The R-390/URR is a general-coverage military HF communications receiver covering 500 kHz to 32 MHz across 32 bands. It was designed and manufactured exclusively by Collins Radio Company, Cedar Rapids, Iowa, from approximately 1950 to 1954 under U.S. Army Signal Corps contract. The receiver was developed in response to Signal Corps specification SCL-1134-B and was initially classified SECRET.

The R-390 preceded the more widely known R-390A, which was introduced in 1954–1955 as a cost-reduction redesign. Despite their similar appearance, the two receivers are mechanically and electrically distinct. The R-390 uses L-C IF filtering, a welded chassis, standard-size BNC internal connectors, a more complex voltage regulator system, and a different sub-assembly arrangement. It is significantly rarer than the R-390A.

Manufacturer

Unlike the R-390A — which was produced by Collins, Motorola, Stewart Warner, Electronic Assistance Corporation, Capehart, Amelco/Teledyne, and Fowler Industries — the R-390 was manufactured by Collins Radio Company only. Any R-390 bearing another manufacturer’s data plate has had its plate replaced or the receiver misidentified.

Restoration Phases

The R-390A was designed as a cost-reduction version of the R-390. Despite superficial similarity, they differ in ways that materially affect restoration procedures. Understanding these differences prevents costly mistakes.

Electrical Differences

• IF filtering: The R-390 uses L-C type IF filtering. The R-390A uses Collins mechanical filters for the narrow bandwidths. Community opinion holds that the R-390’s L-C filtering produces smoother audio.
• Tube complement: The R-390 uses 6AJ5 tubes where the R-390A uses 6AK5; 6BJ6 in place of the R-390A’s 5749. Other tube types also differ.
• Voltage regulator: The R-390 employs a more complex voltage regulator system using 6082 tubes. These generate substantial heat and are a known problem area in the R-390 that does not exist in the R-390A.
• Ballast tube: The R-390 uses a 3TF7 ballast tube — unique to this receiver.
• Crystal oscillators: In the R-390, the first and second crystal oscillators are in a separate sub-assembly driven via a small Oldham coupler from the main gearbox. In the R-390A the crystal oscillators are integrated differently.
• Calibrator: The calibrator sub-assembly in the R-390 is mounted underneath the chassis. In the R-390A it is in a different location.

Mechanical Differences

• Chassis construction: The R-390 chassis is a welded unit. The side and internal panels are permanently attached to the baseplate. The R-390A chassis is different in construction.
• Converter placement: In the R-390, the first and second converters are placed in line with the RF racks. In the R-390A they are behind the racks.
• Internal coax: The R-390 uses standard BNC connectors for internal coax. The R-390A uses a miniature variety that can be difficult to handle.
• The green gearwheel: The R-390 has a green gearwheel on the front of the RF deck gearbox that must be relocated before the RF sub-assembly is removed. This procedure does not apply to the R-390A.
• Manufacturer: Collins only for the R-390. Multiple contractors for the R-390A.

Cosmetic Differences

• Front panel finish: The R-390 front panel uses a semi-gloss non-wrinkle gray enamel per procurement specification TT-C-595 Color No. 2610. It is not a wrinkle finish like the R-388 or the S-Line cabinet, and not a flat finish.
• Panel markings: Restored using a lacquer stick technique — paint over the legends with the topcoat, then fill the legends by drawing a lacquer stick across them. This differs from the silk-screen decal or dry-transfer approaches used on other Collins receivers.
• Data plate: Collins Radio data plate only — no other manufacturers.

The CCA grading scale applies to the R-390 as it does to all Collins equipment. The R-390’s rarity means that even a Good or Fair example has significant value as a restorable unit — do not part out an R-390 that can be restored. A fully cosmetically restored R-390 with correct panel markings can achieve Very Good grade, which is the ceiling for any repainted unit.

Source: collinsradio.org — CCA Grading Standards

Screwed to the front of the RF deck gearbox is a green gearwheel that appears to serve no useful purpose in normal operation. It is, in fact, a timing reference wheel that maintains the mechanical synchronisation of the receiver’s complex gear-driven tuning system when the RF deck is removed from the chassis.

Procedure

1. 1Before removing the front panel or any sub-assembly, locate the green gearwheel on the front face of the RF deck gearbox. It is a distinctive green-painted gear, typically held by a setscrew on a shaft.
2. 2Remove the front panel to gain clear access to the green gearwheel and the large brass gear at the centre of the gearbox.
3. 3Remove the green gearwheel from its current position on the front of the gearbox.
4. 4Reinstall the green gearwheel to the right of the large brass gear at the centre of the gearbox. It will engage a gear behind this position. Ensure it is firmly fixed to the shaft.
5. 5There is an oblong slot in the wheel that must be aligned with the corresponding slot on the shaft. Verify this alignment before tightening. Misalignment means the wheel is not engaged correctly and synchronisation will be lost when the RF deck is moved.
6. 6Do not proceed with RF deck removal until you have verified the green gearwheel is correctly seated in its new position and the shaft slot alignment is confirmed.

The R-390 is a heavy, densely constructed receiver. Complete disassembly breaks the unit into distinct sub-assemblies: the RF deck, IF deck, Audio/VR deck, PTO, Power Supply, Calibrator, front panel, and rear panel with wiring harness. Each sub-assembly can be removed and cleaned independently.

Knob Removal

1. 1R-390 knobs use Bristol (splined) setscrews. Use the correct Bristol wrench — not a hex key, which will damage the splined socket.
2. 2Pull all knobs straight off their shafts. Label and bag each by panel position. The main tuning knob is weighted and should be handled carefully.
3. 3Remove the S-meter pointer carefully — press a thin plastic spudger under the hub base. This pointer is fragile and not easily replaced.

Front Panel Removal

1. 1Perform the green gearwheel procedure (Section 5) before removing the front panel.
2. 2The front panel is secured by machine screws around its perimeter — JIS type. Disconnect all inter-panel wiring connectors and label each before separation.
3. 3Note that several controls pass through the front panel. The bandswitch shaft coupling must be released before the panel can be drawn forward.
4. 4Set the panel face-down on foam padding. Protect the dial glass — it is fragile and difficult to replace.

Sub-Assembly Removal Sequence

1. 1With the front panel removed and the green gearwheel relocated, the RF sub-assembly can be removed. This is the most complex sub-assembly — consult TM 11-856 for the precise removal sequence. The first and second converters are in line with the RF racks and require care to disengage.
2. 2Remove the IF sub-assembly. Note the separate crystal oscillator sub-assembly driven via the Oldham coupler — disconnect the coupler carefully and set the crystal sub-assembly aside.
3. 3Remove the Audio/VR deck. The 6082 voltage regulator tubes run very hot in service — if the receiver has been recently operated, allow 30 minutes cooling before handling the VR deck.
4. 4Remove the PTO. The PTO calibration is a separate procedure requiring the TM — do not adjust PTO calibration during cosmetic restoration work.
5. 5Remove the Power Supply sub-assembly. Discharge all capacitors fully before handling — use a bleeder resistor, not a screwdriver short.
6. 6The Calibrator sub-assembly is mounted underneath the chassis. Remove it last, after all top-deck sub-assemblies are out. It is secured by screws accessible from beneath.

Bare Chassis

With all sub-assemblies removed you will have the bare welded chassis. The baseplate is zinc-coated and will come up clean and shiny after cleaning. The side and internal panels are permanently attached to the baseplate and will have painted areas that require touch-up. The chassis is sufficiently robust to withstand direct water rinsing during cleaning — see Section 7.

The R-390 chassis cleaning approach differs significantly from the S-Line and R-388. The welded chassis and individual sub-assemblies are sufficiently robust for direct water washing, which is the standard community technique. The front panel, however, requires a different approach because of its hard semi-gloss baked enamel (TT-C-595 No. 2610) finish.

Chassis & Sub-Assembly Cleaning

The following procedure is documented by the R-390 community (Dave Medley, r-390.com) as effective for a thorough chassis clean:

1. 1Spray the bare chassis thoroughly with Formula 409 and scrub with 0000 fine steel wool until clean. The zinc-coated baseplate will come up shiny.
2. 2Rinse with a garden hose. In hot climates, the water may initially be very hot — allow the line to run first.
3. 3Follow with a rinse of distilled water to remove tap water mineral residue.
4. 4Dry thoroughly with a hair dryer or in an oven. Any trapped moisture will cause corrosion.
5. 5When dry, repaint any painted chassis areas — scuff lightly with steel wool and apply semi-gloss gray enamel per TT-C-595 No. 2610 (Rustoleum Dark Machine Gray is the community-documented approximation). The zinc baseplate requires no painting.

Each sub-assembly (RF deck, IF deck, Audio/VR deck, Power Supply) can be cleaned using the same 409 spray, hose, and distilled water rinse sequence. The RF deck requires special attention — see the r-390.com RF deck cleaning notes before starting on this sub-assembly.

Front Panel — Baked Enamel

1. 1Clean the front panel surface with 409 and a soft cloth. Remove all grease, grime, and loose material.
2. 2Scuff the entire panel surface with 0000 fine steel wool to break the gloss and provide mechanical adhesion for the new topcoat.
3. 3Wipe down with 99% IPA followed by an acetone wipe. Tack-clean. Prime any bare metal areas exposed by the steel wool.
4. 4Apply semi-gloss topcoat (TT-C-595 No. 2610). The existing enamel panel legends will be painted over — they are restored using the lacquer stick technique after the topcoat is dry. See Section 12.

Corrosion Treatment

Treat any rust on the chassis with Ospho phosphoric acid converter before priming. Allow to dry to a white zinc phosphate residue, then prime immediately. The welded chassis corners and any areas where dissimilar metals meet are common corrosion sites.

With the chassis cleaned and the front panel scuffed, surface prep focuses on ensuring good primer adhesion on any bare metal areas and correct masking before topcoat application.

Dent Repair

Minor dents in the front panel or cabinet side panels can be worked from behind using a hammer and dolly before repainting. Apply Evercoat Metal Glaze finishing putty to any low spots that can’t be hammered out, sand to 220 grit, and re-prime before topcoat. Deep dents that have creased the metal are not practically repairable with body filler on a receiver of this construction — accept and disclose them.

Non-Original Hole Filling

R-390 units sometimes carry field modification holes — additional connector cutouts, bracket mounts, or antenna switch holes. Fill from behind with a small aluminium patch bonded with PC-7 epoxy, fill proud from the front with Metal Glaze, and sand flush. Prime before topcoat.

Final Cleaning Before Topcoat

1. 1Wipe all surfaces with 99% IPA on a lint-free cloth. Allow to flash off (2 minutes).
2. 2Follow with an acetone wipe to remove any silicone contamination.
3. 3Tack-wipe lightly. Prime within 30 minutes of final cleaning on any bare metal areas.

On the R-390 the primer requirement is more limited than on the S-Line or R-388, because the front panel is typically scuffed-and-painted rather than stripped to bare metal. Primer is required on bare metal areas only — typically the chassis, any holes that have been filled, and areas where corrosion was treated.

• Self-etching primer on all bare steel: SEM Self-Etching Primer or 3M 05917 — 2 light coats, no sanding
• High-build surfacer over filler: USC Spray Max 2K High Build Primer — activated aerosol, 3 medium coats, cure and sand to 320 grit
• The front panel over the scuffed baked enamel does not require primer — the scuffed surface provides sufficient mechanical adhesion for the topcoat
• Prime and allow full cure (minimum 2 hours) before topcoat

The R-390 procurement specification called for a semi-gloss non-wrinkle gray enamel matching Federal Standard Color No. 2610 in specification TT-C-595. This is a fundamentally different finish type from the wrinkle finish used on the S-Line cabinets or the R-388 front panel. Many surviving R-390 panels present today as a medium gray with a slight greenish cast — this is still considered within the TT-C-595 2610 gray rather than a different official colour family. The community-documented closest consumer approximation is Rustoleum Dark Machine Gray, as recorded by Dave Medley (r-390.com).

The R-390 front panel takes a semi-gloss enamel topcoat per TT-C-595 Color No. 2610 — not a wrinkle or texture product, and not a flat enamel. Application is straightforward compared to wrinkle-finish work. The key constraint is coat thickness: if the topcoat is applied too heavily over the panel legends, the lacquer stick technique for marking restoration becomes difficult because the legend recesses are filled.

Front Panel Application

1. 1Mask off the dial glass, meter glass, and all bezel hardware before painting. Fine-line tape at the bezel edges, doubled with painter’s tape.
2. 2Apply a thin first coat — thinner than a normal topcoat. The goal at this stage is to seal and provide a base, not full coverage. Allow 30 minutes flash time.
3. 3Apply a second medium coat for full coverage. Allow to dry 1 hour.
4. 4Do not apply a third coat over the panel legends — this will fill the recesses. Two coats is sufficient; the legend restoration (Section 12) provides the visible white fill.
5. 5Allow 24 hours cure at room temperature before proceeding to legend restoration.

Chassis Painted Areas

Painted areas on the welded chassis — internal brackets, vertical side panels — receive the same Rustoleum Dark Machine Gray. Scuff with steel wool, clean, and apply two medium coats. The zinc-coated baseplate does not receive paint — the zinc surface is original and should remain exposed. It will come up shiny after the 409 and steel wool cleaning process.

The R-390 front panel markings are engraved or embossed legends filled with white paint — a different construction from the silk-screened legends of the S-Line or R-388. This is important because it enables the lacquer stick technique, which is the standard community method for restoring R-390 panel markings after repainting.

Lacquer Stick Technique

1. 1Allow the topcoat to cure fully — minimum 24 hours at room temperature. A partially cured topcoat will lift when the lacquer stick is applied.
2. 2Draw a white lacquer stick firmly across the panel surface, working in sections. The lacquer fills the engraved legend recesses.
3. 3Before the lacquer stick fill has fully hardened (typically 5–10 minutes working time), wipe the panel flat surface with a soft cloth dampened with lacquer thinner. The fill in the recesses is protected by the engraving geometry; the excess on the flat surface wipes clean.
4. 4Inspect under a magnifier. Repeat the fill and wipe process for any legends that are incompletely filled.
5. 5Allow 24 hours for the lacquer stick fill to fully cure. Do not apply sealer coat until the fill has hardened completely.
6. 6Apply a single thin coat of Testors Dullcote flat lacquer from 12 inches to seal the markings and unify the panel surface sheen.

Data Plate

The R-390 data plate is a Collins Radio Company plate only — no other manufacturers. Do not overpaint it. If missing, reproduction plates are available from specialist suppliers in the community. The plate carries the contract number and serial number which establishes the production date and should match the receiver’s known production history.

Knob Restoration

R-390 knobs are black phenolic or Bakelite. Do not repaint them. Clean with mild soap and a soft brush. Polish with Novus Plastic Polish #2 followed by #1. White indicator lines in knob grooves can be restored with a white lacquer stick using the same technique as the panel markings — draw across the groove and wipe the flat surface clean.

Internal BNC Connectors

The R-390 uses standard-size BNC connectors for internal coax — an advantage over the R-390A’s miniature BNC connectors which are difficult to handle. Clean BNC bodies with 99% IPA; polish tarnished silver contacts with Flitz Metal Polish. Replace only if the connector body is cracked or the centre pin is damaged.

Rack Hardware

• Original rack mounting screws are 10-32 cadmium-plated — replacements available from Fastenal and Grainger
• Rack ears receive the same TT-C-595 No. 2610 semi-gloss topcoat as the front panel
• Replace any deteriorated rubber gasket strips with 1/16″ closed-cell neoprene foam tape

Gearbox Lubrication

After cleaning, the main gearbox and RF slug rack gearbox require fresh lubrication. Apply Mobil 1 synthetic gear grease sparingly with a small brush — only where needed on gear teeth and cam surfaces. Excess grease migrates onto adjacent components. Oilite bronze bearings need only a single drop of machine oil.

6082 Voltage Regulator Tubes

The R-390’s voltage regulator system uses 6082 tubes that generate substantial heat in normal operation. This heat can degrade nearby wiring insulation and is a known problem area unique to the R-390. During cosmetic restoration, inspect all wiring adjacent to the VR deck for cracked or brittle insulation and replace as necessary. Do not enclose the VR deck without adequate ventilation clearance.

1. 1Apply panel markings (lacquer stick technique, Section 12) and seal with Dullcote before fitting any hardware — knob shafts and screws will damage fresh legend work if installed first.
2. 2Reinstall sub-assemblies in reverse order of removal. The Calibrator sub-assembly goes in last from underneath before the front panel is fitted.
3. 3Reconnect all inter-panel wiring harnesses per your labelled documentation. Route cables exactly as original.
4. 4Fit the front panel, engage the bandswitch shaft coupling, and tighten panel perimeter screws with a JIS driver.
5. 5Reinstall all knobs to their labelled positions. Tighten Bristol setscrews snugly on the shaft flats.
6. 6Install rack ears and the S-meter pointer last.

CCA Grade Self-Assessment

Evaluate the completed restoration under natural daylight against CCA standards. R-390-specific assessment points:

• Panel markings — all legends present, white, filled into recesses, no excess on flat surface
• Colour match — front panel, chassis painted areas, and rack ears visually consistent
• Data plate — Collins Radio only, present, correctly mounted, not overpainted
• Knobs — correct type for R-390 (not R-390A knobs), polished, indicator grooves filled white
• Green gearwheel — returned to original position on gearbox front after reassembly
• Zinc baseplate — clean and shiny, no paint applied over it