Collins 75A-2 Amateur Receiver
Failure Prevention Kit — Component & Modification Design

Section 1 — Circuit Overview, New Features vs 75A-1, and Community Resources

Key Circuit Architecture and New Features vs the 75A-1

The 75A-2 shares the fundamental double-conversion architecture of the 75A-1 but incorporates significant redesigns that create different failure modes:

• New PTO: Collins 70E-12. The 70E-12 has a fundamental output range of 1.955–2.955 Mc; the second harmonic (3.910–5.910 Mc) is used for the 10 and 11 metre bands. This is a different PTO from the 75A-1’s 70E-7 and requires the 70E-12 specific service procedures.
• 160 metre single conversion. A critical design distinction: the 160 metre band uses single conversion, where the RF signal from V1 is fed directly to the second mixer grid through the variable IF filter — the first mixer and crystal injection are bypassed. This is mentioned explicitly in the circuit description and means 160 metre alignment and troubleshooting proceed differently from all other bands.
• 0A2 voltage regulator tube. The 75A-2 introduces a 0A2 neon voltage regulator (approximately 150 V regulation) to provide a stabilised voltage for critical receiver circuits. The 0A2 is a gas-filled neon tube with a finite operating life; when it fails, the regulated circuits lose their stability reference, affecting AVC consistency, S-meter calibration, and frequency stability.
• Crystal phasing notch circuit. The crystal phasing control provides a tunable notch rejection at approximately 455 kc, allowing the operator to notch out a heterodyne within the receiver passband. This uses a variable capacitor (the phasing condenser) across one of the IF transformers. The phasing condenser and associated components are subject to drift and mechanical wear.
• 5-position crystal filter. Extended from 4 to 5 selectivity positions (approximately 5 kc to 200 cps bandwidth). The filter mechanism now uses varying series resistance to control crystal Q across the five positions. More switch contacts than the 75A-1.
• 6BA6-dominated design. The 75A-2 uses six 6BA6 tubes throughout the IF chain, oscillators, and calibrator. This tube type is widely available and generally reliable, but the IF section resistors associated with these stages are the primary site of carbon composition drift.
• FM adapter provision (8F-1). The 75A-2 is designed to accept the optional 8F-1 FM discriminator adapter unit. The adapter connects to the receiver’s IF output and audio input. The optional 8R-1 100 kc crystal calibrator is also available. Neither is built in to the basic 75A-2.
• Drum dial. The 75A-2 introduced a new slide-rule drum dial calibrated in 0.1 Mc intervals, with a vernier dial in 1 kc steps (2 kc on 10 and 11 metres). The slide rule guide pointer is cable-driven, same as the 75A-1.

Tube Complement (17 tubes)

Community Resources

Section 2 — Root Cause Failure Analysis

The 75A-2’s failure modes differ meaningfully from the 75A-1 in three areas: the introduction of the 0A2 voltage regulator tube (a new failure category absent from the 75A-1), the prevalence of carbon composition resistor drift in the IF section (documented from community experience), and the crystal phasing notch circuit (a feature with no equivalent in the 75A-1 that introduces its own drift and alignment requirements).

• 1
  Electrolytic Capacitors (6 Units) — Hum, Ripple, and Power Supply Instability The Collins 75A-2 contains six electrolytic capacitors in its power supply and bias circuits. Community documentation from Antique Radio Forums is specific: “Looks like 6 electrolytics and 8 paper caps need replacing.” The main B+ filter electrolytic (which may be a multi-section can type) is the highest priority. After 70+ years, degraded electrolytics produce: audible 120 Hz hum on all received signals (B+ supply ripple coupling into IF and audio stages); inconsistent AVC action (the AVC filter electrolytics also age); reduced audio volume (cathode bypass electrolytics losing capacitance); and in the case of the 0A2 regulator supply electrolytic, unstable regulated voltage leading to frequency drift and S-meter errors. Replace all six electrolytics simultaneously with 105°C high-ripple rated types at correct capacitance and voltage ratings.
• 2
  Paper Capacitors — “Black Beauties” and Wax-Paper Types (8 Units) — Leaky and Open The 75A-2 contains eight wax-paper and moulded paper capacitors in bypass and coupling roles. Community documentation: “8 paper caps need replacing.” These 70-year-old capacitors develop two distinct failure modes: leakage (a DC resistance much lower than the specification for a capacitor, causing bias shifts in the stages they couple or bypass), and open circuit (complete loss of coupling or bypass function). A leaky coupling cap passes DC to a following grid, biasing it into conduction or cut-off; an open coupling cap removes the signal path entirely. The symptoms of a single leaky coupling cap can be subtle — slightly distorted audio, reduced gain on one band — while a completely open cap gives an unmistakable stage-selective dead symptom. The Antique Radio Forums note that mica capacitors in the PTO section also require attention: the buffer capacitor for the PTO must be mica type (temperature stability critical), while the two PTO bypass caps can be ceramic.
• 3
  Carbon Composition Resistors Drifted High in IF Section — Gain Loss and S-Meter Errors This failure mode is unique to the 75A-2 series in the degree of community documentation. Antique Radio Forums: “Id check the resistors; several in mine were way high.” A subsequent post confirms: “Got the remaining out of spec resistors replaced in the I.F. section. Now, all of the resistance readings match the chart in Sam’s.” Carbon composition resistors in the IF amplifier stages (V5, V6, V7 — the three 6BA6 IF stages) drift upward with age, heat cycles, and moisture absorption. The specific resistors of concern are: plate load resistors (higher value = lower plate current = reduced gain), cathode bypass resistors (incorrect value = altered gain and distortion), and AVC decoupling resistors (incorrect value = AVC time constant error). The symptoms of drifted IF resistors range from reduced sensitivity (incorrect plate loads) to erratic AVC action (incorrect AVC decoupling) and incorrect S-meter readings. Measure every resistor in the IF section against the Sams or Collins service data chart before any alignment attempt.
• 4
  0A2 Voltage Regulator Tube (V16) — Regulated Voltage Collapse and Stability Loss The 0A2 is a cold-cathode neon gas regulator tube that maintains approximately 150 V DC across its terminals by controlling gas discharge current. It stabilises critical circuit voltages in the 75A-2, including portions of the AVC circuit and oscillator supplies. The 0A2 has a finite gas life; after 70+ years, the gas pressure inside the envelope changes, causing the regulation voltage to drift upward or downward, or causing the tube to stop regulating altogether (conducting continuously or not at all). Symptoms of 0A2 failure: S-meter readings that vary with supply voltage changes rather than with received signal strength; AVC action that is inconsistent across the dial; BFO frequency instability; reduced sensitivity that correlates with how long the receiver has been on. Test the 0A2 by measuring DC voltage across the tube: should be approximately 150 V DC ±5 V regardless of primary supply variation. An 0A2 that reads significantly above or below 150 V, or whose reading varies with primary voltage, has failed.
• 5
  Collins 70E-12 PTO Lubrication Failure — Stiff Tuning, Run-Out, and Frequency Drift The Collins 70E-12 PTO (fundamental 1.955–2.955 Mc; second harmonic used for 10/11 metres) uses the same precision lead screw and ferrite slug mechanism as all Collins PTO-equipped receivers. After 70 years, the original lubricant has dried, hardened, or migrated. The Antique Radio Forums thread documents PTO-specific issues: the buffer capacitor inside the PTO must be a mica type (temperature stability critical for accurate frequency calibration), while bypass caps can be ceramic. The community also documents PTO stop adjustment as a calibration task: “I caught this, and adjusted everything so that the stop now functions, and prevents tuning beyond the end(s) of the dial scale.” Initial PTO calibration procedure noted: “going to the 160 meter scale, set the pointer at 2.0 KC, loosened the coupler, turned the PTO clockwise all the way, then counter clockwise for 5 turns, then using the RF signal generator, set the PTO for a strong 2 KC signal.” Complete PTO service per the CCA 70E-12 service document is essential before final alignment.
• 6
  Crystal Phasing Notch Circuit Drift — Non-Functional Heterodyne Rejection The crystal phasing notch is a feature unique to the 75A-2 in the A-series lineup. The phasing condenser (variable capacitor across IF transformer T2) allows the operator to tune a null across the 455 kc IF passband to reject a specific heterodyne. The circuit description explains: an additional set of stator plates on the phasing condenser compensates for detuning effects of T2. After 70 years, the phasing condenser dielectric may have absorbed moisture, shifting its capacitance range; the associated phase-shift network components may have drifted; and the phasing condenser itself may have developed mechanical play or binding. Symptoms: the phasing control has no audible effect on heterodyne rejection (open-circuit phasing path); or the null position has moved completely off the dial range (drifted capacitance or component values). Inspect the phasing condenser for smooth mechanical operation and correct capacitance range before attempting the phase control alignment.
• 7
  AVC Amplifier (V14 — 12AX7) Emission Degradation — S-Meter and AVC Simultaneous Failure The 12AX7 at V14 is the single most critical tube in the 75A-2 because it serves two essential functions: one triode section as the AVC amplifier (producing the AVC bias that controls V1, V5, V6, V7 and drives the S-meter), and the other triode as the first audio amplifier. The Antique Radio Forums documents the characteristic AVC failure in the 75A-2: “The S meter doesn’t seem to be working like it should. It acts like a volt meter, as it seems to just rise and fall with the movement of the RF control. Not sure why this is doing that.” This symptom — S-meter tracking the RF gain control rather than received signal strength — is a reliable diagnostic indicator that the AVC amplifier half of V14 has failed. When V14’s AVC section fails, the AVC voltage collapses; the S-meter then responds to whatever bias source it can find, which in this circuit is the RF gain potentiometer. Test V14 on a calibrated tube tester; replace if either section shows below 70% emission.
• 8
  Band Switch Multi-Section Contact Oxidation — Band-Selective Receive Failures The 75A-2 band switch controls multiple ganged sections simultaneously: RF amplifier tuning (V1), first mixer crystal injection (selecting the appropriate crystal for each amateur band), variable IF tuning, and second mixer injection range. The circuit description notes: “Band switching of RF stages is accomplished by means of a multiple section switch gang. In addition to RF circuits, the band switch selects high frequency oscillator crystals.” After 70 years, the silver or nickel switch contacts develop oxide film producing high-resistance connections. A dirty contact on the crystal injection section produces no signal on a specific band while all others work; on the RF section it produces reduced sensitivity on one band. Additionally, the 75A-2 shares the 75A-1’s characteristic that the crystal filter selectivity switch also develops oxide film across its five positions. Apply DeoxIT D5 to both the band switch and the crystal selectivity switch, cycling through all positions 20 times minimum before any alignment.
• 9
  Power Transformer Thermal Stress from Modern Line Voltage — Premature Ageing The 75A-2 power transformer was designed for a 115 V AC nominal line. Modern US residential AC is 120–125 V, representing a 4–9% overvoltage. The Antique Radio Forums documents this directly: “The power transformer does get hot after the set is on about 2 hours or so. After this period of time, I can still touch it, but it’s hot, and you want your hand off it in a few seconds.” A 10% line overvoltage produces approximately 10% higher filament voltages across all 17 tubes, approximately 20% more transformer core loss, and a measurably higher transformer operating temperature. Prolonged operation at elevated line voltage progressively damages the transformer insulation, increases winding resistance, and can eventually cause transformer failure. A bucking transformer (as documented in the 75A-1 restoration section by WA3DSP) or a Variac set to deliver 112–115 V is the appropriate remedy.
• 10
  Signal Tube Ageing — 6CB6 RF, 6BA6 IF Chain, 6AL5 Detector, and 5Y3 Rectifier The 75A-2’s signal tubes are 70+ years old. Priority testing order: V1 (6CB6 RF amplifier — reduced emission directly reduces receive sensitivity and overload performance); V14 (12AX7 AVC/audio, covered in Failure Mode 7); V8 (6AL5 detector and AVC rectifier — a weak diode section causes AVC loss or reduced audio); V5, V6, V7 (three 6BA6 IF amplifiers — reduced emission reduces gain, though all three must be weak to produce obvious sensitivity loss); V17 (5Y3 rectifier — increased internal drop as the tube ages, reducing B+ voltage). The community documented replacing 6BA6 tubes with NOS 5749 equivalents and the 6CB6 RF amplifier with NOS military equivalents for improved performance. All 6AL5 tubes should be tested by individual section since one diode section may fail while the other remains functional, causing a partial symptom.

Section 3 — Kit Component Reference

Section 4 — Pre-Operational Safety Protocol

Visual Inspection Checklist

• Main filter electrolytic: inspect for leakage, distortion, or colour change before power-up. Any distress: replace before power-up.
• Paper capacitors: inspect all visible wax-paper or moulded paper caps for discolouration, bulging, or evidence of oil migration. Any distress: replace before power-up.
• 0A2 tube (V16): inspect for signs of over-running (glass blackening, internal electrode discolouration). 0A2 tubes in normal operation glow with a characteristic violet/purple discharge; any orange or white glow indicates incorrect operating conditions.
• IF resistors: visually inspect all resistors in the IF section for physical heat damage (discolouration, cracked bodies). Any physically damaged resistor: replace before power-up and investigate why it overheated.
• Dial cable: verify cable intact and pointer tracking drum dial correctly.
• Tubes: reseat all 17 tubes. Check for cracked envelopes or missing shields.
• Line cord: replace if original two-wire type before any power-up.

Section 5 — Circuit Modifications

  COLLINS 75A-2 SIGNAL PATH (double conversion; 160m single conversion)

  ALL BANDS EXCEPT 160m (double conversion):
  Antenna → V1 (6CB6, RF amp, AVC controlled)
           → V2 (6BA7, first mixer)
               + Crystal XCO (HF injection oscillator, band-selected by band switch)
           → Variable first IF (changes across band)
           → V3 (6BA6, variable IF amplifier, gang-tuned)
           → V4 (6BA7, second mixer)
               + V12/V13 (6BA6×2, 70E-12 PTO VFO + buffer, 1.955–2.955Mc fund.)
           → 455 kc fixed second IF
           → V5, V6, V7 (three 6BA6, IF amplifiers, AVC controlled)
               [Crystal filter acts here: 5 selectivity positions]
               [Crystal phasing notch: condenser across T2]
           → V8 (6AL5, AM detector + AVC rectifier)
           → V9/V10 (6AL5×2, noise limiters AM and CW)
           → V14 (12AX7: ½ AVC amplifier → 0A2-stabilised AVC line
                          ½ first audio amplifier)
           → V15 (6AQ5, audio output, 2.5W)
           → Speaker

  160m (single conversion — crystal injection BYPASSED):
  Antenna → V1 (6CB6 RF amp) → directly to V3 variable IF grid
           → V4 (6BA7 second mixer) + 70E-12 PTO → 455kc IF → chain continues

  0A2 REGULATOR (V16) DIAGNOSTIC:
  ┌──────────────────────────────────────────────────────────────┐
  │ Normal: ~150V DC across V16, stable, violet/purple glow     │
  │ Failed: >155V or <145V, or varies with primary voltage       │
  │ S-meter tracking RF gain control = V14 AVC half failed      │
  │ IF sensitivity loss = check K-003 resistors before tubes    │
  └──────────────────────────────────────────────────────────────┘

Figure 1. Collins 75A-2 signal path (standard and 160m modes), 0A2 regulator diagnostic, and key fault indicators.

Section 6 — Installation Sequence

• 1
  Documentation and visual inspection Obtain 75A-2 service manual and Sams chart from collinsradio.org archives or BAMA. Inspect electrolytics and paper caps for distress. Inspect 0A2 (V16) tube. Replace line cord before any power-up.
• 2
  Measure all IF section resistors against Sams chart (K-003) — before any power-up This must be done before power-up. Replace every drifted resistor in V5/V6/V7 IF stages. This step affects alignment validity; alignment cannot be performed reliably until all IF resistors are within specification.
• 3
  Replace all 6 electrolytics and 8 paper caps (K-001, K-002, MOD-1) Work section by section: power supply first, then IF, then RF, then audio, then oscillators. PTO buffer cap: silver mica only; bypass caps can be ceramic.
• 4
  Install grounded line cord, safety bypass caps, and bucking transformer (K-005) Three-wire cord with X2 bypass caps. Buck transformer to reduce mains to 112–115V. Target filament voltage 6.0–6.3V.
• 5
  PTO service (K-006) Remove 70E-12 PTO per CCA document. Clean lead screw. New synthetic grease. Adjust end-of-travel stop. Verify run-out below specification.
• 6
  Switch cleaning and phasing condenser inspection (K-007) DeoxIT D5 on crystal selectivity switch (5 positions) and band switch (all sections). Inspect phasing condenser mechanical action. Verify dial cable intact.
• 7
  Tube testing — priority V14, V16, V1, V8 (K-008, K-009) Test V14 (12AX7) first — both sections. Test V16 (0A2) under power after initial power-up. Test V1 (6CB6 RF), V8 (6AL5 detector, both sections). Then test all 17 tubes.
• 8
  First Variac power-up and B+ / 0A2 verification Raise mains from 0 to full over 10 minutes. Measure B+ under load. Measure V16 (0A2) voltage — target ~150V stable. Verify filaments at 6.0–6.3V. No burning smell.
• 9
  Full alignment and crystal phasing notch (MOD-3, MOD-4) 455 kc IF; variable IF; RF all bands; 160m single conversion path separately; PTO/drum dial calibration; AVC and S-meter; BFO pitch and phase; crystal phasing notch alignment.
• 10
  All-band verification and performance documentation Verify signal reception on all bands including 160m. Test all 5 selectivity positions. Verify phasing notch on each band. Document baseline sensitivity and S-meter calibration. Consider MOD-2 (5Y3 replacement) if transformer heat remains after line voltage bucking.

Section 7 — Verification Tests

0A2 Regulator Voltage Check

AVC and S-Meter Function — V14 Verification

Crystal Phasing Notch Depth

160m Single Conversion Receive