R-390/URR
Failure Prevention Kit — Component & Modification Design

Section 1 — R-390/URR Design: How It Differs from the R-390A

Understanding the R-390’s unique architecture is essential before any restoration work. Every R-390-specific failure mode in this document has its root in one of the five fundamental design differences listed below.

Section 2 — Root Cause Failure Analysis

The following ten failure modes account for the overwhelming majority of R-390/URR restoration casualties. They are presented in priority order — the order in which they must be addressed. All are unique to the R-390’s design or manifest differently from their R-390A equivalents.

• 1
  6082 Voltage Regulator Tubes — Unregulated B+ Destroys Downstream Tubes The two 6082 double-triode tubes in the audio module are the heart of the R-390’s B+ regulation circuit. They are heated by a 28-volt filament (considerably higher than most receivers) and run continuously at high dissipation. Failure modes: loss of regulation (most common) where the regulated B+ reads the full raw supply of approximately 300 V instead of the specified +180 V; or low regulation where B+ falls to 130 V or below. Loss of regulation is the dangerous failure: with 300 V on regulated supply rails, the downstream 6AJ5 audio output tubes and other tubes in those circuits can be destroyed within minutes. This failure has a known and insidious misrepair: some technicians have replaced the 6082 (28-volt filament) with a 6080 (12-volt filament). The 6080 will appear to regulate for a short period, then fail irreversibly, often taking other tubes with it. Always verify filament voltage before trusting any 6082 replacement. Replace only with genuine 6082 or solid-state equivalent (see MOD-1).
• 2
  5651 Voltage Reference Tubes — Regulation Circuit Loss The two 5651A neon-glow voltage reference tubes establish the reference voltage that the 6082 circuit regulates to. When working correctly, both 5651A tubes glow brightly at power-up, then dim as the regulation circuit pulls the B+ down to target and the current through the reference tubes stabilises. If the 5651A tubes remain at full brightness after warm-up, the regulation is not functioning. Common associated faults: leaky C-608 capacitor (causes low B+ rather than loss of regulation), and out-of-specification values in resistors R-625 (1 kΩ), R-626 (2.7 kΩ), and R-624 (520 kΩ). All three must be checked before condemning the 5651A tubes themselves. New 5651A tubes are still available as NOS and at reasonable cost.
• 3
  47 Ω Cathode Resistors for the 6082 Tubes — Thermal Failure The four 47 Ω cathode resistors in the 6082 regulator circuit (in the audio module) are stressed thermally by the high continuous dissipation of the regulator tubes. They fail open or drift significantly high, causing the regulated B+ to read incorrectly or for the regulator to lose control. These resistors are physically small and are often not visibly burned even when electrically dead. Always measure them in-circuit with power removed and compare to specification. Replace all four as a set with 5-watt wire-wound types at the correct values. The original values are specified in TM 11-856.
• 4
  Series Filament String Open Circuit — Silent Receiver with No Obvious Cause The R-390’s 26-volt AC filament supply runs several groups of tubes in series. The VFO oscillator tube and the BFO oscillator tube share a series string with the 3TF7 ballast tube. If any tube in a series string develops an open filament, every tube in that string goes dark simultaneously — but tubes in other strings continue to glow normally. The result: a receiver that appears mostly normal (tubes lit, B+ present) but produces no output at all. The BFO and VFO failing together is a classic sign of a series-string open. Systematic diagnosis requires pulling tubes one at a time and testing the filament with a continuity meter, starting with the most failure-prone types (3TF7, 26Z5W, then the oscillator tubes).
• 5
  3TF7 Ballast Tube — BFO and VFO Simultaneous Silence The 3TF7 (Amperite ballast tube) is a current-stabilising device in the filament circuit serving the PTO (VFO) oscillator and the BFO oscillator, both wired in series through it. When the 3TF7 fails open, both the BFO and the VFO go silent simultaneously: the BFO disappears (no CW or SSB reception) and the PTO produces no output (the receiver may receive AM but with no stable LO). This tube is the most expensive and least available in the entire R-390 design — new equivalents cost $15–$25. Test the original by measuring filament continuity. Several solid-state and resistor substitutes exist (see Modification section and r-390a.net) and should be considered as a permanent replacement rather than hoping to find a NOS 3TF7.
• 6
  26Z5W Rectifier Tubes — B+ Supply Failure The 26Z5W is the R-390’s full-wave rectifier tube, running on a 26-volt filament. These tubes age and fail like any rectifier: weak emission reduces the unregulated B+ (the raw DC that feeds into the 6082 regulator circuit), and an open filament kills B+ entirely. When one 26Z5W plate has weak emission, the regulation circuit may still maintain correct B+ under light load but collapse under normal operating current. Always test 26Z5W tubes on a tube tester before trusting them, and keep tested spares available — the 26Z5W is not interchangeable with any common rectifier and requires specific sourcing.
• 7
  IF Module Paper Capacitors — Universal Leakage Across All Axial Types The R-390 IF module was thoroughly documented in one restoration account that tested all 18 axial-lead paper capacitors after removal: every single one leaked. Most had microscopic cracks near the body seams that were invisible until magnification. These are the classic “Brown Beauty” (Sprague Vitamin-Q) oil-impregnated paper dielectric types. Their failure manifests as bias voltage offsets in the IF stages, reduced sensitivity, poor S-meter tracking, and background noise that modulates with signal strength (classic leakage-induced AVC corruption). All axial-lead paper capacitors in the IF module must be replaced with modern polypropylene or polystyrene film types, regardless of measured DC resistance — in-circuit resistance measurement cannot reliably detect the partial leakage that degrades performance.
• 8
  Large Oil-Filled IF Capacitor — Above-Chassis Oil Can Mounted above the IF module chassis is a large oil-filled capacitor unique to the R-390 (not present in the R-390A). This capacitor handles higher voltage than most of the other IF bypass/coupling types and its failure mode is dielectric breakdown — a hard short or high leakage. Test it at its rated voltage (400–500 V DC) via a 100 kΩ series resistor before any power-up. Leakage above 100 µA at rated voltage is a failure. Replacement requires sourcing a film-type capacitor at the correct capacitance and voltage rating; the physical size is not critical as it is strap-mounted above the chassis.
• 9
  Plug-In Multi-Section Filter Capacitors C-603 / C-606 — Dried Electrolyte Like the R-390A, the R-390 uses two multi-section electrolytic filter capacitors that plug into octal tube sockets. After 70 years the electrolyte has dried, effective capacitance has dropped dramatically, and ESR has risen. Symptoms are identical to the R-390A version: 60 Hz hum on all received signals, B+ sag under load, and a distorted or compressed audio output. Reformation is possible if the capacitors are not yet severely degraded. The same Hayseed Hamfest replacement plug-in units designed for the R-390A are compatible with the R-390 sockets, as the octal plug-in format is shared between the two models. Verify pin assignments match before fitting.
• 10
  RF Module Mechanical Synchronisation — Green Locking Gear Misuse The R-390 RF module must not be removed without first engaging the green locking gear mounted on the module front. This gear locks the slug-rack drive to prevent the main frame’s KC tuning drive from becoming unsynchronised with the RF module’s band-change mechanism. A receiver where a previous technician removed the RF module without using the locking gear will have KC and band-change synchronisation errors that require a complete mechanical re-alignment — a multi-hour procedure involving the Oldham coupler, the crystal oscillator module phasing, and a full RF/IF alignment. The first check on any unknown R-390 is to verify the green locking gear is present and undamaged. If it is missing, do not remove the RF module under any circumstances until a replacement is sourced.

Section 3 — Kit Component Reference

The table below lists all components in the R-390/URR failure prevention kit. Items are specific to the R-390’s design — these are not the same as the R-390A component kit. Tier 1 components are mandatory pre-power replacements. Tier 2 items address secondary aging. Modification components are listed separately.

Section 4 — Pre-Power Safety Protocol

Visual Inspection Checklist

• Locate and confirm the green RF module locking gear is present. If missing, do not remove the RF module for any reason until a replacement is found.
• Check the rear panel for B+ fuses F102 and F103. If your unit has them, test both. If F103 is blown, locate and resolve the sub-chassis fault before power-up.
• Inspect C-603 and C-606 (the plug-in electrolytic filter capacitors) for swelling, electrolyte staining, or cracked seals. Any of these means the can has already failed — replace before power-up.
• Check the two 6082 tubes and confirm they are marked 6082 (not 6080). The 6080 is a 12-volt filament type that will fail within months in this application.
• Inspect the AC mains cord. If it is a 2-wire type or shows cracked/brittle insulation, install K-007 before any power connection.
• Look for any sign of previous catastrophic failure: burned board sections, heat-discoloured resistors, melted wire insulation in the audio module area. If found, treat as a fault investigation before applying power.
• Confirm all module interconnecting cables and mini-BNC connectors are properly seated. P218 connects to J518, P213 to J513 — verify the pairing logic before closing the receiver.

Section 5 — Tier 1 Mandatory Replacements

Diagnosing the 6082 Circuit Before Replacement

The 6082 circuit is located in the audio module. With the receiver powered via Variac at 50% mains and B+ monitored at the test point specified in TM 11-856:

• B+ reads ~300 V (full raw supply): Regulation has failed entirely. Check 6082 tube filament and emission first. If 6082 tubes check out, check 5651A reference tubes and then R-625, R-626, R-624.
• B+ reads <130 V: Check C-608 for leakage and R-623 and R-618 in the 6BH6 amplifier circuit. Leaky C-608 is the most common cause of low regulated B+.
• 5651A tubes stay bright permanently: Regulation is not engaging. Recheck 6082 emission and all four 47 Ω cathode resistors. At least one is likely open.

  R-390/URR VOLTAGE REGULATOR CIRCUIT — DIAGNOSTIC SUMMARY

  V605, V606 (6082) — Regulator triodes, 28V filament, in audio module
     Normal: Regulation maintains B+ at +180 VDC ± 5V
     Failure (open 6082): B+ rises to full unregulated ~300 VDC → kills 6AJ5 etc.
     Failure (low): B+ drops below 150 VDC → sensitivity loss across all bands

  V608, V609 (5651A) — Voltage reference tubes
     Normal: Bright glow at power-up, dims within 30 seconds as regulation engages
     Failure: Stays bright → regulation not engaging → check 6082, cathode resistors

  Key resistors in regulator path:
     R625 = 1 kΩ    Check for opens
     R626 = 2.7 kΩ  Check for opens
     R624 = 520 kΩ  Check for opens
     R623, R618     In 6BH6 (V607) amplifier section — check if low B+ problem

  Key capacitor:
     C608           Leaky → causes LOW regulated B+ (130V or less)
                    Replace with film type at correct value and voltage

Figure 1. R-390 voltage regulator diagnostic chart.

Tracing a Series Filament String Open

When a series filament string is open, all tubes in that string will be dark while other strings appear normal. The methodical approach:

1. With the receiver powered off and unplugged, pull each tube in the suspect string one at a time.
2. Measure filament resistance across the filament pins of each pulled tube with a DMM on resistance range. A good filament will read a few ohms. An open filament reads infinite resistance.
3. Start with the 3TF7 (ballast tube) as it is in the BFO/VFO string and is the most common series-string failure. Then test the 26Z5W rectifier pair (power supply string).
4. Once the open tube is identified and replaced, verify all other tubes in the string with the same test before powering up.

Pull both plug-in filter cans and inspect. If either shows physical damage, replace immediately with K-006 units. If both appear physically intact, attempt reformation via the Variac procedure in Section 4. A properly reformed or replaced can will result in B+ within specification with <1 V AC ripple on the regulated supply.

Section 6 — Circuit Modifications

  MOD-3 — MWO 11-5820-294-35/2 WIRING CHANGE: Audio Tube XV603

  BEFORE modification:
  XV603 pin 2 (suppressor grid) — connected to GND via jumper
  XV603 pin 7 (cathode) — connected to GND via cathode resistor
  Result: suppressor at same potential as cathode except for cathode
          resistor voltage drop → parasitic oscillation path exists at 340 MHz

  AFTER modification:
  Remove jumper from pin 2 to GND.
  Connect pin 2 (suppressor grid) directly to pin 7 (cathode).
  Result: suppressor held at same potential as cathode at all times →
          parasitic oscillation path eliminated ✓

  Wire: 22 AWG solid copper, 75 mm maximum length.
  Solder both joints cleanly. No substitution or partial completion.

Figure 2. MOD-3 XV603 suppressor-to-cathode wiring change.

Section 7 — Installation Sequence

Work one module at a time. The R-390’s modular construction is its greatest restoration asset. Verify each module before moving to the next.

• 1
  Documentation and inventory Photograph every module, every connector, and every tube label before touching anything. Locate and confirm the green RF module locking gear. Consult TM 11-856 throughout — this receiver has subtleties not in any other manual.
• 2
  Install K-007 — 3-wire mains cord (MOD-4) Replace the mains cord before any other work. Verify earth continuity to the chassis with a DMM on resistance range.
• 3
  Verify 6082 tube types (K-001) Pull both regulator tubes and confirm they are marked 6082, not 6080. If 6080 tubes are fitted, replace immediately with genuine 6082 before any power-up. If solid-state replacement module is being installed (MOD-1), do so now before any power is applied.
• 4
  Replace 47 Ω cathode resistors (K-003) and check C-608, R-624–626 Remove the audio module. Measure all four cathode resistors and C-608. Replace any out-of-specification resistors and C-608 (K-010) while the module is accessible on the bench. Reinstall audio module.
• 5
  Replace C-603, C-606 (K-006) and 26Z5W rectifiers (K-005) Pull the plug-in filter cans and fit replacements. Fit tested 26Z5W rectifiers. Do not power up yet.
• 6
  3TF7 inspection and MOD-2 decision Pull the 3TF7 and test filament continuity. If good, re-fit and continue. If open, install the solid-state substitute (MOD-2) rather than sourcing a replacement 3TF7 — the solid-state version is more reliable long-term. If MOD-2 is installed, also rewire the VFO and BFO oscillator tube filaments per the modification documentation.
• 7
  First Variac power-up — regulator verification Connect via Variac. Advance to 25% mains and pause 10 minutes — monitor B+ continuously. Advance to 50%: B+ should be within 15 V of +180 V. Advance to 75%: B+ should be at specification. Advance to 100%. If B+ exceeds 250 V at any point: stop and diagnose. After 30 minutes at full voltage, confirm 5651A tubes have dimmed properly, B+ is stable, and audio module is running at expected temperature.
• 8
  Series filament string audit With receiver at full voltage and 5 minutes warm-up: darken the room and inspect every tube socket for dark (unlit) tubes. Any dark tube indicates a series string open. Trace the string systematically with power removed and DMM on resistance across filament pins. Identify and replace the open tube.
• 9
  IF module paper capacitor replacement (K-008) Remove IF module. Replace all 18+ axial-lead paper capacitors with polypropylene or polystyrene film types. Test the large above-chassis oil capacitor (K-009) at rated voltage. Replace if leakage fails. Reinstall IF module.
• 10
  MOD-3 — XV603 suppressor-to-cathode wiring change With the receiver powered down: locate XV603 in the audio module area, remove the pin-2-to-ground jumper, and install pin-2-to-pin-7 wire per MOD-3 procedure. Resolder both joints.
• 11
  Selenium rectifier replacement (K-012) Locate the small selenium rectifier under the power transformer. Remove and replace with silicon diode bridge. Verify polarity. Test break-in relay function after replacement by keying the STANDBY switch and confirming the antenna relay engages.
• 12
  Full alignment and performance verification Perform full IF and RF alignment per TM 11-856. The R-390’s LC-tuned IF stages require careful alignment to achieve specified selectivity and minimum insertion loss. Verify sensitivity at ≤1 µV on 14 MHz (the R-390 should comfortably achieve this with fresh tubes and properly aligned IF). Verify all six selectivity positions produce audible narrowing. Verify 100 kHz calibrator operation. Record a post-restoration baseline.

Section 8 — Verification Tests

B+ Regulator Verification

Series Filament String Verification

Sensitivity and IF Alignment Verification

RF Module Synchronisation Verification