R-388/URR
Failure Prevention Kit — Component & Modification Design

Section 1 — Root Cause Failure Analysis

The following ten failure modes account for the overwhelming majority of R-388/URR restoration casualties. They are presented in priority order — the order in which they must be addressed before any power is applied to an unknown unit.

• 1
  C-136 Leakage — The Mixer Tube and AVC Killer C-136 is a 100 pF mica capacitor that injects the 100 kHz calibration oscillator signal into the first mixer tube grid. When it becomes leaky — as it does with high frequency in R-388 units — it places over 70 V of positive DC bias from the crystal calibrator oscillator plate directly onto the mixer tube control grid. This collapses the AVC (Automatic Volume Control) line voltage, sends the receiver into full-gain operation, and typically destroys the mixer tube within a short period of operation. Because the receiver continues to produce audio (just badly distorted and over-driven), the fault is often misdiagnosed as a tube or alignment problem while the mixer tube continues to degrade. Collins service centre engineers specifically identified C-136 as the primary cause of mixer tube failures and AVC malfunctions across all 51J-3 and R-388 production runs. It must be replaced before any power-up regardless of its measured DC resistance. The dangerous batch was primarily Micamold-branded green cases, but red and other-coloured 100 pF micas from this era have also been found leaky — replace by type and value, not colour.
• 2
  C-204 Failure — Loss of AVC Action C-204 is a 100 pF mica coupling capacitor from the detector plate to the AVC amplifier grid. When it fails open — a common failure mode — the AVC rectified voltage no longer reaches the AVC amplifier. The result: the receiver runs at full gain regardless of signal level, the S-meter pegs at full scale on any received signal, and weak signals are completely obliterated by overload from even moderate-strength stations. When C-204 is merely leaky rather than fully open, it introduces a partial AVC voltage offset that reduces dynamic range and produces non-linear S-meter readings. Like C-136, this is a 100 pF mica capacitor of the era that fails at a rate far exceeding other values in the same production run. It is the second most commonly reported restoration fault in R-388/URR sets after C-136 itself.
• 3
  All 100 pF Mica Capacitors — Batch Defect, Value-Specific The leakage problem in R-388 sets is specific to the 100 pF value. Collins service engineers noted this at the time and it has been confirmed by restorers across decades of work with these receivers: all other mica capacitor values in the same chassis remain well within specification while the 100 pF types in multiple circuits show elevated and increasing leakage. The root cause is likely a defective dielectric formulation in the Micamold supplier’s 100 pF production run, though other-branded caps of the same value have also been found problematic. The correct prevention is to replace all 100 pF mica capacitors in the receiver, not just C-136 and C-204. Additional critical locations include the IF transformer coupling trimmer networks and the crystal filter phasing circuits. Using silver mica replacements is preferred over disc ceramic for stability, though C0G/NP0 ceramics are acceptable.
• 4
  Multi-Section Filter Capacitor — B+ Rail Collapse and Hum The R-388/URR uses a multi-section plug-in electrolytic capacitor (the “filter can”) for power supply filtering. After 70 years, the electrolyte in these cans has dried, the dielectric oxide layer has degraded, and effective capacitance has fallen to a fraction of rated value. Symptoms: 60 Hz hum superimposed on all received signals, B+ rail drooping under normal tube current load, and in severe cases, elevated AC ripple visible on the HT rail that destabilises the 0A2 voltage regulator tube. The filter can must be either replaced with a modern equivalent or re-stuffed with new individual capacitors before power is applied. Hayseed Hamfest produces a purpose-built replacement plug-in unit for the 51J/R-388 series that is the simplest solution. The identical hum symptom can be produced by a failing 5V4 rectifier, so both should be checked together.
• 5
  5V4 Rectifier Failure — B+ Supply Collapse The 5V4 full-wave rectifier tube is the single most critical active device in the power supply. Its failure mode after decades of storage is typically weak emission, low peak-inverse voltage margin, or complete open-circuit of one or both diode plates. A weak 5V4 causes reduced and unstable B+, degraded sensitivity across all bands, and worsened hum (misdiagnosed as filter cap failure). A fully failed 5V4 produces zero B+ and a silent receiver. The 5V4 should be regarded as a consumable — always replace with a known-good NOS or new equivalent (5AR4 is an acceptable high-quality substitute in most applications). Test the original before discarding; a strong 5V4 can be kept as a spare.
• 6
  0A2 Voltage Regulator Failure — Cascaded B+ Instability The R-388 (and 51J-3) added an 0A2 gas-discharge voltage regulator tube that was absent from the earlier 51J-1 and 51J-2. This tube regulates a portion of the B+ supply for critical circuits. After long storage, 0A2 tubes fail dark (no ionisation), fail with excessive voltage drop, or develop noisy ionisation that introduces wideband noise into regulated circuits. A failed 0A2 typically manifests as oscillation or instability in the crystal calibrator circuit, erratic AVC behaviour, and sensitivity degradation on the lower bands where the regulated supply feeds the crystal oscillator circuit. Replace the 0A2 as a matter of course alongside the 5V4. New-production 0A2 equivalents are available.
• 7
  6AK5 Front-End Tubes — Gas Development and AVC Corruption The RF amplifier and mixer stages of the R-388 use 6AK5 (EF95) pentode tubes. After decades of storage, 6AK5 tubes frequently develop gas contamination inside the envelope. A gassy 6AK5 exhibits a characteristic blue internal glow and introduces positive ion current to the control grid, effectively offsetting grid bias in the direction of increased plate current. In the R-388 context, this produces exactly the same symptom as a leaky C-136 — corrupted AVC voltage, overdriven mixer, and reduced dynamic range. Always inspect 6AK5 tubes in a darkened room after warm-up. Any blue glow indicates a gassy tube requiring replacement. The 5654/6AK5W military-grade variant is a reliable and affordable substitute. Budget for replacing all front-end 6AK5s as a precaution rather than testing selectively.
• 8
  Bathtub Capacitors — Tar-Sealed Oil-Paper Leakage The R-388 contains several tar-sealed “bathtub” capacitors (rectangular metal cans sealed with tar at one end) in various bypass and coupling roles throughout the chassis. These are oil-impregnated paper-foil types that develop leakage resistance as the oil migrates and the paper absorbs moisture over decades. The critical test is an HV leakage measurement: apply 450–600 V DC via a series resistor and measure the leakage current. Units C-205 and C-214 have been found to retain their specifications in some examples; others in the same chassis have not. Do not assume — test each bathtub capacitor at rated voltage before condemning or passing it. Failed bathtub caps typically produce bias voltage offsets in the stages they serve, manifesting as stage gain collapse or grid blocking.
• 9
  70E-15 “M” PTO End-Point Drift — Irreversible Slug Permeability Change The 70E-15 permeability-tuned oscillator (PTO) used in the R-388, 51J-3 and 51J-4 uses slug-tuned coils wound on ferrite cores supplied by Aladdin. These cores exhibit an unstable permeability characteristic over long time periods: the effective permeability of the slug material drifts, changing the VFO coil inductance at each end of the 1 MHz tuning range by different amounts. The result is that end-point accuracy — the alignment of the mechanical dial to the actual PTO frequency at the start and end of its 10-turn range — cannot be corrected by the normal end-point adjustment screw because the adjustment range is exhausted before the correct end-point is reached. The non-standard correction is to open the PTO housing and remove one or two turns from one of the end-point correction coils (L-002) inside the PTO, then re-set the external trimmer. This is precision work requiring frequency counter verification. An alternative is to acquire a 70E-15 “CR” variant PTO (used in later 51J-4 units) which uses a different correction mechanism and is generally more tractable.
• 10
  500 kHz Crystal Phasing Circuit — Crystal Aging and Rejection The R-388 uses crystal phasing (a Collins innovation) to provide its selectivity steps, using a 500 kHz fixed IF with a quartz crystal filter. The 500 kHz crystals in the phasing circuit can age out of tolerance, crack, or lose activity after 70 years. A failed phasing crystal produces either no selectivity narrowing on the affected selectivity step, or in severe cases, complete IF blocking when that selectivity position is selected. The symptom can be confused with a selector switch contact failure. Test by listening for proper audio on each selectivity position while monitoring S-meter deflection. The 500 kHz crystals required are still available as special-order items from crystal suppliers. Do not discard the original crystals without testing — many are still within specification.

Section 2 — Kit Component Reference

The following table lists every component in the failure prevention kit. Tier 1 components are mandatory pre-power replacements. Tier 2 components address secondary aging mechanisms and are recommended but may be installed during the restoration phase. Modification components are listed separately.

Section 3 — Pre-Power Safety Protocol

Visual Inspection Checklist

• Remove the chassis top cover (if fitted). Dust covers on the tuning slug compartments may be removed for storage prevention if the receiver will be used in a shack environment; leave in place during transport.
• Inspect all tubes for visible damage: cracked envelopes, loose internal elements, blackened getters. A blackened getter indicates the tube envelope has been breached — that tube is dead.
• Inspect the filter capacitor can for visible bulging, electrolyte staining on the surrounding chassis, or cracked seals. Any of these means the can has already failed and must be replaced before any power is applied.
• Locate C-136 and C-204. These are small green or red “postage stamp” mica capacitors. If either is visibly cracked, bulged, or has a burnt appearance, treat the chassis as a fault investigation, not a routine restoration.
• Check the AC mains cord for cracked or brittle insulation. An R-388 with a 2-wire mains cord must have K-008 installed before any power connection.
• With a DMM on resistance range: measure chassis to centre-pin of each RF connector. Should read open circuit. Any low resistance reading indicates a direct or near-direct short somewhere that must be traced before power-up.

Section 4 — Tier 1 Mandatory Replacements

Replacing C-136, C-204, and all remaining 100 pF mica capacitors is the single highest-priority action for any R-388 restoration. These replacements must be completed before any mains power is applied to the chassis.

Locating C-136

C-136 is located in the mixer/oscillator section of the chassis, near the crystal calibrator oscillator tube (V-105, a 6AK5). It is a small green or red mica “postage stamp” capacitor with leads going to the first mixer tube grid and the calibrator oscillator plate circuit. The R-388 chassis is deep and tightly packed — access requires patience and correct tools. Use hemostats or small-diameter curved-tip needle-nose pliers to reach the component. Do not use excessive force on adjacent component leads.

Locating C-204

C-204 is in the detector/AVC section, coupling the detector diode plate to the AVC amplifier grid. It is one of the most photographed fault components in R-388 restoration documentation. Access is moderately difficult due to chassis depth — hemostats are essential. Use proper soldering iron temperature control; the surrounding components include the AVC time-constant electrolytics that are heat-sensitive.

  R-388/URR — CRITICAL 100 pF MICA CAPACITOR LOCATIONS

  C-136  Crystal calibrator oscillator plate → First Mixer V-104 grid
         Failure: +70 V positive bias on mixer grid → AVC collapse → mixer tube destruction
         Location: Mixer/oscillator section, near V-105 calibrator tube
         Access: Difficult — chassis depth requires hemostats or curved pliers

  C-204  Detector plate → AVC amplifier grid
         Failure (open):   No AVC action → full gain, S-meter pegged
         Failure (leaky):  Partial AVC offset → reduced dynamic range
         Location: Detector/AVC section near V-110 (12AX7 detector/AVC)
         Access: Moderately difficult

  All other 100 pF micas:
         IF transformer trimmer networks — inspect and replace systematically
         Crystal phasing circuit auxiliary capacitors — verify values after replacement
         Replace ALL 100 pF instances regardless of apparent condition

Figure 1. C-136 and C-204 fault paths and chassis locations.

Filter Can Replacement

The plug-in filter can is the easiest major component to replace in the R-388 — it pulls straight out of its socket on the chassis floor. Before ordering a replacement, measure the existing can with a capacitance meter if possible: some examples still test within 20% of rated value and retain acceptable ESR. However, any can that has visible electrolyte staining, swelling, or that was stored without power for more than a decade should be replaced without hesitation. The Hayseed Hamfest replacement module is the recommended solution, providing the correct plug-in footprint with modern capacitor elements in an original-style housing.

If re-stuffing the original can, note that the pin assignments must be maintained exactly — reversing the common (negative) pin with a capacitor pin will connect the capacitors in reverse polarity. Verify pin assignments against the TM 11-854 schematic before soldering.

5V4 and 0A2 Tube Replacement

Pull both the 5V4 rectifier and the 0A2 regulator and replace with known-good units. Test the originals on a calibrated tube tester — a 5V4 reading above 70% emission can be kept as a spare. Do not return a weak 5V4 to service: its failure mode under load is gradual voltage sag that produces multiple symptoms simultaneously, making it extremely difficult to diagnose.

With the receiver powered via Variac at approximately 50% mains voltage, allow the tubes to warm up for 5 minutes. Then darken the room and inspect each 6AK5 tube for internal blue glow — an indicator of gas contamination. Any blue-glowing tube must be replaced before the receiver is operated at full power. Replace with 5654/6AK5W types for improved long-term reliability in this application.

Section 5 — Circuit Modifications

  MOD-1 — AVC QUIESCENT VOLTAGE CHECK AND CORRECTION

  Measurement point: AVC bus at AVC amplifier output (V-110 pin 6 area)
  Condition: No antenna, RF Gain max, BFO off, no signal, 5 minutes warm-up

  Specification per TM 11-854:        -1.40 V (target)
  Lankford improved specification:    -1.60 to -1.80 V (preferred)
  If measured value is > -1.40 V:     Indicates leaky C-136 or C-204 — fix first
  If measured value is < -2.0 V:      AVC circuit fault — investigate before proceeding

  Effect of correct AVC quiescent voltage:
    IF amplifier stages biased slightly further off at no signal
    → Higher instantaneous gain available for weak-signal work
    → More headroom before strong-signal AGC onset threshold
    → Improved S-meter linearity across received signal range

Figure 2. MOD-1 AVC quiescent voltage measurement and specification.

Section 6 — Installation Sequence

The sequence below is designed so that each step creates a verified baseline before the next. Do not compress steps or skip ahead. The R-388’s modular layout is an asset for disciplined restoration — work one circuit area at a time and verify before moving on.

• 1
  Documentation and visual inspection Photograph every tube location and every capacitor before touching anything. Note any evidence of previous servicing: replaced components, solder bridges, modified wiring. Build a pre-restoration baseline record. Consult TM 11-854 and its four Change documents before proceeding — the Change documents contain important corrections and clarifications to the original alignment procedures.
• 2
  Install K-008 — 3-wire mains cord (MOD-3) Replace the mains cord before any other work. This ensures the chassis is properly earthed throughout the restoration process. Verify earth continuity from the earth pin of the new plug to the chassis with a DMM before proceeding.
• 3
  Replace C-136 (K-001) and C-204 (K-002) Locate and replace both critical 100 pF mica capacitors. Work slowly and carefully in the tightly-packed chassis. After replacement, verify with a DMM that the mixer grid circuit (accessible at the tube socket pin) reads no DC voltage with the receiver unpowered. A reading other than zero indicates an additional leakage path to investigate.
• 4
  Replace all remaining 100 pF mica capacitors (K-003) Systematically work through the IF, crystal oscillator, and detector sections replacing every 100 pF mica instance. Mark each replacement on a photocopy of the TM 11-854 schematic as you go. Do not proceed to power-up until this is complete.
• 5
  Replace filter can (K-004), 5V4 (K-005), and 0A2 (K-006) Pull the original filter can and fit the Hayseed replacement or restuffed can. Fit the new 5V4 and 0A2. Test original tubes on a tube tester and record results. Remove and store the top dust covers (MOD-4).
• 6
  First Variac power-up — filter can reformation With all Tier 1 components installed, connect to a Variac set to 0 V. Slowly raise to 25% mains over 5 minutes, pause 10 minutes. Advance to 50%, pause 10 minutes. Advance to 75%, pause 10 minutes. Advance to 100%. Monitor current throughout: instability or sudden increase indicates a fault. At full voltage, measure B+ rail against TM 11-854 specifications.
• 7
  6AK5 gassy-tube inspection Allow the receiver to operate at full voltage for 5 minutes. Darken the room and inspect V-101, V-102, V-103, and V-105. Replace any blue-glowing 6AK5 with 5654/6AK5W types (K-007). Repeat the visual check after each replacement to verify the blue glow has not returned from an adjacent tube.
• 8
  Bathtub capacitor HV leakage test (K-009) With the receiver powered down and mains disconnected: remove each bathtub capacitor, apply 500 V DC via a 100 kΩ series resistor, and measure leakage current after 60 seconds. Fail threshold: >100 µA. Re-stuff all failed units. Re-install all units.
• 9
  AVC voltage check and MOD-1 With no antenna connected, RF gain maximum, BFO off, and receiver at full voltage: measure the AVC bus voltage. Compare against the −1.40 V specification and the −1.60 to −1.80 V Lankford target. If the measured value is more positive than −1.40 V despite C-136 and C-204 having been replaced, investigate the AVC chain before proceeding with MOD-1. Record the result.
• 10
  PTO calibration and end-point verification Set the main tuning to the beginning of any calibrated band and verify PTO output frequency with a frequency counter. Check end-point accuracy at both the start and end of the 1 MHz range. If end-point cannot be corrected by the standard trimmer: implement the PTO coil turn-removal procedure per Radio Boulevard Western Historic Radio Museum documentation (Part 2). Record pre- and post-calibration accuracy figures.
• 11
  PTO internal capacitor replacement (MOD-2) and K-011 Open the 70E-15 PTO and replace internal bypass and feedback capacitors with C0G/NP0 types. Seal and re-install the PTO. Verify PTO end-point and linearity have not shifted from the Step 10 baseline. If they have, re-trim.
• 12
  Full alignment and performance verification Perform the complete alignment procedure per TM 11-854 and all Change documents. Verify: sensitivity at ≤2 µV for 10 dB S/N across the amateur HF bands; all five selectivity positions operative; 100 kHz calibrator locking; S-meter tracking with increasing signal level. Record a post-restoration performance baseline.

Section 7 — Verification Tests

C-136 / C-204 Replacement Verification

PTO End-Point and Linearity Verification

Sensitivity Verification

AVC Range Verification