Collins 32S-3 Transmitter

Single-Band Loss: Symptom Analysis, Root Cause Diagnosis & Troubleshooting

Collins Radio · Technical Reference Series
Collins 32S-3 Transmitter
Single-Band Loss: Symptom Analysis, Root Cause Diagnosis & Troubleshooting
80 METRES VFO INJECTION 1ST MIXER BAND SWITCH
Reported Case Summary

All three 80 metre band positions are non-functional. The transmitter exhibits identical symptoms whether operated from its own VFO/crystals or in transceive with a 75S-3B receiver. All tubes have been verified good.

  • Grid meter pegs hard on all three 80m positions
  • Mike gain adjustment has no effect on the grid reading
  • Exciter tuning within the 80m range has no effect on grid meter
  • At the converter jack, exciter tuning does change output frequency
  • VFO has no effect on output frequency at the converter jack
  • Cleaning the band switch contacts produced no improvement
1  ·  What the Symptoms Reveal

Before chasing components, the symptom pattern itself points precisely at the fault location. The 32S-3 uses a dual-conversion architecture: the VFO injects into the first balanced mixer, which combines it with a 455 kHz IF signal to produce a bandpass IF. That signal passes to the second mixer, which uses crystal oscillator V12 injection to translate to the final operating frequency.1

KEY OBSERVATION 1 — VFO HAS NO EFFECT AT CONVERTER JACK In a correctly operating 32S-3, the exciter tuning dial is mechanically coupled to the VFO capacitor.2 If exciter tuning changes output frequency but VFO has no influence, the VFO’s RF signal is not reaching the 1st mixer on the 80m positions. The frequency change you see is from mechanical coupling to the output tank — not from VFO injection.
KEY OBSERVATION 2 — PEGGED GRID METER, IMMUNE TO MIKE GAIN When drive at the PA grid is unaffected by MIC GAIN, the signal driving the PA is not passing through the normal modulator/balanced mixer chain.3 A raw crystal oscillator signal — or a mixing product of it — is leaking directly into the driver stage. The 6146 PA tubes cannot resonate at the wrong frequency, so the meter pegs regardless of tuning.

Conclusion: The fault lies in the VFO-to-1st-mixer injection path on the 80m band switch positions — upstream of the 2nd mixer and the entire PA chain.

2  ·  Symptom / Root Cause Matrix
SYMPTOM ROOT CAUSE MECHANISM LIKELY STAGE PROB.
VFO no effect on 80m output Mechanically broken contact on band switch VFO injection wafer — finger sprung away, cracked substrate, or cold solder joint on terminal lug4 Band switch
VFO wafer		 HIGHEST
VFO no effect on 80m output Shorted bypass capacitor in 80m VFO injection network — grounds VFO signal before it reaches 1st mixer grid; well-documented age failure in 60-year-old paper and ceramic capacitors5 80m injection
LC network		 HIGH
VFO no effect on 80m output Open coupling capacitor — signal cannot pass to 1st mixer grid; characteristic of failed paper capacitors whose DC-blocking function has been lost with age6 80m injection
coupling cap		 HIGH
Grid pegged; immune to mike gain Crystal oscillator V12 running uncontrolled or parasitically on 80m — raw crystal signal floods driver chain bypassing normal first mixer conversion7 80m crystal
oscillator V12		 MED
All 3 × 80m positions identical Open series resistor in 80m VFO injection path — single component shared by all three 80m positions; aged carbon composition resistors drift open after decades of service 80m injection
series resistor		 MED
Cleaning produced no improvement Cold/cracked solder joint on band switch terminal + leaky crystal oscillator coupling cap — silver sulfide tarnish on Collins contacts is conductive; it is mechanical failure that DeOxit cannot remedy8 Band switch
terminal / lug		 LOWER
⚑ Note on the 2nd Mixer: In the 32S-3 architecture, crystal oscillator V12 injects into the second mixer.9 A 2nd mixer failure would still permit VFO tuning action at the converter jack — just at wrong frequencies or with mixing images. The complete absence of VFO influence points firmly to the 1st mixer injection path, upstream of the 2nd mixer entirely.
3  ·  Step-by-Step Troubleshooting Procedure

Follow in sequence. Each step confirms or eliminates a candidate root cause. Throughout, refer to the 32S-3 7th Edition service manual schematic as the authoritative circuit reference.10

1
Obtain the Schematic — Trace the 80m VFO Injection Path

Before touching anything, open the 32S-3 service manual schematic. The Collins Collectors Association (CCA) hosts all 32S-3 manual editions free of charge, licensed from Rockwell Collins.10 The BAMA archive is a secondary free source.11 Identify every component unique to the 80m band switch position in the VFO injection path — from VFO output through the band switch wafer to V5 (1st balanced mixer) grid.

For real-time expert input, the CCA Reflector email list has over 1,000 subscribers and is free to join — it is the primary community resource for S-Line troubleshooting.12

2
Scope Probe: Confirm VFO Signal at V5 1st Mixer Injection Grid

With a 10× high-impedance probe, measure VFO signal level at the V5 (6U8A) 1st mixer injection grid on 40m (a known-good band) first, noting amplitude. The 32S-3 Typical RF Voltage Table provides expected levels at each node.13 Then switch to each 80m position in turn and compare.

Expected result: VFO signal normal on 40m, absent or very low on all three 80m positions. This single measurement confirms the fault lies between the VFO output and the 1st mixer grid — the same oscilloscope signal-tracing methodology used in documented S-Line repair work.14
3
Visual Inspection: Band Switch VFO Injection Wafer — Beyond Cleaning

Cleaning removes environmental crud; it cannot repair mechanical failure. Silver sulfide tarnish on Collins silver-plated contacts is actually conductive — mechanical damage is what DeOxit cannot fix.8 With a strong loupe or stereo magnifier, inspect the wafer deck that routes VFO injection for 80m:

  • Cracked wafer substrate — phenolic and SRBP wafers become brittle with age, especially near mounting posts and rivet holes15
  • Contact finger sprung away from its rotor position — may look normal statically but fails under the light spring pressure of the rotor
  • Cold or cracked solder joints on the wafer terminal lugs — dull, grey, cratered fillet rather than a bright concave joint
  • Broken terminal wire near its solder point — stress fractures at the first bend are common in gear that has been transported

For a cracked wafer: slow-cure clear epoxy (Araldite or equivalent) applied to the fracture surface only — never on contacts — cured under gentle clamping pressure has a high reported success rate.15

4
In-Circuit Check: Bypass and Coupling Capacitors in 80m Injection Network

Power off, band on 80m. Use a DMM (Ω range) from the 80m VFO injection node to chassis ground. Repeat on 40m for reference. In documented 32S-1 restoration (predecessor model), every paper capacitor in the unit had failed its DC-blocking function after 50+ years — the same failure class applies to any 32S-3 of the same age.6

Near-zero ohms to ground on 80m → shorted bypass capacitor. Substitute with silver mica or polypropylene film type.
Floating/open node on 80m → open coupling capacitor. Substitute and verify.
5
Measure Crystal Oscillator V12 on 80m — Verify Frequency and Stability

In the 32S-3, V12 is selected by band switch S14; its output feeds the second mixer. Crystal frequency is always 3.155 MHz above the lower edge of the desired transmit band.7 Measure V12 output on an 80m position with a frequency counter or calibrated receiver and confirm correct frequency, stable output, and normal amplitude. Excessive V12 level on 80m can flood the driver chain and produce the pegged grid symptom.

6
Substitute-Test the 80m Injection Coil; Escalate if Needed

If steps 2–5 have not identified the fault, the 80m injection coil in the LC network may have an internal open. Unsolder and bench-test for continuity; compare inductance against a working band’s coil.

If the fault remains elusive, the CCA maintains a directory of experienced S-Line service technicians with OEM parts stocks and alignment equipment.16 The CCA Technical Bulletins — including “Repairing the 32S-3,” “32S-3 ALC Instability,” and “32S-3 Out-Of-Band Spur” — cover edge-case fault modes that may accompany or mimic this symptom set.17

4  ·  Quick Reference — Diagnosis at a Glance
Fault Location
VFO injection path, band switch wafer — 80m positions only
Most Likely Component
Mechanically broken band switch contact or shorted bypass capacitor in 80m injection network
Stage Exonerated
2nd mixer — fault is upstream; 2nd mixer failure would not eliminate all VFO influence
First Measurement
Scope probe at V5 1st mixer injection grid — compare VFO signal level on 80m vs 40m
References & Further Reading
  1. Collins 32S-3 — Dual Conversion Architecture and Circuit Description. Collins Collectors Association historical archives. Describes the complete dual-conversion signal path: VFO → 1st balanced mixer → 2.155–2.955 MHz bandpass IF → 2nd balanced mixer + V12 crystal oscillator → output.  collinsradio.org — 32S-3 Equipment Page ↗
  2. VFO and First Mixer Operation in the 32S-3. CCA 32S-3 equipment page. The VFO produces a variable signal mechanically coupled to the exciter tuning dial; it injects into the 1st balanced mixer grids in parallel. Loss of VFO injection on a single band is a band-switch level fault.  collinsradio.org — 32S-3 Equipment Page ↗
  3. 32S-3 Grid Current Behaviour and MIC GAIN Control. Antique Radio Forums — Collins 32S-3 Grid Current thread. Documents normal PA grid current behaviour and the MIC GAIN control’s role in regulating drive to the PA stage. A pegged grid immune to MIC GAIN indicates signal bypassing the normal modulator chain.  antiqueradios.com — 32S-3 Grid Current Discussion ↗
  4. Collins Band Switch Contact Cleaning — Grounding and Contact Cleaning Recommendations. CCA Technical Reference, compiled by Bill Carns N7OTQ/K0CXX. Distinguishes between conductive silver-sulfide tarnish (not the failure cause) and mechanical contact damage or cold joints that cleaning cannot fix. Provides correct DeOxit methodology for Collins wafers.  collinsradio.org — Grounding & Contact Cleaning PDF ↗
  5. Capacitor Failure in Vintage Collins S-Line Equipment. Gary White W5GW — Collins S-Line Troubleshooting and Repair Adventure. Documents shorted bypass capacitors causing band-specific signal injection loss in S-Line equipment after 50+ years of service.  w5gw.com — Collins S-Line Repair Adventure PDF ↗
  6. Paper Capacitor DC-Blocking Failure — Systemic S-Line Age Fault. Gary White W5GW — Collins S-Line Repair. In a 32S-1 restoration, every paper capacitor in the unit had failed its DC-blocking function; the same failure class applies to the 32S-3 after equivalent service life.  w5gw.com — Collins S-Line Repair Adventure PDF ↗
  7. V12 High-Frequency Crystal Oscillator — Function and Band Assignment. CCA 32S-3 equipment page. V12 is controlled by band switch S14; it feeds the second mixer. Crystal frequency is always 3.155 MHz above the lower edge of the desired transmit band. Crystal frequency is fundamental for bands below 12 MHz, doubled in plate circuit for bands above 12 MHz.  collinsradio.org — 32S-3 Equipment Page ↗
  8. Silver Sulfide Tarnish Conductivity vs. Mechanical Contact Failure. CCA Grounding and Contact Cleaning document. Explicitly states: the tarnish on Collins contacts is Silver Sulfide — conductive and not the contact problem. Environmental crud and mechanical failure are the actual fault causes; cleaning helps only the former.  collinsradio.org — Grounding & Contact Cleaning PDF ↗
  9. Second Mixer Injection Architecture in the 32S-3. CCA 32S-3 equipment page. V12 crystal oscillator output feeds the signal input cathode and one grid of the second balanced mixer. The bandpass IF feeds the other grid. VFO absence upstream of the first mixer renders the second mixer irrelevant to this diagnosis.  collinsradio.org — 32S-3 Equipment Page ↗
  10. Collins 32S-3 Service Manual — 7th Edition, June 1969 (P/N 523-0755011-006311). Primary circuit reference for all 32S-3 troubleshooting. Available free from the CCA under licence from Rockwell Collins. Includes full schematics, alignment procedures, and RF voltage table.  collinsradio.org — Equipment Manuals Index ↗  |  Direct PDF: 32S-3 7th Edition PDF ↗
  11. BAMA — Boat Anchor Manual Archive (Collins section). Secondary free source for Collins manuals, maintained by community contributions.  bama.edebris.com — Collins Manuals ↗
  12. CCA Collins Reflector (CCAR). Free email list with 1,000+ subscribers for Collins troubleshooting assistance. Open to members and non-members. Hosted on groups.io.  collinsradio.org — CCA Reflector Info ↗  |  groups.io/g/cca ↗
  13. Collins 32S-3 Typical RF Voltage Table. Separate document published by the CCA alongside the 32S-3 service manual. Provides expected RF signal levels at all key signal nodes — essential reference for scope-probe comparisons when signal-tracing across band switch positions.  collinsradio.org — Manuals Index (search “32S-3 Typical RF Voltage Table”) ↗
  14. Oscilloscope Signal-Tracing Methodology for S-Line Transmitters. Gary White W5GW — Collins S-Line Troubleshooting and Repair. Documents injection of calibrated signals stage-by-stage using manual signal-tracing procedure; identified a bad second mixer by absence of drive at the 455 kHz and 3055 kHz IF nodes. Validates the scope-probe methodology recommended in Step 2 above.  w5gw.com — S-Line Repair Adventure PDF ↗
  15. Switch Wafer Crack Repair — Phenolic and SRBP Rotary Wafers. G3OOU (QSL.net). Comprehensive guide to epoxy repair of cracked rotary switch wafers: jig construction, degreasing, application of slow-cure clear epoxy (not metal-filled), clamping and cure time. Community forum discussion at Antique Radio Forums (Wafer Switch Repair) confirms high success rate over multiple decades.  qsl.net/g3oou — Switch Wafer Repair Guide ↗  |  antiqueradios.com — Wafer Switch Repair Discussion ↗
  16. Collins S-Line Service and Repair — CCA Vendor Directory. CCA-maintained listing of experienced S-Line service technicians including Exline Signal LLC (WA9Z, Exline IA), Steve Berman N6HK (Precision Collins Services, Chehalis WA), and others with OEM parts stocks and full laboratory test equipment.  collinsradio.org — S-Line Service Directory ↗
  17. CCA Technical Bulletins — 32S-3 Fault Documentation. The CCA RX page lists specific 32S-3 technical articles: “Repairing the 32S-3 — Carns and Bud K7RMT,” “32S-3 ALC Instability,” “32S-3 Chirp Problem in CW Mode,” “32S-3 Mod Produces Out-Of-Band Spur: How to Find & Fix,” and “32S-3 Trouble Shooting Voltage Table.” These cover edge-case fault modes that may accompany or mimic the single-band loss described in this article.  collinsradio.org — RX for Your Collins ↗
Mike Peace VK6ADA  ·  r-390a.net Administrator
vk6ada.com.au  ·  Boatanchor Legends & Technical Reference Series
Collins 32S-3 Technical Reference
Single-Band Loss Diagnostic · 80 Metres

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