The R-388/URR is the United States military designation for the Collins 51J-4 high-frequency communications receiver. Manufactured by Collins Radio Company between approximately 1951 and 1958, it covers 500 kHz to 30.5 MHz across 30 switched bands with a permeability-tuned oscillator (PTO) providing 1 MHz of continuous coverage per band. The receiver was adopted by all branches of the US Armed Forces and saw extensive Cold War service in fixed and mobile HF communications installations.

Cosmetically, R-388 units range from pristine NOS examples still in original packaging to heavily abused rack fillers stripped of hardware and resprayed in field-expedient colours. The restoration task varies accordingly. This guide addresses the complete cosmetic workflow — from a field-worn unit requiring full strip-and-repaint through to touch-up and conservation of a largely intact original finish.

Variants Covered

• R-388/URR — standard military production unit, Collins Radio Company
• Collins 51J-4 — commercial equivalent; identical cabinet and panel, civilian data plates
• R-388A/URR — late production variant with minor circuit changes; identical externally
• 51J-3 — predecessor model; similar cabinet treatment, slightly different front panel layout

Restoration Phases

The Collins Collectors Association grading scale applies to the R-388 and 51J-4 as it does to all Collins amateur and commercial equipment. A fully stripped and correctly refinished R-388 can realistically achieve Very Good — the highest grade open to a repainted unit — provided all hardware is present, the panel markings are correct, and the finish quality is indistinguishable from original in normal viewing conditions.

Source: collinsradio.org — CCA Grading Standards

Markings-Specific Supplies

The R-388 front panel markings are a critical restoration element with no good shortcut. The following are specifically required for legends work:

• Tamiya flat white XF-2 — fine lettering touch-up
• Artist’s ruling pen or 0.2 mm Rotring isograph — line restoration
• Dry-transfer lettering sets (Letraset or equivalent) — full legend replacement
• Reproduction silk-screen decal sets — available from specialist suppliers (see Section 13)
• Magnifier (10×) — legend inspection and detail work
• Clear flat lacquer (Testors Dullcote or equivalent) — markings sealer

The R-388 chassis is substantially heavier than a typical S-Line unit — a complete receiver weighs approximately 35 lbs. Two people are strongly recommended for handling during disassembly. Photograph every assembly from multiple angles before removing any fasteners.

Rack Ear Removal

1. 1The rack ears are secured by four 10-32 machine screws per ear through the front panel side flanges. Remove all eight screws and set ears aside. Note the orientation — the upper mounting slot is offset rearward on most production runs.
2. 2Inspect ear mounting surfaces for corrosion. The aluminium ears in contact with the steel panel often develop galvanic corrosion — clean with a brass brush and treat with Ospho before reassembly.
3. 3Bag all rack screws together — they are a specific thread pitch and length and should not be mixed with chassis hardware.

Knob Removal

1. 1R-388 knobs use a Bristol (splined hex) setscrew in the knob skirt, typically 3/32″ or 1/8″. Use the correct Bristol wrench — a hex key will damage the splined socket. Loosen only, do not remove the setscrew fully.
2. 2Pull knobs straight off the shaft — never twist or rock. The main tuning knob and bandswitch skirt are largest and require a firm straight pull. Use a knob puller with padded jaws if available.
3. 3Bag and label each knob by its panel position. The R-388 uses several different knob body sizes — they are not interchangeable between positions.
4. 4Remove the S-meter pointer by pressing a thin plastic spudger under the hub base. This pointer is extremely fragile — treat it as irreplaceable.

Front Panel Removal

1. 1The front panel is secured to the main chassis by six to eight 6-32 machine screws around the perimeter. These are typically JIS type — use a JIS #2 driver to avoid damaging the heads.
2. 2Disconnect the bandswitch shaft coupling before attempting panel removal — the bandswitch extends through the front panel and will bind if the panel is pulled without releasing it first.
3. 3Label all inter-panel wiring connectors with masking tape before disconnecting. The PTO tuning shaft, AVC control, and several multi-pin connectors all pass between chassis and panel.
4. 4Set the panel face-down on foam padding. Place foam between the dial glass and the work surface — the dial scale is hand-printed and irreplaceable on many units.

Cabinet & Chassis Separation

1. 1Remove the top cover (four screws) and bottom pan (six to eight screws). Slide each off the main chassis frame — do not force; look for any hidden retaining screws at the rear corners.
2. 2The rear panel carries the power connector, audio and RF output connectors, and the BFO compartment. Remove the four perimeter screws and set aside. Bag all rear panel hardware separately.
3. 3Remove all rubber grommets at cable entry points and store them — original rubber grommets in good condition are worth preserving.
4. 4Remove the data plate and contract tag if present. These are historically significant — do not paint over them. They are typically secured by two small screws or rivets.

R-388 units in military service often received multiple field repaints in incorrect colours — various grays, flat black, and incorrect wrinkle colours have all been documented. Getting back to bare metal is almost always necessary for a quality restoration. The original factory finish is a relatively thin wrinkle coat over a primer, so stripping is straightforward on unmodified surfaces.

Front Panel — Special Considerations

If original panel markings are present and worth conserving, mask them with multiple layers of fine-line tape before applying stripper to the panel edges and knob mount areas. The silk-screened legends are solvent-sensitive and will lift if stripper contacts them directly.

Chemical Strip Procedure

1. 1Work outdoors. Apply a thick coat of Klean-Strip Aircraft Remover with a cheap paintbrush. Cover with plastic sheeting and allow 20–45 minutes dwell.
2. 2Scrape lifted paint with a plastic body spreader. Never use metal scrapers on the front panel — the aluminium substrate is easily gouged.
3. 3Re-apply a second coat to stubborn areas. R-388 field repaints are often oil-based enamel that requires two applications to lift completely.
4. 4Neutralise with a water rinse, then wipe with 3M PrepSol. Dry immediately with a heat gun to prevent flash rust on steel panels and chassis.
5. 5Residual wrinkle texture in corners: remove with maroon Scotch-Brite and lacquer thinner. Work into seams with a natural-bristle brush — nylon dissolves in methylene chloride.

Media Blasting

Walnut shell (40–80 mesh) is safe for both the steel chassis and the aluminium front panel. Glass bead (100–120 mesh) gives a better anchor profile for primer on chassis steel but may be too aggressive for the thin front panel aluminium — use walnut shell only on the panel. After blasting, prime within 30 minutes.

Corrosion Treatment

Military R-388 units frequently have corrosion at the rack ear mounting points and along the bottom chassis pan where moisture pooled during storage. Treat all rust with Ospho phosphoric acid converter, allow to dry to a white residue, then prime immediately. Treat galvanic corrosion at aluminium-to-steel interfaces with a zinc chromate primer before topcoating.

The R-388 front panel is cast or formed aluminium — relatively thick and dimensionally stable compared to the S-Line sheet steel cabinet. Dent repair on the panel uses the same glazing putty approach but the material is more forgiving. The steel chassis and covers are thin-gauge and require the same care as S-Line metalwork.

Non-Original Hole Filling

Many R-388 units were modified for additional connectors or bracket mounts during service life. Extra holes in the front panel are common. Fill from behind with a small aluminium backing patch bonded with PC-7 epoxy, then fill proud from the front with Evercoat Metal Glaze and sand flush. For holes in the chassis steel, a TIG braze by a body shop gives the most durable result.

Data Plate Recesses

The data plate recess on the front panel is a machined pocket. If the original data plate is missing, the recess must be filled flush before painting or the void will remain visible through the topcoat. Use Metal Glaze, sand level, and sand to 400 grit. If an original or reproduction data plate will be reinstalled, leave the recess at full depth.

Final Cleaning Sequence

1. 1Wipe all surfaces with 99% IPA on a lint-free cloth. Allow 2 minutes to flash off.
2. 2Follow with an acetone wipe — removes silicone contamination IPA cannot address.
3. 3Tack-wipe with a quality tack cloth, barely grazing the surface.
4. 4Prime within 30 minutes. Do not allow bare aluminium to sit — it forms an oxide layer that degrades primer adhesion.

The aluminium front panel and steel chassis require different primer chemistry. Self-etching primer bonds chemically to both, but zinc chromate is the historically correct choice for aluminium surfaces where corrosion protection is critical. A two-stage system is used throughout.

Stage 1 — Self-Etching Primer

• Product: SEM Self-Etching Primer (aerosol) or 3M 05917
• For aluminium surfaces: Zinc Chromate Primer (MIL-P-8585 type) is historically correct and available from aircraft suppliers — apply in place of or over the self-etching primer
• Apply 2 light coats, 10-minute flash between coats
• Do not sand — sanding removes the chemical bond mechanism
• Allow 1 hour dry before top-primering

Stage 2 — High-Build Surfacer

• Product: USC Spray Max 2K High Build Primer — activated aerosol, no spray gun required
• 3 medium coats, 15-minute flash between coats
• Cure 2–4 hours at 65–70°F ambient
• Block sand 320 grit → 400 grit wet on flat areas
• Spot-fill remaining low spots with Metal Glaze, blend, re-prime affected areas

The R-388/URR finish is the defining cosmetic characteristic of the receiver. The correct colour name, used consistently in the Collins restoration community, is St. James Gray — a wrinkle-finish paint applied across Collins military and commercial equipment of the era. The R-388 and S-Line are both St. James Gray but are not the same shade — do not use S-Line paint references for the R-388. The front panel, chassis, covers, and rack ears all receive the same colour.

Wrinkle-finish application on the R-388 follows the same technique as the S-Line cabinet work, with one significant addition: the front panel is aluminium and requires more aggressive heat activation than sheet steel to develop the wrinkle pattern. Aluminium is an excellent heat conductor and draws heat away from the surface during curing — compensate with a longer heat application time.

Application Sequence

1. 1Temperature: Ideal ambient is 65–80°F. Pre-warm the panel with a heat gun to ~90°F before the first coat. Cold aluminium is the leading cause of poor wrinkle formation on R-388 panels.
2. 2Apply a heavy first coat — significantly heavier than a standard topcoat. Wet and thick, not a mist coat. Allow 5 minutes skin-over time.
3. 3Apply a second heavy coat. Allow 10 minutes.
4. 4Heat activation: Apply heat gun at 400–450°F from 6 inches, moving continuously. Aluminium panels require 30–60 seconds longer than steel to fully develop the wrinkle — don’t stop too early because the surface appears to have cured.
5. 5Allow to cool fully before handling. Full cure: 24 hours at room temperature. The aluminium panel will feel harder sooner than steel but wait the full cure time before fitting legends or hardware.

Rack Ears

The rack ears are typically aluminium extrusions. They receive the same wrinkle finish as the front panel. Hang them vertically during painting to avoid runs — the flat rack face is the primary visible surface and must be free of sags. Both sides of the ear receive finish; the rear face is visible when the receiver is racked.

Chassis & Covers

The chassis, top cover, and bottom pan are steel and receive the same wrinkle finish as the panel. These surfaces are not visible during normal operation but are clearly seen during inspection and affect grade assessment. Apply the same two-coat wrinkle technique and heat-activate with a heat gun or oven.

The R-388 front panel legends — white silk-screened text and symbols on the gray wrinkle finish — are among the most restoration-critical features of the receiver. Correct, legible markings are required for Very Good CCA grade. Lost, incorrect, or crudely touched-up legends will hold an otherwise excellent restoration at Good grade regardless of paint quality.

Assessing Existing Markings

Under a 10× loupe, original R-388 legends show a slightly raised white print with sharp edges. Field touch-ups and brush-applied repairs show irregular edges, inconsistent opacity, and brush marks. Photocopy a good original panel and print at 1:1 scale as a reference for letter size and weight before starting any repair work.

Touch-Up Technique

1. 1Clean the area with IPA on a cotton swab. Allow to dry fully.
2. 2Mix Tamiya flat white XF-2 to a flowing consistency — slightly thinner than the tube consistency. Test on scrap material first.
3. 3Apply with a 000 artist’s brush for letters, or a ruling pen for straight lines. Use a bridge rest to keep your hand off the panel surface.
4. 4Allow each character to dry 30 minutes before adjacent work. Wet paint smears easily.
5. 5Clean up overspray with a fine cotton swab barely damp with Tamiya thinner — work from outside the legend inward to avoid lifting the white paint.
6. 6When touch-up is complete and fully dry (24 hours), seal the entire markings area with a single thin coat of Testors Dullcote sprayed from 12 inches.

Full Legend Replacement

When markings must be fully replaced, the options in decreasing order of authenticity are:

• Reproduction silk-screen decals — available from specialist suppliers; applied over the finished topcoat and sealed with Dullcote. Most accurate result.
• Waterslide decal sheets — custom-print the legend artwork on waterslide film, apply wet, position carefully, allow to dry, seal. Artwork files for R-388 legends are available on r-390a.net.
• Dry-transfer lettering (Letraset or equivalent) — for individual characters and abbreviated legends. Requires careful spacing and a burnishing tool.
• White paint stencil — least preferred; produces visible edges at the stencil boundary that are difficult to eliminate.

Data Plates

Original contract data plates are aluminium stampings with black-filled lettering. Do not overpaint them — they are a primary authenticity indicator. If the data plate is missing, reproduction plates are available from a small number of community suppliers. The contract number and serial number range should match the receiver’s production period.

Knob Restoration

R-388 knobs are black phenolic or Bakelite — the same material used throughout Collins military production. They are not painted and should not be repainted. Clean with mild dish soap and a soft brush. Polish crazes and surface oxidation with Novus Plastic Polish #2 followed by #1. A light coat of Meguiar’s PlastX after polishing restores depth and colour.

• Knob skirt indicator lines are machined grooves filled with white paint — touch up with Tamiya XF-2 using a 0.3 mm ruling pen
• The main tuning knob is weighted and should not be disassembled — clean externally only
• Missing knobs: NOS examples occasionally appear on eBay; the CCA Support Directory lists several vendors with R-388-compatible knob stock
• Do not substitute S-Line knobs — the shaft bore and skirt diameter differ from R-388 production

Military Connectors

The R-388 uses a mix of BNC, PL-259 (SO-239 panel mount), and military binding post connectors for audio and RF. These should be cleaned and re-used wherever possible — replacement with modern commercial connectors degrades CCA grade.

• Clean BNC and SO-239 bodies with 91% IPA and a cotton swab — avoid abrasives on the plating
• Polish tarnished silver-plated contacts with Flitz Metal Polish on a cotton swab
• Binding posts: clean with fine brass brush; replace only if the threaded post is stripped
• The AC power connector is a military twist-lock type — NOS replacements are available from Fair Radio Sales and Surplus Sales of Nebraska

Rack Hardware

• Original rack mounting screws are 10-32 × 3/4″ pan-head cadmium-plated — replacements available from Fastenal and Grainger
• Rack ear finish should match the front panel — do not leave ears as bare aluminium
• Original felt or rubber gasket strips behind the rack ears should be replaced if deteriorated — 1/16″ closed-cell neoprene foam tape is acceptable
• The rear panel handles (if fitted) are typically black wrinkle-painted aluminium — treat identically to the rack ears

Fastener Preservation

As with all Collins military equipment, original cadmium-plated fasteners are preferred over modern zinc or stainless replacements. Clean with a soft brass brush and Flitz before deciding to replace. Coat all reinstalled fasteners lightly with NoOx-ID A-Special to prevent future corrosion at panel contact points.

1. 1Apply panel markings and seal with Dullcote before fitting any hardware. Knob shafts and mounting screws will damage fresh legend work if installed first.
2. 2Reconnect all inter-panel wiring harnesses per your labelled documentation. Route cables exactly as original — incorrect routing can cause the bandswitch to bind or create RF interference paths.
3. 3Fit the front panel to the chassis, engaging the bandswitch shaft coupling before tightening panel perimeter screws. Use JIS driver to avoid cam-out damage on original screw heads.
4. 4Install all knobs to their labelled positions. Tighten Bristol setscrews snugly on the shaft flat — do not over-torque phenolic knob bodies.
5. 5Fit rack ears with original screws. Verify the mounting slot orientation is correct — the offset slots face rearward on standard 19″ rack mounting.
6. 6Install the S-meter pointer last. Press gently and straight onto the spindle — do not use tools.

CCA Grade Self-Assessment

Evaluate the completed restoration under natural daylight against CCA standards. The R-388-specific assessment points are:

• Panel markings — all legends present, white, sharp-edged, and correctly positioned
• Colour match — panel, chassis, covers, and rack ears must be visually identical in colour and texture
• Data plate — present, correctly mounted, not overpainted
• Knobs — correct type, polished, indicator lines white and sharp
• Hardware — no modern stainless fasteners visible; no incorrect connectors
• Dial scale — undamaged, correctly aligned with the PTO position