The Collins 70K-2 Permeability Tuned Oscillator is the frequency-determining heart of every Collins S-Line receiver, transmitter, and transceiver — and after 60+ years, virtually every surviving 70K-2 needs service.[1] The two most common symptoms are stiff or uneven tuning feel (dried lubricant on the lead screw and coil bore) and excessive warm-up drift exceeding several hundred hertz (aged capacitors and a degraded 1N34A diode inside the sealed PTO enclosure).[2]
The caution against opening a Collins PTO was reasonable in the 1960s when the hermetic seal was intact, the internal desiccant was still dry, and Collins had replacement PTOs readily available. None of this is true today.[3] Modern restorers routinely service 70K-2 PTOs with excellent results — the assembly is surprisingly forgiving if approached methodically. The reward is a PTO that tunes like silk and settles within a couple of hundred hertz inside 30 minutes, rather than the 2+ kHz drift over an hour that is typical of an unserviced unit.[2]
The 70K-2 PTO is used across the entire Collins S-Line and KWM-2 family. While minor circuit variations exist between early and late production units, the mechanical assembly — lead screw, permeability core, coil bore, shaft seals, and lubrication system — is fundamentally identical across all models.[4] The procedure described in this guide applies to every 70K-2 installation.
| Model | Type | PTO Output Range | Years | Notes |
|---|---|---|---|---|
75S-1 | Receiver | 2.5–3.5 MHz | 1958–1962 | First S-Line receiver |
75S-2 | Receiver | 2.5–3.5 MHz | 1960–1962 | Shortwave listener variant |
75S-3 / 3A | Receiver | 2.5–3.5 MHz | 1962–1968 | Most common S-Line receiver |
75S-3B / 3C | Receiver | 2.5–3.5 MHz | 1968–1978 | Late S-Line receiver |
32S-1 | Transmitter | 2.5–3.5 MHz | 1958–1962 | First S-Line transmitter |
32S-3 / 3A | Transmitter | 2.5–3.5 MHz | 1962–1978 | Most common S-Line transmitter |
KWM-2 / 2A | Transceiver | 2.5–3.5 MHz | 1959–1978 | Shares identical 70K-2 |
312B-4 / 312B-5 | Console | 2.5–3.5 MHz | 1962–1978 | External VFO for KWM-2 |
Stiff or uneven tuning: The original factory lubricant in the lead screw, core bore, and shaft bearings dries out and hardens over decades. This manifests as a tuning knob that requires excessive force, feels “gritty,” or has irregular drag across the tuning range. In severe cases the tuning mechanism can bind completely. Forcing a dry PTO risks scoring the precision lead screw threads and the coil bore surface — damage that is extremely difficult to repair.[1]
Excessive warm-up drift: A healthy 70K-2 should drift less than a few hundred hertz and stabilize within approximately 30 minutes. PTOs drifting 2 kHz or more that take an hour or longer to settle have aged capacitors — particularly C302 (1000 pF), C303 (3000 pF), and bypass capacitors C306/C309/C310 — that have become temperature-sensitive with age. The 1N34A germanium switching diode (CR301) is also a common drift contributor.[2]
Frequency jumps: Intermittent frequency steps of hundreds of hertz or more during operation typically indicate a capacitor with an internal fault that is RF-voltage-dependent, or a cracked solder joint on the delicate internal connections.[5]
Pinch-drive slippage: Before opening the PTO, check the split pinch roller assembly at the front of the tuning shaft. The oblong bushing on the front escutcheon can be rotated to raise or lower the shaft, adjusting pinch-roller pressure. This simple external adjustment resolves some “stiff tuning” complaints without opening the PTO.[2]
Lubricant Selection: White lithium grease is the most commonly recommended lubricant in the community. However, some experienced restorers report that Lubriplate-type greases can harden and bind over time.[6] Synthetic greases (e.g., Valvoline Synpower) have been used successfully as an alternative. Whichever lubricant you choose, use sparingly — just enough to produce smooth travel without excess that could migrate onto electrical contacts.[7]
⚠ Critical: The shaft shim stack determines axial play and end-stop alignment. If these parts are lost or reassembled in the wrong order, the PTO will not track correctly across its range. Lay them out sequentially and photograph them.
This is the core of the re-lubrication procedure. The sequence is always: clean first with denatured alcohol, then lubricate with fresh grease. Never apply new lubricant over old dried grease — it will produce an inconsistent, gritty action.[2]
The Golden Rule: Alcohol removes. Grease protects. The sequence is non-negotiable: clean first, then lubricate. Applying grease over old hardened lubricant creates an abrasive paste that accelerates wear on the precision lead screw.
If the PTO exhibits excessive drift or frequency jumps, capacitor replacement inside the sealed enclosure is strongly recommended. The original mica and bypass capacitors become temperature-sensitive and RF-voltage-dependent with age.[5] The 1N34A germanium diode (CR301) should also be replaced with a modern 1N4148 silicon type, which has lower capacitance and eliminates a common source of irregular frequency jumps.[5]
| Ref. Des. | Original | Replacement | Function | Priority |
|---|---|---|---|---|
C302 | 1000 pF mica | 1000 pF dipped silver mica | Tuned circuit — frequency-determining | High |
C303 | 3000 pF mica | 3000 pF dipped silver mica | Tuned circuit — frequency-determining | High |
C304 | 200 pF mica | 50 pF dipped mica (per Change 20B) | Grid-to-ground coupling | Medium |
C306 | 0.02 µF | 0.022 µF film | B+ bypass | Medium |
C309 | 0.02 µF | 0.022 µF film | B+ bypass | Medium |
C310 | 0.02 µF | 0.022 µF film | B+ bypass | Medium |
CR301 | 1N34A (Ge) | 1N4148 (Si) | USB/LSB switching diode | High |
C304 Change 20B: Per the 9th edition KWM-2 instruction book (Collins, 1978), C304 should be changed from 200 pF to 50 pF. C304 couples V301’s grid-to-ground capacitance into the tuned circuit. The reduction in capacitance causes a predictable frequency rise of approximately 900 Hz, which is absorbed during PTO end-point recalibration.[5]
⚠ Do not close the PTO when hot. A warm PTO will develop internal pressure as it cools, making the cover extremely difficult to remove again if the gasket seals well. Allow the PTO to reach room temperature before refitting the cover.[8]
A properly serviced 70K-2 PTO will restore the Collins S-Line to the smooth, accurate tuning that made these receivers famous. The difference between a dry, drifting PTO and a freshly serviced one is immediately obvious on the air — it transforms the entire operating experience.[1]
- Collins Reflector / QTH.net. 70K-2 PTO — Every S-Line receiver, transmitter or transceiver I’ve had I’ve done the PTO reconditioning. November 2007. qth.net — Collins Reflector
- Miller, Jim N4BE. 70K-2 Stability Service. Cleaning, re-lubrication, and capacitor replacement procedure for the Collins 70K-2 PTO. Collins Collectors Association. collinsradio.org — 70K-2 Stability Service
- Antique Radio Forums. Collins 75A-3 70E-12 PTO repair — update. Discussion of modern PTO service rationale — original factory cautions against opening no longer apply. February 2014. antiqueradios.com — 70E-12 PTO repair
- Collins Collectors Association — PTO Service and Specifications. Master index of PTO resources: 70E-15/51J PTO Service, 70K-2 Stability Service, 70E-8 PTO Rebuild, W5HTW Tutorial (Parts I–III), Emilio Ciardiello PTO Overview, and Service Bulletins SB1007/SB1011. collinsradio.org — PTO Service and Specifications
- Borowski, Gerd DJ7HS. Another Collins 70K-2 PTO Repair Story. Detailed capacitor-by-capacitor replacement in PTO #14289 with before/after drift measurements; C302 RF-voltage-dependent fault analysis; C304 Change 20B implementation; CR301 replacement with 1N4148. QSL.net/DJ7HS, 2018. qsl.net/dj7hs — Another 70K-2 PTO Repair
- Antique Radio Forums. Recommended Collins PTO Lubricant. Discussion of Lubriplate hardening problems; alternative lubricant experiences. February 2016. antiqueradios.com — PTO Lubricant
- 70K-2 PTO Lubrication & Assembly. Collins Collectors Association. Practical experience report on lubrication, pinch roller reassembly challenges, and three-target dial calibration convergence. collinsradio.org — 70K-2 PTO Lubrication & Assembly
- Malinen, Jarmo OH5IY. Repair and calibration of the Collins PTO 70E-15. Gasket lubrication with silicone oil, pressure warning when closing hot, post-service aging requirements. jarmomalinen.fi — 70E-15 PTO Repair
- Borowski, Gerd DJ7HS. A Collins 70K-2 PTO Repair Story. C302 mica capacitor failure producing 30 kHz frequency shift; RF-voltage-dependent fault diagnosis. QSL.net/DJ7HS, 2018. qsl.net/dj7hs — 70K-2 PTO Repair Story
- W2PA. Collins S/Line Restoration Notes. Complete chassis cleaning, PTO handling during restoration, Krud Kutter technique. w2pa.com — S-Line Restoration Notes
- White, Gary W5GW. A Collins S-Line Troubleshooting and Repair Adventure. Comprehensive S-Line restoration including PTO service. w5gw.com — S-Line Repair Adventure (PDF)
- Collins Collectors Association — RX For Your Collins. Master index of S-Line technical articles, PTO tracking and accuracy procedures, frozen trimmer repair, and general maintenance resources. collinsradio.org — RX For Your Collins
- Dennis Brothers / Soo, Floyd W8RO. Collins 75S-3 and 32S-3 Repair DVD. Four-hour video including 70K-2 PTO removal and repair procedure. Electric Radio Magazine. ermag.com — S-Line Repair DVD
- Zook, Glen K9STH. Repairing Frozen Ceramic Trimmers in Collins S-Line / KWM-2. Trimmer disassembly, cleaning, and reassembly procedure — applicable when C308 trimmer is seized during PTO alignment. Collins Collectors Association. collinsradio.org — Frozen Trimmers (PDF)
- Collins Service Bulletin SB1011 — S-Line PTO Dial Assembly Instructions. June 26, 1963. Official Collins procedure for dial removal and replacement. collinsradio.org — SB1011 (PDF)
Jim Miller, N4BE — For the foundational 70K-2 Stability Service article that has guided hundreds of PTO restorations. The denatured alcohol cleaning and white lithium grease lubrication sequence documented by N4BE remains the standard community procedure.
Gerd Borowski, DJ7HS — For two meticulously documented PTO repair stories with before/after drift measurements, capacitor-by-capacitor replacement analysis, and the critical insight that C302 failures can be RF-voltage-dependent.
Collins Collectors Association (CCA) — For maintaining the master PTO reference library including the N4BE procedure, W5HTW three-part tutorial, Emilio Ciardiello PTO Overview, annotated schematics, and Service Bulletins SB1007/SB1011.
Jarmo Malinen, OH5IY — For documenting 70E-15 PTO gasket handling and the critical warning about pressure buildup when sealing a hot PTO.
W5HTW — For the comprehensive three-part PTO assembly tutorial published through the CCA, providing the most detailed photographic reference for 70K-2 disassembly and reassembly.
Glen Zook, K9STH — For the frozen ceramic trimmer repair procedure that is essential when C308 is seized during PTO alignment.
Dennis Brothers & Floyd Soo, W8RO — For the four-hour Collins S-Line repair video including PTO service demonstration.