Reader Question · from Rich, WA6KNW
Driving a Collins KWM-2A or 75S-3B with an Electronic Specialty Products DDS VFO
Q: “Does anyone have the manual for hooking up the Electronic Specialty Products DDS VFO (the N3ZI DDS-based unit) to a Collins radio — KWM-2A or 75S-3B?” — Rich, WA6KNW

Good question, Rich — and a popular one, because dropping a stable DDS in place of a drifting analogue PTO changes how one of these classics behaves on the bands. Here is where the documentation actually lives, the connection scheme for both radios, and the single setup detail that causes most of the grief.

First, Confirm What You Actually Have

Electronic Specialty Products offers two different Collins-oriented products, and they connect to completely different points[1]:

  • A digital frequency display — passively reads the existing PTO, HFO and BFO and shows you the operating frequency. It taps those nodes; it does not drive them.
  • A DDS local-oscillator / VFO — generates injection and replaces the analogue PTO (and optionally the HFO for general coverage).

Before you wire anything, be certain which unit is on the bench. A display hangs off the oscillator nodes; a VFO has to feed them. Connecting a VFO into display taps — or the reverse — is the first mistake to head off.

Where the Documentation Lives

The three sources worth having

  • ESP’s own Collins install page is the authoritative source for the unit itself: electronicspecialtyproducts.com/ddscollins.html[1]
  • The N3ZI synthesizer manuals. The unit is built around the N3ZI “DDS” synthesizer designed by Doug Pongrance, N3ZI; the base-synth documentation (encoder, RIT, VFO A/B, IF-offset and output-level details) lives on his site: pongrance.com/manuals.html[2]
  • The AOR DDS-2A manual. The closest fully-documented analogue — and an excellent reference even if you are not running one. It covers the KWM-2A, 75S-3B and 32S-3 explicitly and is a free download from AOR USA and on ManualsLib. Every one of these external DDS units follows the DDS-2A injection scheme.[3]

Connecting to the KWM-2A — the Easy One

The KWM-2A already has an external-VFO interface built in: J17 on the chassis — the same connector the Collins 312B-5 external PTO plugs into.[4] A properly designed external DDS VFO goes in there with no permanent modification to the transceiver. Switch the radio to external VFO and the DDS takes over the PTO function for both transmit and receive.[3] This is the clean, fully reversible route, which is one reason the KWM-2(A) is the friendliest Collins host for one of these units.

Connecting to the 75S-3B — Plan for a Receiver Mod

The S-Line receiver is less plug-and-play. The relevant nodes are the receiver’s VFO injection jack (J203) and the crystal / HFO injection jack (J205) on the chassis.[5] A display can simply tap those points. A VFO has to actually drive them, which means lifting the receiver’s internal PTO — and, if you want general coverage, the crystal HFO — out of the path so they are not fighting the DDS.

That receiver modification is exactly what the ESP (and AOR) S-Line instructions document.[3] Follow the specific steps written for your unit rather than improvising the lift. And if you are running a 32S-3 alongside for transceive, remember the transmitter and the DDS want the same VFO and HFO nodes — tee them deliberately, and bring the receiver back into alignment with everything connected.[5]

⚠ The detail that causes most of the trouble — injection level

Take the injection level from ESP’s documentation, not from the bare N3ZI defaults. The raw N3ZI synthesizer output is only on the order of a couple of hundred millivolts peak-to-peak — fine as a signal source, but well below what the Collins mixers want to see, which is why N3ZI documents an add-on buffer amplifier for exactly this purpose.[6] The correct level into J17 (or the S-Line injection jacks) is unit-specific.

Too little drive and you will chase phantom sensitivity and alignment faults that are not really there; too much and you invite spurs and mixer overload. Set it per the manual, then confirm against a known-good signal across all bands.

Quick Reference

Radio Connection point Mod required? Notes
KWM-2A J17 external-VFO connector None Same jack as the 312B-5; switch radio to external VFO
75S-3B VFO inj. jack (J203) + crystal inj. jack (J205) Yes Lift internal PTO/HFO; coordinate with 32S-3 if transceiving
Both Injection level per ESP doc — Bare N3ZI output low; outboard buffer amplifier required

If anyone reading this has a scanned copy of the ESP Collins install sheet, send it along and I’ll add it to the reference library here so the next person isn’t hunting for it. And Rich — once you have it running, I’d be glad to hear how the receive performance stacks up against the original PTO.

Sources, References & Credits

  1. Electronic Specialty Products — DDS for Collins S-Line / KWM installation page. electronicspecialtyproducts.com/ddscollins.html ↩
  2. Doug Pongrance, N3ZI — N3ZI Kits, DDS VFO synthesizer manuals and technical notes. pongrance.com/manuals.html ↩
  3. AOR Ltd. / AOR USA — DDS-2A External VFO Instruction Manual (External VFO for Collins KWM-2(A), 75S-, 32S-). Documents the no-modification KWM-2(A) install and the S-Line receiver modification. AOR USA download · ManualsLib · WA3KEY product notes ↩
  4. Collins Radio Company — KWM-2 / KWM-2A Instruction Book, external VFO power cable to J17 (interconnection notes). Via the Collins Collectors Association manual archive and worldradiohistory.com. ↩
  5. S-Line injection nodes (J203 VFO, J205 crystal/HFO) — per Collins 75S-family digital-display interconnection notes and Antonio Vernucci, I0JX, “Digital Frequency Display for the Collins S-Line” (qsl.net/i0jx). ↩
  6. N3ZI output level & buffer amplifier — N3ZI DDS technical notes (buffer-amp page) and the “A Digital VFO for Vintage Transmitters” article by Latta, frostburg.edu/personal/latta/ee/vfo, which documents the low bare-output level and a two-transistor buffer. ↩
Credits: Synthesizer design — Doug Pongrance, N3ZI. External-VFO injection scheme and S-Line modification reference — AOR Ltd./AOR USA (DDS-2A). Collins documentation — Collins Radio Company, via the Collins Collectors Association archive and worldradiohistory.com. S-Line injection detail — Antonio Vernucci, I0JX. Drive-level / buffer reference — Latta (frostburg.edu). Thanks to Rich, WA6KNW, for the question.
Mike Peace VK6ADA · r-390a.net Administrator

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