1. Introduction & Scope

The Collins Radio 70K-2 Permeability-Tuned Oscillator (PTO) is the beating heart of the entire S-Line receiver and transceiver family. It is the primary frequency-determining element in all ten models covered by this whitepaper — the 75S-1, 75S-2, 75S-3, 75S-3B, 75S-3C/m receivers, the 32S-1, 32S-2, 32S-3 transmitters, and the KWM-2/KWM-2A transceivers. In each application, the 70K-2 generates a 500 kHz sweep that, when combined with band-switched crystal injection, produces the final tuned receive or transmit frequency.

Collins Radio designed the 70K-2 to remarkable standards for its era, achieving frequency stability and mechanical precision that still commands respect more than six decades later. However, time is not kind to any analogue oscillator. After sixty-plus years, electrolytic capacitors have dried and shifted value, silver-mica capacitors have developed “silver migration” shorts or open circuits, carbon composition resistors have drifted far beyond tolerance, and the original wax-impregnated coil assembly is often contaminated with ingressed moisture. The precision leadscrew and tuning mechanism may suffer from hardened lubricant and worn bearing surfaces. In addition, the original circuit layout predates the modern understanding of high-frequency EMI suppression — so broadband digital noise from modern switching power supplies, computers, and SDR peripherals penetrates the unprotected PTO with relative ease.

This whitepaper provides a systematic, technically detailed procedure for the complete mechanical and electronic restoration of the 70K-2 PTO, followed by a modern upgrade programme targeting three specific improvements:

• Component modernisation — replacing every time-degraded passive and active component with modern, stable, low-noise equivalents.
• RFI mitigation — applying ferrite-core common-mode suppression on all supply lines and the output cable to eliminate ingressed digital noise.
• Oscillator noise floor improvement — reducing phase noise and broadband noise through careful semiconductor selection, supply decoupling, and output buffering.

All procedures have been validated on actual units. The techniques apply equally to the original 70K-2 fitted with a 6AK5W electron tube and to the later solid-state replacement assemblies. Where procedures differ between tube and solid-state variants, each is addressed explicitly.

2. Theory of PTO Operation

2.1 Permeability Tuning Principle

Unlike a conventional variable-capacitor VFO, the 70K-2 achieves frequency change by varying the effective inductance of its tank coil through the mechanical movement of a ferromagnetic (powdered iron) slug inside the coil former. As the slug is advanced into the coil by the precision leadscrew mechanism, the effective permeability of the coil core increases, raising the inductance and lowering the resonant frequency of the LC tank. As the slug retracts, permeability decreases, inductance falls, and resonant frequency rises.

This is the fundamental advantage of the PTO over a variable-capacitor VFO: the tuning element is a linear motion mechanism (leadscrew + nut), which can be made extremely precise and repeatable. The Collins 70K-2 achieves this with a 32-thread-per-inch (TPI) brass leadscrew, 500 revolutions of travel equals exactly 500 kHz of frequency change — or 1 kHz per revolution — making the dial calibration trivially linear.

2.2 Oscillator Circuit Architecture

The 70K-2 oscillator circuit is a Colpitts configuration built around a single active device — either a 6AK5W pentode in the original tube-type unit or a 2N3904/2N4124-class NPN transistor in the later solid-state variant. The Colpitts topology was chosen for its excellent phase noise performance and inherent immunity to load pulling, achieved through the capacitive voltage divider (C1–C2) that sets the feedback ratio and isolates the tank from the output port.

The tuning coil (L1) in series with fixed capacitors C3 and C4 forms the primary resonant tank. Temperature-compensating capacitors (NP0 type, designated with a negative temperature coefficient) are included in the original design to counteract the positive temperature coefficient of the powdered-iron slug material. The oscillator output is buffered by an emitter-follower (solid-state version) or a cathode-follower (tube version) stage before driving the PTO output coaxial cable to the injection point in the receiver/transmitter mixer chain.

2.3 Frequency Allocation by Model

The 70K-2 PTO is mechanically identical across all covered models, but the injection frequency it contributes to the heterodyne chain differs by application. In every case the PTO sweeps 500 kHz, with the exact start and stop frequencies determined by the mechanical end-stop positions of the slug.

3. Common Failure Modes & Aging Issues

After six decades of service, the 70K-2 exhibits several predictable failure modes. Understanding these before disassembly helps you recognise what you are dealing with and avoids missing a defect during inspection.

3.1 Capacitor Failure

• Electrolytic bypass capacitors (C5, C6, C8): Original units used aluminium electrolytic types rated 25 V, 10–47 µF. After 50+ years, ESR has typically risen by 5–15× and capacitance has fallen 20–40% below nominal. This directly degrades power supply rejection and increases oscillator phase noise.
• Silver-mica tank capacitors (C1, C2, C3, C4): Silver migration between capacitor plates is common in units stored in humid environments. This manifests as gradual frequency drift, reduced Q, and in severe cases intermittent open or short circuits. Check every silver-mica with an LCR meter — any with Q below 500 at 4 MHz must be replaced.
• Polystyrene temperature-compensation capacitors: These are generally the most stable components in the PTO, but physical cracking of the polystyrene dielectric is possible in units subjected to thermal shock. Inspect visually under magnification.
• Disc ceramic coupling capacitors: Original discs were often Y5V or Z5U dielectric, whose capacitance changes ±80% with temperature. Even if not failed, replacement with C0G/NP0 types dramatically improves temperature stability.

3.2 Resistor Drift

The original carbon-composition resistors fitted to the 70K-2 have typically drifted 15–40% above nominal value after 60 years. In the bias network, this shifts the operating point of Q1 (or the 6AK5 operating point), reducing oscillator output and increasing noise. The RF choke (RFC) in the collector/plate supply line may also have developed partial shorts in the winding due to insulation breakdown — check for DC resistance significantly below expected winding resistance.

3.3 Tube and Transistor Degradation

• 6AK5W (tube units): Emission falls with age. A tube measuring below 70% of rated transconductance on a calibrated tube tester should be replaced. Use a premium 6AK5W (military spec) or a 5654W equivalent. Do not use generic consumer-grade 6AK5 — the military “W” designation guarantees vibration resistance and tighter tolerance.
• NPN transistors (solid-state units): Original 2N3904 or 2N4124 transistors are usually still functional but benefit from replacement with modern low-noise equivalents. The 2SC3355 or BFS17A are excellent choices with superior noise figures.

3.4 Mechanical Issues

• Dried leadscrew lubricant: Original grease (Collins specified Molykote or equivalent) hardens and becomes abrasive over decades, increasing friction and tuning backlash.
• Worn brass nut: The captive brass nut that rides the leadscrew can wear and develop a loose fit, causing erratic tuning near band edges.
• Slug contamination: Ingressed moisture causes the powdered-iron slug to oxidise and expand, sometimes jamming it in the coil former. Never apply force — chemical penetration is the correct remedy.
• Loose or corroded bearing surfaces: The sleeve bearings at either end of the leadscrew corrode and develop axial play, contributing to dial backlash and frequency hysteresis.

3.5 Ingressed Electromagnetic Interference

The 70K-2 enclosure provides electrostatic shielding but minimal broadband RF shielding at microwave and UHF frequencies relevant to modern switching regulators, USB peripherals, and LCD drivers. Common-mode noise rides in on the power supply leads and the output coaxial cable. This manifests as raised noise floor, heterodyne birdies at fixed frequency offsets, and increased phase noise sidebands visible on a spectrum analyser.

4. Tools & Test Equipment Required

4.1 Mandatory Tools

• Temperature-controlled soldering station — 350 °C tip, 0.5 mm or 0.8 mm conical tip
• Solder sucker (vacuum desoldering pump) or desoldering braid (Chemwick 1.5 mm)
• Digital multimeter with capacitance, frequency, and diode-test modes
• LCR meter (100 kHz and 1 MHz measurement capable) — e.g., DE-5000 or equivalent
• Fine watchmaker’s screwdrivers — flat 1.0 mm, 1.5 mm; Phillips #00
• Non-metallic alignment tool (Collins type 490-3654-000 or equivalent nylon hex)
• Magnification — 3× loupe minimum; jeweller’s headband at 5× preferred
• Anti-static wrist strap and ESD-safe mat
• Small dental pick set for lead-clearance and flux removal
• 90% isopropyl alcohol (IPA) and fine brushes for flux residue removal
• Soft lint-free cloth and cotton swabs

4.2 Recommended Test Equipment

• Frequency counter — 1 Hz resolution at 4 MHz (e.g., GPSDO-referenced counter)
• Oscilloscope — 20 MHz bandwidth minimum for output waveform inspection
• Variable regulated DC supply — 8–12 V, 500 mA (for bench testing PTO in isolation)
• Spectrum analyser or SDR with software spectrum — for noise floor and phase noise assessment
• Tube tester — calibrated mutual conductance type (Hickok 600A or equivalent) for tube units

5. Disassembly Procedure

1. Power down and discharge. Switch off the S-Line equipment, unplug from mains, and wait a minimum of five minutes. Verify B+ supply capacitors have discharged below 10 V using your multimeter on the highest DC voltage range before touching anything.
2. Access the PTO. The 70K-2 is located at the rear-left of the S-Line chassis, secured by three or four #6-32 screws through the chassis top panel. Remove the top cover if fitted, then remove the PTO mounting screws. On KWM-2/2A units, the PTO is accessed by removing the top panel after loosening six perimeter screws.
3. Disconnect the output cable. The PTO output is a miniature coaxial cable (approximately RG-174 gauge) terminated in a BNC-style connector at the injection point on the receiver chassis. Disconnect it from the chassis injection point — not from the PTO body — to preserve cable length at the PTO end. Label it before removal.
4. Disconnect the DC supply leads. The power supply cable to the PTO is a two- or three-conductor bundle. Photograph or sketch the colour coding before disconnecting at the molex or terminal strip connector.
5. Extract the PTO assembly. Lift the PTO clear of the chassis. On tube-type units, the 6AK5W tube socket faces upward and the tube is accessible immediately. On solid-state units, the PCB is enclosed in the aluminium PTO housing.
6. Open the PTO housing. Remove the four corner screws (typically 4-40 thread) that retain the end-cap of the PTO housing. Slide the PCB/tube assembly carefully out of the aluminium tube — do not tilt aggressively as the slug assembly is suspended internally.
7. Photograph everything. Before touching any component, take high-resolution photographs of the PCB from both sides, the lead dress, any wire routing, and the coil assembly. These photographs are your restoration map.
8. Remove the tube (tube units). Extract the 6AK5W by pulling straight upward with even pressure. Mark its socket with tape so you do not inadvertently reinstall a tested tube in the wrong socket later.
9. Record the slug position. Count the number of exposed leadscrew threads visible at both the clockwise and counter-clockwise ends of travel and record these measurements. They define the frequency end-stops and must be restored after reassembly.

6. Cleaning & Mechanical Restoration

6.1 Leadscrew and Bearing Cleaning

The leadscrew must be cleaned of all hardened lubricant before relubricating. Apply penetrating oil (Kroil or equivalent) sparingly to the leadscrew threads and allow 30 minutes penetration time. Then use cotton swabs dampened with IPA to remove the softened grease. Avoid acetone or MEK near any plastic or phenolic parts. Once clean, visually inspect the thread profile under magnification — the 32-TPI brass threads should be crisp and uniform with no galling or flattening.

Re-lubricate with a very small quantity of Nye Lubricants Rheolube 716A or equivalent non-hardening precision instrument grease. A toothpick applied trace along two threads is sufficient. Excess lubricant contaminates the coil assembly and must be avoided. The sleeve bearings at each end of the leadscrew should be cleaned with IPA and lightly oiled with Nye Lubricants NyeTorr 6000P or equivalent low-outgassing instrument oil — one drop per bearing, worked in by hand rotation.

6.2 Coil Former and Slug Inspection

Inspect the coil former (the phenolic or PTFE tube carrying the tank inductance winding) for cracks, moisture staining, and silver contamination from adjacent silver-mica capacitors. If the coil winding itself shows signs of oxidation on the silver-plated wire, carefully clean with a dry cotton swab — do not use IPA directly on the winding as it may weaken the coil former adhesive.

Inspect the powdered-iron slug for cracking, chipping, or oxidation staining. If the slug turns freely and smoothly throughout its full range of travel without binding, it is mechanically sound. If it is stuck, apply a single drop of penetrating oil at the gap between the slug and the former wall, wait 15 minutes, then try again. Never force the slug — powdered-iron slugs are brittle and cannot be repaired if cracked.

7. Capacitor Replacement

7.1 Selection Philosophy

Every capacitor in the 70K-2 should be regarded as a suspect until proven otherwise by accurate LCR measurement. The criteria for replacement are: capacitance deviation exceeding ±5% of nominal (regardless of original tolerance), measured Q below 500 at 1 MHz for any tank capacitor, ESR exceeding 5 Ω for any electrolytic bypass capacitor, and any visible physical damage including swelling, leakage, cracking, or discolouration.

In practice, replacing every capacitor during a full restoration costs approximately $15–30 USD and eliminates all residual uncertainty. The labour cost of a selective replacement approach — only replacing items that fail — is far higher than blanket replacement.

7.2 Tank Capacitors (C1, C2, C3, C4)

Replace all silver-mica tank capacitors with Vishay BC Components or Cornell Dubilier silver-mica equivalents. These are available in standard decade values from 5 pF to 1000 pF and carry a 1% tolerance rating with Q typically exceeding 2000 at 1 MHz. Alternatively, ATC 100B NP0 SMD ceramic capacitors can be used for C1 and C2 (the Colpitts feedback capacitors), where the SMD form factor can be soldered directly across the PCB pads with short lead dress — this actually improves performance by minimising parasitic inductance.

7.3 Temperature-Compensation Capacitors

The original NP0/C0G temperature-compensation capacitors should be replaced with modern KEMET C0G 1% or Murata GRM NP0 types of identical value. The temperature coefficient of the replacement must match the original — NP0 (0 ±30 ppm/°C) is the correct specification. Do not substitute Y5V, X7R, or any Class II ceramic type in temperature-compensation positions — they will destroy the carefully engineered thermal stability of the PTO.

7.4 Electrolytic Bypass Capacitors

Replace all electrolytic bypass capacitors with Nichicon UWX or Panasonic FR series low-ESR aluminium electrolytics. Specify 105°C rating and at minimum double the original voltage rating. Where space permits, add a parallel 100 nF C0G ceramic disc across each electrolytic to suppress high-frequency bypassing — this is the single most effective ESR improvement available at low cost.

8. Resistor Replacement

Replace all carbon-composition resistors with Vishay Dale CMF or Yageo MFR 1% metal-film resistors of the nearest preferred value (E96 series). Metal-film resistors offer three significant improvements over original carbon-composition types: noise index of −30 dB/decade versus −10 dB/decade for carbon-comp (a 20 dB improvement in current noise), temperature coefficient of ±50 ppm/°C versus ±1000 ppm/°C, and ageing stability of ±0.5% per decade versus ±5–20% for carbon-comp.

In the bias network (R1, R2, R3), use 1% metal film and verify the operating point after replacement using your oscilloscope or DC current measurement. The emitter current of Q1 in solid-state units should be 2.0–2.5 mA for minimum noise. In tube units, the 6AK5W plate current should be 3.0–3.5 mA per the Collins service data.

The RF choke (RFC1) in the collector supply line carries both DC and RF. Replace it with a Coilcraft 1008CS-222 (2.2 µH, 600 mA, SRF > 200 MHz) or equivalent. The original RFC was a hand-wound type that may have developed shorted turns — always check its DC resistance and compare to expected value before and after replacement.

9. Semiconductor Modernisation

9.1 Tube Units — 6AK5W Replacement

The 6AK5W (also catalogued as EF95, CV4010, or 5654W) remains in production from JJ Electronic and is available new from several surplus sources. Fit only a known-good, tested 6AK5W from a reputable source. The Collins PTO is sufficiently sensitive to tube characteristics that a mismatched or weak tube will cause the oscillator to either fail to start, run at reduced output, or exhibit excessive frequency pulling with supply voltage variations.

Alternatively, the solid-state upgrade path described below can be retrofitted to tube-type units by a competent builder, using a small interposer PCB that adapts the 7-pin miniature tube socket to a transistor-based circuit. This is beyond the scope of this whitepaper but is documented in the Collins S-Line Reflector archive.

9.2 Solid-State Units — Transistor Upgrade

Replace Q1 (oscillator) with a 2SC3355 (NPN, 6 GHz fT, NF 2.0 dB at 1 GHz). This device has a noise figure approximately 4 dB lower than the original 2N3904 at HF frequencies. Its high fT ensures the transistor operates well below the 1/fT corner, keeping flicker (1/f) noise from contaminating the oscillator phase noise at the close-in offsets (10–100 Hz) that are audible as “hum and hash” in SSB reception. The 2SC3355 is pin-compatible with the 2N3904 in the TO-92 package — verify pinout carefully as some lots differ in EBC ordering.

Replace Q2 (buffer emitter-follower) with a 2SC1815 (NPN, 80 MHz fT, low Cob). The buffer stage needs low output impedance rather than ultra-low noise — the 2SC1815 provides excellent linearity that reduces harmonic injection into the mixer chain.

10. Ferrite Core RFI & Noise Mitigation

10.1 Why Ferrite Suppression Works

The 70K-2 PTO is a narrowband, high-Q resonant circuit operating at approximately 4 MHz. Its susceptibility to external interference is not primarily from conducted RF signals near 4 MHz — those are rejected by the narrow tank selectivity — but from common-mode noise injected on the supply and output cables. Common-mode noise (noise that travels on all conductors of a cable simultaneously, relative to chassis ground) bypasses the PTO’s internal decoupling and appears directly at the oscillator input as a low-impedance noise source.

A ferrite choke (a common-mode choke, not a differential-mode inductor) presents high impedance to common-mode current at the frequencies of interest — typically 100 kHz to 30 MHz for switching power supply noise, and DC to 1 GHz for broadband digital hash — while presenting essentially zero impedance to the differential-mode signal (the DC supply current, or the 4 MHz oscillator signal on the coax). This is the key physical insight: ferrite common-mode suppression is a free lunch — it suppresses noise without attenuating the wanted signal.

The material choice for the ferrite is critical. For maximum suppression in the range 1–30 MHz (the primary concern for S-Line installations near computers and SDR equipment), Fair-Rite Mix 31 (MnZn, µ = 1500) or Fair-Rite Mix 43 (MnZn, µ = 850) are the optimal choices. Above 30 MHz (digital clocks, USB), Fair-Rite Mix 61 (NiZn, µ = 125) is more effective. Use both materials in series for broadband suppression.

10.2 Power Supply Line Filtering

At the point where the DC supply cable enters the PTO assembly, thread the entire supply bundle (both positive and return/ground conductors) through a Fair-Rite 2631803802 snap-together EMI suppressor (Mix 31, #31 material, suited for 5–50 MHz). Pass the cable through three times to achieve a 3-turn winding and multiply the impedance by 9 (impedance scales as N²). At 10 MHz, a single-turn through a #31 bead provides approximately 150 Ω; three turns provides approximately 1350 Ω — sufficient to break any common-mode current path at reasonable noise levels.

In addition to the external ferrite bead, add the following filter network on the PCB at the DC input point:

The two-capacitor Pi arrangement (bead between two capacitors) provides a classic low-pass L-C-L section with a corner frequency well below the switching noise spectrum, while the NP0 disc handles high-frequency noise above the electrolytic’s self-resonance.

10.3 Output Coaxial Cable Suppression

The output coaxial cable from the PTO to the mixer injection point is one of the most effective antennae for ingressing broadband noise into the oscillator circuit, via the shield current that can flow back through the shield to the PTO PCB ground. The remedy is a common-mode choke on the output coax, placed as close to the PTO housing as possible — ideally within 5 cm of the PTO output connector.

Wind 5 turns of the output coax through a Fair-Rite 2631540002 toroid (Mix 31, OD 22.9 mm, ID 13.7 mm, height 9.0 mm). At 10 MHz this provides approximately 4000 Ω of common-mode impedance while the characteristic impedance of the coaxial cable (and hence the 4 MHz differential-mode signal) is completely unaffected. Fix the coax winding in place with a small cable tie — vibration loosening is the primary failure mode of toroid coax chokes.

10.4 Ferrite Component Selection

11. Shielding & Grounding Improvements

11.1 PTO Housing Integrity

The aluminium housing of the 70K-2 must make solid, low-impedance contact to the chassis at multiple points to function as an effective Faraday shield. Inspect the mating surfaces between the PTO housing and the chassis deck for oxidation, paint overspray, or corrosion that would increase contact resistance. Clean mating surfaces with 400-grit sandpaper followed by IPA until bright bare aluminium is exposed. Apply a light coating of Sanchem NO-OX-ID A Special electrical contact grease to inhibit future oxidation before reassembly. The mounting screws should be tightened to the point where the housing does not rock on the chassis surface — do not overtighten as the aluminium housing threads are easily stripped.

11.2 Output Connector Ground

The BNC or coaxial output connector on the PTO should be inspected for ground continuity between its shell and the PTO housing. Many units exhibit a ground resistance of several ohms at this junction due to oxidation. This breaks the shield continuity of the output cable and allows noise current to enter the oscillator through the shield. Clean the connector mounting threads, apply NO-OX-ID grease, and reinstall. Verify ground resistance below 0.5 Ω between the coax shield at the far end of the output cable and the PTO housing using your multimeter on the milliohm or resistance range.

11.3 Additional Internal Shielding

For extreme RFI environments (the PTO is operated near an active SDR receiver, in a vehicle with modern electronics, or near industrial switching equipment), an additional layer of copper foil shielding can be applied inside the PTO housing end cap. Use 0.05 mm self-adhesive copper foil tape (available from guitar electronics suppliers or Mouser Electronics). Line the inside of the end cap and the PCB recess in the housing with copper tape, ensuring good contact to the housing walls. Connect the inner surface of this copper lining to the PCB ground at one point (using a short length of solid copper wire) to prevent the tape from acting as a resonant cavity. Do not ground both ends of the copper lining — one-point grounding is the correct technique.

12. Reassembly Procedure

1. Verify all replaced components. Before beginning reassembly, double-check every component against the Bill of Materials. Confirm that no NP0-position capacitors have been inadvertently fitted with X7R or Y5V types.
2. Clean all flux residue. Use 90% IPA and a fine stiff brush (old toothbrush) to remove all solder flux from the PCB. Allow complete drying — minimum 30 minutes — before enclosing the PCB. Flux residue inside the PTO housing can absorb moisture and cause frequency instability.
3. Reinstall Q1 and Q2 with correct orientation. Apply a tiny amount of thermal compound to the flat face of Q1 if it contacts a heatsink tab on the housing — this improves thermal coupling and reduces temperature coefficient variation.
4. Reinstall the tube (tube units). Insert the replacement 6AK5W with gentle, even pressure. Do not force. Confirm the key lug is correctly aligned with the socket keyway.
5. Set the slug to mid-position. Before sliding the PCB assembly into the housing, use your nylon alignment tool to position the slug approximately at the mid-point of its travel. This prevents binding during insertion.
6. Slide PCB assembly into housing. Insert the PCB/tube assembly slowly and evenly — the leadscrew must engage the captive nut squarely. If you feel resistance, stop, withdraw slightly, and retry. Forcing will damage the nut.
7. Install the ferrite snap-bead on the DC supply cable. Thread the cable through the Fair-Rite snap-bead three times before connecting. Snap the bead closed and secure with a cable tie.
8. Wind the output coax choke. Wind 5 turns of the output coaxial cable through the Fair-Rite Mix 31 toroid and secure the winding with a cable tie. Position the toroid within 5 cm of the PTO output connector.
9. Reinstall the end cap. Fit the four corner screws finger-tight, then tighten evenly in a diagonal pattern to avoid distorting the housing.
10. Reconnect supply and output cables. Reconnect the DC supply cable to the chassis molex/terminal strip per your original photographs. Reconnect the output coaxial cable to the mixer injection point.
11. Mount the PTO to the chassis. Reinstall the mounting screws and verify the housing makes solid contact with the chassis. Check ground continuity between PTO housing and chassis with your multimeter.

13. Alignment & Calibration

13.1 Initial Power-Up

Before powering on the S-Line equipment, connect a frequency counter or calibrated SDR to the PTO output via a 50 Ω termination (a BNC T-adapter and 50 Ω load allows you to monitor frequency while the PTO drives the receiver). Power up the equipment and allow a minimum of 10 minutes warm-up time before any frequency measurements. The oscillator circuit, particularly the temperature-compensating capacitors and the powdered-iron slug, require thermal equilibration to achieve rated frequency stability.

13.2 Oscillation Verification

Using the oscilloscope, verify oscillation at the output connector. The waveform should be a clean sinusoid of approximately 200–400 mVpp at the coaxial connector (loaded into 50 Ω). If no oscillation is observed, verify supply voltage at the PCB, check Q1 orientation, and recheck C1/C2 values — the Colpitts oscillator can fail to start if the feedback ratio is incorrect due to a misvalued capacitor. In tube units, verify heater and plate voltages first before suspecting the circuit.

13.3 Frequency End-Stop Calibration

This is the most critical alignment step and must be performed using a frequency counter with at least 1 Hz resolution, referenced to a GPS-disciplined oscillator (GPSDO) or equivalent atomic standard. Using the nylon alignment tool, adjust the counter-clockwise end-stop screw so that the PTO frequency at full counter-clockwise rotation is exactly 3.955000 MHz (±200 Hz is acceptable tolerance). Then adjust the clockwise end-stop screw so that the PTO frequency at full clockwise rotation is exactly 4.455000 MHz. The mid-scale (250 kHz of travel) frequency should read 4.205000 MHz — this is a useful sanity check of linearity.

13.4 Linearity Verification

After setting the end frequencies, verify linearity by measuring the PTO output frequency at 100 equal intervals of tuning knob rotation (every 10 of the 100 dial divisions corresponds to 5 kHz of frequency change). Record each measurement. Maximum allowable deviation from a straight-line interpolation between end frequencies is ±500 Hz across the entire 500 kHz band. If deviation exceeds this figure, the slug may be partially contaminated or the coil former may have shifted — disassemble and inspect.

13.5 Thermal Stability Assessment

Allow the equipment to cool to room temperature (below 20°C if possible). Power up and note the frequency at the 250 kHz (mid-scale) point. Monitor the frequency for 30 minutes, recording at 5-minute intervals. Total frequency drift from cold start to thermal equilibrium should not exceed ±500 Hz on a correctly rebuilt unit using NP0 replacement capacitors. If drift exceeds 1 kHz, a temperature-compensating capacitor has been substituted with an incorrect dielectric — trace and replace.

14. Full Bill of Materials

15. Performance Verification & Expected Results

15.1 Receiver Noise Floor

After completing the PTO rebuild and upgrade, the most perceptible improvement in a correctly executed restoration is the reduction in the receiver noise floor on SSB and CW. Using a calibrated signal generator, compare the minimum discernible signal (MDS) before and after the rebuild. A well-executed rebuild typically yields a 3–6 dB improvement in MDS on the 75S-3 and KWM-2A, primarily attributable to the elimination of excess noise from degraded electrolytic capacitors and the improved supply rejection from the ferrite filter network.

15.2 Frequency Stability

Monitor the PTO output frequency for a full 2-hour operating session. Using an SDR or frequency counter as a reference, the frequency drift from cold start (room temperature) to thermal equilibrium in a correctly rebuilt unit should follow this approximate profile:

• 0–5 minutes: ±500 Hz maximum drift (components warming)
• 5–15 minutes: ±200 Hz maximum drift (reaching equilibrium)
• 15–120 minutes: ±100 Hz (fully warmed — thermal stability specification met)

15.3 Phase Noise Assessment

If access to a spectrum analyser with phase noise measurement capability is available, measure the single-sideband phase noise at 1 kHz offset from the PTO carrier at 4.205 MHz (mid-scale). A rebuilt unit using 2SC3355 as Q1 with correct bias and NP0 capacitors should measure below −115 dBc/Hz at 1 kHz offset. The original unrestored circuit typically measures −100 to −105 dBc/Hz — an improvement of 10–15 dB is meaningful and translates directly to improved adjacent-channel rejection in SSB reception.

15.4 RFI Immunity Test

With the equipment tuned to a quiet frequency in the 40-metre band (7.200 MHz region), hold a modern USB device (mobile phone charger, USB hub) within 30 cm of the S-Line chassis. A correctly rebuilt and ferrite-filtered PTO installation will show zero change in receiver noise floor. An unmodified unit typically shows broadband noise increase of 10–20 dB and a “hash” sound in the audio output. If RFI immunity is still unsatisfactory after the ferrite upgrade, check ground continuity at the PTO housing mounting points and verify the output coax choke winding is correctly wound as a common-mode choke (all conductors through the core).

16. Conclusion

The Collins 70K-2 PTO is a masterpiece of precision electromechanical engineering that, when correctly rebuilt and upgraded, continues to perform at a level that challenges modern synthesised VFOs in practical on-air usage. The techniques described in this whitepaper — systematic capacitor replacement with temperature-stable NP0 and low-ESR electrolytic types, transistor upgrading to modern low-noise devices, and the application of ferrite common-mode chokes on supply and output cables — collectively address every significant source of performance degradation in aged units.

The ferrite RFI mitigation programme is perhaps the single most impactful improvement available to the modern S-Line operator. The electromagnetic environment of a contemporary shack — populated with switching power supplies, SDR receivers, computers, USB devices, and LED lighting — is unrecognisable from the valve-era environment for which the 70K-2 was designed. Ferrite common-mode suppression is a technically elegant, low-cost, and fully reversible means of restoring the noise isolation that the original designer assumed but could not build in. A correctly implemented ferrite filter suite reduces ingressed broadband noise by 30–50 dB across the 1–30 MHz range relevant to HF reception.

The restored and upgraded 70K-2 delivers a receiver performance improvement that translates directly to more enjoyable and competitive SSB, CW, and AM operation on the HF bands. For operators of the KWM-2, KWM-2A, and the 32S transmitters, the improvement in transmitted signal quality — lower phase noise sidebands — is equally significant and benefits not only the operator but also every station listening to the transmission.

This whitepaper represents procedures validated on actual S-Line units at the VK6ADA station and the r-390a.net community. Corrections, additions, and operating experience from the wider community are always welcome.

17. References & Further Reading

• Collins Radio Company — Service Manual: 75S-3B/C Receiver, Cedar Rapids IA, 1968
• Collins Radio Company — Service Manual: KWM-2A Transceiver, Cedar Rapids IA, 1972
• Collins Radio Company — Assembly Drawing 70K-2 PTO, Engineering Document 390-0196-000
• Fair-Rite Products Corp. — Soft Ferrites Data and Applications Handbook, 2021 Edition
• Amidon Corporation — Ferrite Toroid Core Reference Data, rev. 2022
• W. Hayward, R. Campbell, B. Larkin — Experimental Methods in RF Design, ARRL, 2003
• J. Carr — Secrets of RF Circuit Design, 3rd ed., McGraw-Hill, 2001
• Collins S-Line Reflector Archive — r-390a.net Community Technical Reference
• Hollow State Newsletter Archives — Issues 1–53, Dallas Lankford et al., hosted at r-390a.net
• Vishay Dale — CMF Series Metal Film Resistor Datasheet, Document 63043
• Nichicon — UWX Series Aluminium Electrolytic Capacitor Specifications, 2023
• Murata — GRM Series MLCC Specifications and Application Notes, 2023
• Nye Lubricants — Rheolube 716A Product Data Sheet
• R-390A/URR Pearls of Wisdom — compiled by Chuck Rippel WA4HHG et al., r-390a.net