the Generation 1 service data for your serial number. Send them to Dan with your enquiry (Section 8); he tested and adjusted most of these amplifiers personally and can tell you whether they are where the factory left them.¹⁰

6. Field Failures Reported on 4CX1600B-Era Units

Fifteen eHam reviews, the Amps reflector archive and the WorldwideDX repair thread together give a consistent picture of what goes wrong. None of it is exotic.

7. Keep the 4CX1600B, or Convert to the GU84B?

This is the question every first-generation owner eventually faces, and it is a conversation to have with Dan before buying anything. Three facts frame it.

• The 4CX1600B is NOS-only. Emtron said so in 2005, and nothing has changed. Prices and provenance vary; an unconditioned tube of unknown storage is a gamble.
• The GU84B is the proven replacement, but it is not a drop-in. It needs a 27 V / 3.7 A heater against the 4CX1600B’s 12.6 V / 4.4 A, a different socket, a different chimney and different bias and screen set-points. Emtron’s own replacement module handled this as a package; Dan still lists a socket board for the GU74/GU84 family, but the filament supply has to be solved separately.^8,10
• The conversion changed the amplifier’s character. The owner who ran both reported the GU84B version was the better amplifier overall, with heavier-duty components, but with a softer HV supply under digital-mode duty.⁹

For an amplifier with a healthy tube, the answer is to keep it, run the heater slightly low as Svetlana suggests, use the EBS, keep the airflow clean and never let the screen exceed 350 V. For an amplifier with a dead tube, obtain Dan’s opinion on the specific serial number before choosing between an NOS 4CX1600B and a GU84B module.

8. Who Can Still Help

8.1 What to Put in the Email

• Model, serial number and the tube fitted (4CX1600B), plus whether the metering is analogue or LED.
• The AMPC board version from the silk-screen, and which soft-start board (earlier or late) is fitted.
• The six as-found measurements from Section 5.1.
• Clear photographs of the tube deck, the control board and the HV module.
• The specific question — a factory set-point, a part, or a conversion opinion. Dan’s time is free; make it efficient.

8.2 Spares Dan Currently Lists for the DX Range

New Version 7 control boards (compatible with all Emtron amplifiers built after 1996), display boards, early- and late-model soft-start boards and TRIACs, HV power modules, glitch-resistor modules, GU74/GU84 socket boards, QSK modules with vacuum relay, DX-1/DX-2/DX-3 blowers, plate chokes (1 in. OD × 6 in.), and screen-regulator protection boards. Band switches, variable capacitors and tubes are not stocked, but he can usually say where to look.¹⁰

8.3 Other Doors

• Emtron North America / Dishtronix handled US distribution and service in the 2000s; a 2018 forum thread found no authorised US service facility remaining.⁴
• The Amps reflector archive at lists.contesting.com holds Emtron’s own 2005 technical statement and the 1999 DX-2a owner report; both are searchable.
• eHam reviews for the DX-2 contain fifteen owner reports across both tube generations.
• This site — the DX-2 (GU84B) guide covers the modules shared with the DX-2a in far more depth: HV module service, safety capacitors, AMPC known failures, cabinet and hardware.

9. Safe Discharge Procedure — 4CX1600B Deck

  STEP 1 ── OPR/STBY to STBY; POWER to OFF.
               │
  STEP 2 ── DISCONNECT ALL REAR-PANEL LEADS.
            Mains cord, antenna, exciter coax, PTT, ALC.
               │
  STEP 3 ── Wait a minimum of 5 minutes.
            The bleeder string discharges ~58 uF from ~2.5 kV;
            do not rely on the panel meter alone.
               │
  STEP 4 ── ANODE DISCHARGE:
            Insulated probe, 10 kohm / 25 W series resistor,
            clip lead to chassis. Touch the AC-2 anode
            connector on top of the 4CX1600B. Hold 10 s.
               │
  STEP 5 ── HV MEASUREMENT:
            4,000 V-rated DMM, (+) to the HV filter string
            positive, (-) to chassis. CONFIRM < 50 V.
               │
  STEP 6 ── SCREEN CHECK:
            EG2 pin (blue wire) on the AMPC board to chassis.
            Confirm < 50 V. Remember the screen bypass
            capacitor lives inside the SK3A socket.
               │
  STEP 7 ── BIAS CHECK:
            Negative bias supply to chassis. Confirm < 10 V.
               │
  STEP 8 ── RE-VERIFY the HV string: < 10 V.
               │
  STEP 9 ── Now safe to work. One hand behind your back
            whenever a probe is near the tube deck.

Figure 1. Discharge sequence for a first-generation DX-2a. The anode connector and socket differ from the GU84B deck; the procedure does not.

10. Build Your Own Service File

Because no factory document describes the Generation 1 DX-2a, the owner has to assemble one. Done once, it turns every future fault into a comparison against known-good numbers instead of a guess.

1. Download the January 2003 PDF from radiomanual.info and the standalone FCC Schematics and Parts List exhibits. Keep all three; they do not fully overlap.
2. Record the six as-found measurements from Section 5.1 on the inside cover of the manual, dated.
3. Photograph the tube deck, the socket wiring, the AMPC board (both sides), the soft-start board and the HV module. Note every board version from its silk-screen.
4. Trace the metering: which meter reads what, through which shunt. Emtron had no drawing of the analogue metering on the sister DX-1b;¹³ assume none exists for the DX-2a and make your own sketch.
5. Note the mains tap in use, the fuse ratings and the date of the last tube change.
6. Write down Dan’s answers verbatim when they arrive. Those are the only factory numbers you will ever get.

11. Parts Sources & Reference Documents

• emtrondv.com — Dan Voican (former Emtron technician) — emtrondv.com — Advice, Version 7 control boards, soft-start, HV module, socket boards, QSK, blowers, plate chokes. Contact form only.
• Emtron DX-2 Operating Manual, January 2003 — radiomanual.info PDF / ManualsLib / mods.dk.
• FCC ID QYNDX-2 exhibits — fccid.io/QYNDX-2 — Users Manual, standalone Schematics PDF, Parts List / Tune-Up Info.
• Svetlana 4CX1600B application data — Technical Bulletin 20 (W6FR, 1995) and Technical Bulletin 35 (W5LAJ, 1998) — heater, screen, grid and cooling limits; SK3A socket; protection requirements.
• 4CX1600B tube sources (NOS only) — ask Dan first; then RF Parts Co. (rfparts.com) and the European Russian-tube dealers. Condition any NOS tube heater-only before applying HV.
• Mouser / DigiKey — Class Y2 and X2 safety capacitors, LMC555, TDA1085C equivalents, LM358, 1,000 pF/6 kV disc ceramics, 275 V MOVs, A106-class clamp diodes.
• Jennings TJ1A-26S — QSK vacuum relay as used in Emtron QSK modules; RF Parts Co. or Jennings direct.
• Companion guide — Emtron DX-2 Linear Amplifier: Restoration & Service Guide (GU84B) on this site.