Hammarlund Restoration Series Hammarlund SP-600 JX-1 A complete electrical & mechanical rebuild reference Signal Corps R-274A/FRR  ·  Hammarlund Model 31406-1  ·  by Mike Peace VK6ADA / r-390a.net Administrator

The Hammarlund SP-600 is one of the great commercial-military communications receivers of the tube era — a 20-tube, double-preselection, dual-conversion superheterodyne covering 540 kHz to 54 MHz in six ranges, famous for its massive build and silk-smooth tuning. Roughly thirty-nine variants were produced between 1951 and 1972, almost all on military contracts.¹ This guide covers the rebuild of the very first of the line.

The JX-1 was built on a 1951 U.S. Army Signal Corps contract and delivered in September 1951 as the R-274A/FRR. Hammarlund’s internal model number is 31406, with a dash suffix identifying the variant, so the JX-1 is formally Model 31406-1.² The two suffix letters tell you what you have: “J” indicates JAN-grade (Joint Army-Navy) components throughout, and “X” indicates the selectable six-position crystal-controlled HF oscillator, into which the user installs HC-6/U crystals to lock the front end to chosen frequencies. The receiver must still be manually peaked so the RF and mixer stages track; the crystal only stabilises the high-frequency oscillator.³

Variant note Many of the earliest receivers were stamped simply “SP-600-JX” before the dash-number scheme settled in, so a genuine JX-1 may carry either marking. The data plate is on the cover of the tuning-condenser (RF unit) assembly. Mechanically and electrically the JX-1 is the baseline against which all later JX engineering changes were made, so these procedures apply equally to the JX-7, JX-17, JX-21 and most other early-to-mid production receivers, with only minor wiring differences. If your set lacks the crystal-oscillator switch entirely (J-3, J-4, J-25, etc.) it is a 19-tube receiver and the JX crystal-oscillator section below does not apply.

1.  Quick Specifications

Parameter	Value
Frequency coverage	540 kHz – 54 MHz, six bands
Circuit	Dual-conversion superhet; single conversion Bands 1–3, dual conversion Bands 4–6 (above 7.4 MHz)
Intermediate frequencies	First IF 3955 kHz (dual-conv. bands); final IF 455 kHz
Conversion crystal	3500 kHz (second-conversion oscillator)
Selectivity (−6 dB)	13 / 8 / 3 kHz (LC) and 1.3 / 0.5 / 0.2 kHz (crystal filter)
Antenna input Z	95 Ω nominal
Sensitivity	AM ≈ 2 µV; CW ≈ 0.75 µV (10 dB S/N)
Audio output	≈ 2 W into 600 Ω at <10% THD (single 6V6GT)
Voltage regulator / rectifier	0A2 (+150 VDC regulated) / 5R4G B+ rectifier
Power	8 primary taps, 95–260 VAC, ≈ 130 W
Weight	Chassis ≈ 65 lb; in table cabinet ≈ 92 lb

Specifications per the factory data and Signal Corps TM 11-851.³

2.  Tube Complement (JX, 20 tubes)

Knowing what each tube does makes fault-finding far faster. The lineup below is the common early-JX configuration; check yours against the schematic for your serial range, as Hammarlund changed details across contracts.³

Stage	Tube	Function
1st / 2nd RF	6BA6 × 2	Cascaded TRF amplifiers (double preselection)
1st mixer	6BE6	Converts via dual transformer T1 to 455 kHz or 3955 kHz
VFO (HFO)	6C4	Variable high-frequency oscillator
Crystal osc. (JX)	6AC7 (later 6AH6)	Selectable fixed-frequency crystal oscillator — the “X” option
Gate	6BA6	Routes 455 kHz IF on single-conversion Bands 1–3
2nd mixer	6BE6	Combines 3955 kHz with 3500 kHz crystal osc. → 455 kHz (Bands 4–6)
Conversion osc.	6C4	3500 kHz second-conversion crystal oscillator
1st / 2nd IF	6BA6 × 2	455 kHz tuned IF amplifiers
IF driver	6BA6	Boosts level ahead of the detector
Detector / AVC	6AL5	Dual diode — detector + separate AVC rectifier
BFO	6C4	Permeability-tuned beat oscillator
BFO buffer	6BA6	Isolation and injection-level control
1st AF / IF cath. flwr	12AU7	First audio stage + low-Z (70 Ω) 455 kHz IF output
Audio output	6V6GT	2 W into 600 Ω balanced output transformer
Meter rect. / limiter	6AL5	AF meter rectifier + clipper-type noise limiter
Bias rectifier	6AL5	Generates −51 VDC bias for RF gain and dividers
B+ rectifier	5R4G	Full-wave HV rectifier
Voltage regulator	0A2	+150 VDC for BFO, HFO, 1st mixer, crystal osc.

3.  Safety First

The SP-600 carries lethal B+ and a heavy transformer. Treat every rebuild as live until proven otherwise.

⚠  High-voltage and shock hazards B+ runs around +250 VDC and is present on the filter capacitors and the 5R4G socket. Always bleed the filter caps with an insulated resistor (e.g. 10 kΩ / 10 W) and confirm 0 V before reaching in. Work with one hand behind your back when probing live circuits, and never work alone on an energised chassis. Power the receiver through an isolation transformer during testing. Bring the set up slowly on a Variac the first time (see §4.3). Do not slam a 70-year-old electrolytic to full line voltage.

Vintage solder is leaded, and some early panel finishes and potting compounds are unfriendly. Wash hands after handling, and ventilate when desoldering or heating potting tar.

4.  Initial Assessment — Do Not Power Up Cold

An unrebuilt early SP-600 will almost certainly have leaky molded paper capacitors. The circuit keeps working with leaky caps, which fools owners into running them — but the leakage current quietly cooks resistors and can damage transformers. Resist the urge to plug it in.⁴

4.1  Document everything

• Record the data-plate model/contract number and serial, and photograph the chassis top and bottom before you touch anything.
• Photograph the catacomb (turret) wiring and every sub-assembly you will open, from several angles — half the capacitors are hidden inside assemblies and you will want the reference.
• Note the existing power-transformer primary tap before changing anything.

4.2  Visual inspection

• Capacitors: identify the black Sprague “Black Beauty” and gray Cornell-Dubilier molded tubulars — the failure-prone paper units. Look for cracks, bulging, or guts hanging from the leads.
• Resistors: look for charred or discoloured carbon-comp resistors, especially screen and plate droppers near leaky caps.
• Electrolytics: inspect the filter can for leakage, bulging, or dried seals.
• Chokes / transformers: look for tar weep. A known SP-600 failure is potting compound in the filter chokes becoming conductive.⁸
• Mechanical: check the dial drive for the chronic slippage problem (§9) and that the turret indexes cleanly on all six bands.

4.3  Cautious first power-up (after the rebuild)

Do the full recap before any extended power-up. When you do bring it up:

1. Pull the rectifier and audio output tubes; bring the chassis up slowly on a Variac to re-form the filter electrolytics if original (15–20 minutes, watching for heat or hum).
2. Re-form or, better, replace the can capacitor (§5.1). Replacement is strongly preferred over re-forming a 70-year-old can.
3. Reinstall tubes; bring up on the Variac while monitoring B+, heater current, and the 0A2 strike. Confirm +150 V regulated and roughly +250 V B+.
4. Only then proceed to signal tests and alignment.

5.  The Capacitor Rebuild — the Heart of the Job

Every early SP-600 needs a complete replacement of the molded paper capacitors. There are roughly fifty of them and over half are hidden inside sub-assemblies — the catacomb, the IF transformer cans, and the crystal-filter and oscillator assemblies must be dismounted to reach them. Plan the job around that reality.⁴

5.1  Why these caps fail

Two molded paper-dielectric types appear, used interchangeably by Hammarlund: the black Sprague “Black Beauty” and the gray Cornell-Dubilier tubular. Both develop high leakage current with age — the gray C-D units are among the worst capacitors ever made for leakage. The leakage doesn’t just hurt performance; it forces excess current through plate and screen resistors, which is why burned resistors are so common in unrebuilt sets.⁴

5.2  What to replace them with

• Coupling, bypass and decoupling caps: modern film (“orange drop”) or, the most cost-effective and durable choice, 1 kV ceramic disc capacitors. Hammarlund’s own service bulletin advised substituting 0.01 µF for the original 0.022 µF tubulars, so 0.01 µF 1 kV discs are a fine universal replacement throughout the audio/IF bypass positions.⁵
• RF / catacomb caps: use the original values for tuned-circuit and oscillator positions; do not “round off” values that affect tracking or injection.
• Voltage rating: 500 V minimum on signal-path caps, 1 kV where they sit on B+ lines; 1 kV discs are cheap insurance.

Replace every molded paper cap — no exceptions It is tempting to replace only the obviously bad ones. Don’t. The whole population is degraded, and the hidden ones are the ones most likely to bite you later because you won’t want to go back in. Do them all in one pass while the assemblies are open.

5.3  Hidden-capacitor locations

Budget time for sub-assembly removal. The molded caps you cannot see from the top or bottom are typically inside:

• The catacomb / turret RF tuning assembly (coil-strip bypasses).
• The 455 kHz IF transformer cans.
• The crystal-filter assembly.
• The BFO and HFO/oscillator compartments — the BFO area in particular holds Black Beauties (the C136 region) notorious for cracking.

Work one assembly at a time, photograph before disturbing, and keep a marked-up schematic so you can tick off each cap as it is changed.

5.4  Mica capacitors — a measured approach

Do not blanket-replace the mica capacitors. Genuine silver-mica units have a very low failure rate, and wholesale replacement risks detuning RF/IF circuits and introducing new faults. “Silver-mica disease” is real but over-reported. The sensible rule:⁹

• Replace a mica cap only when it tests bad or the symptom points to it.
• The one recurring exception is the open-leaf-construction mica caps used in some Hammarlund IF transformers — for a stubborn low-gain or drifting IF stage, suspect the small micas on the transformer end caps.
• Where a mica has DC leakage but holds value, a large series capacitor can block the DC without affecting RF function — useful for hard-to-reach precision units.

6.  Power Supply

6.1  Filter electrolytics

The original filter is a multi-section can, typically 20/20/20 µF at 450 V, feeding a two-stage filter with two chokes. Replace it. New-manufacture twist-tab cans (22/22/22 µF at 450 V, 105 °C) are a drop-in from Hayseed Hamfest, who also offer a complete SP-600 recap kit. If you must keep the original can for appearance, restuff it with modern electrolytics.⁷

6.2  Filter chokes

⚠  Check the chokes for conductive potting A documented SP-600 failure is the filter-choke potting tar becoming contaminated and conductive over time, leaking B+ to the chassis. If a choke runs hot, weeps tar, or measures leakage from the winding to the frame, it must be rebuilt or replaced — it can masquerade as a shorted filter cap.8

6.3  Rectifier, regulator and bias supply

• 5R4G B+ rectifier: test or replace; a weak rectifier shows up as low B+ and poor sensitivity.
• 0A2 regulator: confirm it strikes and holds +150 V. The regulated rail feeds the BFO, HFO, first mixer and the JX crystal oscillator — drift here causes oscillator instability.
• 6AL5 bias rectifier: provides −51 VDC for the RF gain control and bias dividers. A missing bias supply gives runaway gain and distortion.

6.4  Set the primary tap for modern line voltage

The eight-tap primary lets you match today’s higher line voltage. Most receivers are wired to the 117 V tap (No. 4), but modern mains often sit at 124 V or more. Move the primary to the 130 V tap (No. 5). The receiver runs cooler, tube life improves, and oscillator stability is noticeably better. Confirm B+ stays in spec after the change.⁶

7.  Resistors and the Catacomb

7.1  Resistors

Carbon-composition resistors drift high with age and heat, and the SP-600’s leaky-cap history means many have been cooked. Check resistors as you recap — you are already in each assembly.

• Measure in-circuit first, then out-of-circuit if suspect. Replace anything more than ~20% high or visibly charred.
• High-value AVC and grid-return resistors (1–10 MΩ) drift the most and directly affect AVC action and sensitivity — replace with modern metal-film.
• Keep wattage ratings equal or higher; inspect screen/plate droppers near former leaky caps closely — the usual burn victims.

7.2  The catacomb / turret RF assembly

The rotating turret (“catacomb”) carries the band-specific coil strips and is the RF heart of the front end. It is precision-built and rewards gentle work.

• Clean the turret roller/wiper contacts with a quality contact cleaner. Dirty or oxidised contacts cause intermittent or dead bands and erratic tracking. Do not over-bend the contact fingers.
• Inspect for green corrosion on the silver-plated coil-strip contacts; clean gently and protect afterward.
• Verify the slug-tuned cores turn freely. Never force a seized iron/ceramic core — ease it with a trace of core lubricant to avoid cracking.
• Replace any catacomb bypass caps while the strips are accessible (§5.3) — the worst place to have to return to. Handle coil strips by the frame and keep them in band order.

8.  Tubes and Lamps

• Test every tube; replace weak or gassy units. Watch the oscillator tubes (6C4s, crystal-osc. 6AC7/6AH6) and the 6BA6 RF/IF chain, where weak tubes rob sensitivity.
• Heat upgrade: the nickel-plated tube shields trap heat. Period military/industrial practice — and a worthwhile upgrade — is to fit IERC heat-dissipating shields where they fit, lowering tube temperatures and improving longevity and stability.¹⁰
• Dial lamps: the standard No. 47 lamps are bright and short-lived; GE No. 1847 long-life lamps are a direct, dimmer, cooler-running substitute.¹⁰

9.  Mechanical Restoration

9.1  The chronic dial-slippage fix

The SP-600’s gear-driven tuning is famously smooth — when it works — but dial slippage is the single most common mechanical complaint, caused by hardened lubricant and relaxed clutch tension.¹¹

1. Strip the old, hardened grease from the gear train and tuning-capacitor drive completely — hardened lube is usually the root cause.
2. Clean the friction-drive surfaces; a trace of the correct dressing restores grip without making it greasy.
3. Re-tension the drive clutch only enough to eliminate slip while preserving the smooth feel — over-tightening makes tuning stiff and stresses the dial cord and gears.
4. Lubricate sparingly: light non-migrating grease on gear teeth, light oil on shaft bearings. Keep all lubricant off friction-drive surfaces.

9.2  Bandswitch and cosmetics

• Clean and lightly lubricate the turret index/detent so each band clicks home positively; verify the articulated dial-pointer linkage and band-indicator window read correctly.
• The cast front panel responds well to careful cleaning — only strip and repaint if the finish is beyond saving. Match the original gray and preserve the engraved logging dial and original knobs.

10.  Alignment

Do the recap and tube work before alignment — aligning a leaky receiver is wasted effort. You need at minimum an accurate AM-modulated signal generator, a non-metallic alignment tool, and an output indicator (the receiver’s meter, or a meter across the audio/AVC line). A sweep generator plus oscilloscope makes IF alignment far easier but is not mandatory.¹¹

1. Allow a full warm-up (30+ minutes) so oscillators are stable.
2. 455 kHz IF first. Inject 455 kHz at the first mixer grid and peak the IF transformers for maximum output, working from the detector back toward the mixer. Keep injection low to stay out of AVC compression.
3. Crystal filter / phasing. With the crystal filter selected, set phasing to centre (the “<>” mark) and adjust per the manual so maximum selectivity coincides with the marked position and the heterodyne null falls correctly. Trim the crystal-filter capacitor for symmetry.
4. BFO. Zero-beat at the centre of the IF passband with the BFO control centred; set injection via the buffer for clean CW/SSB without pulling the IF.
5. Second-conversion / 3500 kHz crystal oscillator. Confirm it oscillates and is on frequency — essential for Bands 4–6.
6. RF tracking, per band. At the high-frequency alignment point adjust the trimmers; at the low-frequency point adjust the slug-tuned cores (oscillator, mixer, RF). Iterate high–low until tracking holds across each band. Do all six bands.
7. Crystal oscillator (JX) check. For each installed HC-6/U position, confirm reliable start-up and correct tuning when switched from VFO to crystal.

Tip: don’t chase the frequency readout The SP-600’s main dial readout is deliberately approximate — that’s why it has a precision logging scale on the right. Align for peak performance and tracking, not for perfect agreement between the dial and the generator.

11.  Troubleshooting Quick Reference

Symptom	Likely cause / where to look
Dead, no B+	5R4G rectifier; open filter choke; blown fuse; check the 130 V tap wiring (§6.4).
Hum / poor filtering	Aged or failed filter electrolytics (§6.1).
B+ leaks to chassis	Conductive choke potting tar (§6.2); shorted bypass cap.
Burned resistor(s)	Adjacent leaky molded paper cap pushing DC — replace cap and resistor (§5, §7.1).
Low sensitivity overall	Weak RF/IF 6BA6s; misalignment; low B+; degraded AVC resistors.
One or more dead bands	Dirty turret contacts (§7.2); seized slug core; broken catacomb bypass.
Oscillator drift / instability	0A2 not regulating; weak 6C4; line voltage too high (set tap to 130 V).
No / weak BFO	6C4 BFO or 6BA6 buffer; cracked Black Beauty in the BFO area (§5.3).
IF stage low-gain / drifting	Open-leaf mica on the IF transformer end cap (§5.4).
Scratchy tuning, slips	Hardened drive grease and clutch tension — the dial-slip fix (§9.1).
Runaway gain / distortion	Missing −51 V bias — check the 6AL5 bias rectifier (§6.3).
No audio, B+ OK	6V6GT or 12AU7; open audio output transformer winding.

12.  Performance Expectations and Optional Upgrades

A properly rebuilt and aligned SP-600 delivers excellent sensitivity, ultra-smooth tuning, and “communications-grade” audio that is pleasant but not hi-fi (the 6V6 into 600 Ω is voiced for intelligibility). Don’t expect modern dial accuracy — use the logging scale for repeatable retuning.

• IERC tube shields and No. 1847 lamps — both improve thermal behaviour with no circuit change (§8).
• 130 V primary tap — the easiest stability and longevity improvement (§6.4).
• SSB reception: the low-Z 455 kHz IF output (12AU7 cathode follower) feeds an external SSB adapter such as the period SPC-10 converter, giving good SSB without modifying the receiver — the cleanest reversible route for a JX-1.
• Quality film caps in the audio path if you want the best audio, while using 1 kV discs elsewhere.

Keep it reversible. The JX-1 is the first of the line and historically significant as the original R-274A/FRR. Favour authentic, documented work and avoid irreversible modifications that diminish a receiver this early in the series.

13.  Parts Sources

Item	Source / note
Filter can (20/20/20 µF 450 V) & full recap kit	Hayseed Hamfest — new-manufacture twist-tab cans and SP-600 discrete kits
1 kV ceramic disc / film caps	Mouser, DigiKey; 0.01 µF per the Hammarlund bulletin
Metal-film resistors	Any major distributor — for drifted AVC/grid-return positions
IERC heat-dissipating tube shields	Surplus / specialist tube vendors
GE No. 1847 dial lamps	Lamp specialists; direct No. 47 substitute
HC-6/U crystals (JX positions)	Custom crystal houses for desired net frequencies

A note on scope This guide is a consolidated working reference. Always verify pin-outs, component values and alignment frequencies against the schematic and manual for your specific serial range — Hammarlund changed details across the ~39 SP-600 variants and across contracts within a single dash number. When in doubt, the factory manual and Signal Corps TM 11-851 are authoritative.

Notes & Citations

1. Production span, variant count (~39 versions, 1951–1972) and the predominantly military distribution: Henry Rogers (WA7YBS), “Hammarlund SP-600 Series of Receivers,” Western Historic Radio Museum, radioblvd.com; and Fred Osterman, Shortwave Receivers Past & Present. ↩
2. JX-1 = first contract, delivered September 1951 as the R-274A/FRR; Hammarlund Model 31406 with dash-number suffix scheme: Les Locklear, “SP-600 Designations List,” hammarlund.info/sp600.html; corroborated by Rogers, op. cit. ↩
3. Suffix meanings (J = JAN, X = selectable crystal HFO), circuit topology (3955 kHz & 455 kHz IFs, 3500 kHz second-conversion crystal, dual conversion above 7.4 MHz, 0A2 +150 V regulated rail, 5R4G rectifier, 6AL5 −51 V bias rectifier), tube complement and specifications: Rogers, op. cit.; U.S. Army Signal Corps Technical Manual TM 11-851 (R-274A / SP-600-JX). ↩
4. The molded paper-dielectric capacitor problem (Sprague “Black Beauty” black and Cornell-Dubilier gray tubulars), the ~50-capacitor count with over half hidden in sub-assemblies, and leakage-current-induced burned resistors: Rogers, op. cit. ↩
5. Hammarlund service bulletin advising 0.01 µF in place of the original 0.022 µF tubulars, and the 1 kV ceramic disc recommendation: Les Locklear, Antique Radio Forums (SP-600 capacitor-replacement discussion), antiqueradios.com. ↩
6. Moving the power-transformer primary to the 130 V tap (No. 5) for cooler running and improved stability under modern line voltage: Les Locklear, Antique Radio Forums, op. cit. ↩
7. Replacement filter can (original 20/20/20 µF @ 450 V) and the complete SP-600 recap kit: Hayseed Hamfest LLC, hayseedhamfest.com. ↩
8. Filter-choke potting compound becoming conductive as a documented SP-600 failure mode: Rodger (WQ9E), Antique Radio Forums, op. cit. ↩
9. Measured approach to mica capacitors and the view that “silver-mica disease” is over-reported (replace on demonstrated failure): Rodger (WQ9E) and others, Antique Radio Forums, op. cit. The open-leaf IF-transformer mica exception is also discussed at Radiomuseum.org. ↩
10. IERC heat-dissipating tube shields and GE No. 1847 long-life lamp substitution as operator practice: documented owner restoration notes at The SWLing Post, swling.com. ↩
11. Chronic dial-slippage diagnosis/remedy and the sweep-IF alignment procedure: Henry Rogers (WA7YBS), Western Historic Radio Museum, op. cit. ↩

Credits & Acknowledgements

This guide synthesises decades of community knowledge. Particular thanks are due to:

• Henry Rogers, WA7YBS — whose Western Historic Radio Museum SP-600 article is the definitive published rebuild reference, including the hidden-capacitor survey, circuit descriptions, the dial-slippage cure and the sweep-IF alignment procedure.
• Les Locklear — for the SP-600 designations list at hammarlund.info and his long record of practical SP-600 guidance in the community.
• The contributors of the Antique Radio Forums (notably Rodger, WQ9E) for hard-won detail on capacitors, chokes and the mica debate.
• Hayseed Hamfest LLC — for keeping correct-spec replacement capacitors in production for this and other classic gear.
• The U.S. Army Signal Corps (TM 11-851) and Hammarlund Mfg. Co. — whose original documentation remains the authoritative source for any specific receiver.

Any errors of synthesis are the author’s own. Manufacturer and vendor names are used for identification only.

Mike Peace VK6ADA r-390a.net Administrator  ·  Hammarlund Restoration Series  ·  vk6ada.com.au