Collins 516F-2 speakers – Bluetooth Conversion

1. Project Overview

This post documents a complete Bluetooth audio upgrade for the Collins 516F-2 speaker cabinet, adding dual power modes: mains 110 V AC and an internal rechargeable Li-Ion battery pack rated for a minimum five-day autonomous runtime. The design preserves the 516F-2’s vintage aesthetic while delivering modern Bluetooth 5.0 audio with aptX HD fidelity.

The 516F-2 cabinet houses a single full-range 8 Ω speaker. All electronics are self-contained within the existing chassis space, charged from a new rear-panel IEC inlet. No permanent alterations are required to the original power supply transformer or HV circuitry — the standard community practice of speaker-only use applies.

2. System Architecture

2.1 Signal & Power Flow

The system operates as a self-contained Bluetooth mono speaker. Power management is handled by the battery board, which arbitrates between AC and battery sources seamlessly:

• AC connected: 110 V → 24 V DC switching adapter → LBB-5Sv2 battery board → KAB-250v4 amp → 8 Ω speaker. Batteries charge while playing.
• Battery only: LBB-5Sv2 (5× 18650) → KAB-250v4 amp → 8 Ω speaker. AC adapter absent; battery output sustains operation automatically.
• Transition: Plugging or unplugging AC is hot-switchable with no audio interruption. The LBB-5Sv2 manages priority internally.

2.2 Functional Block Diagram

3. Battery Runtime Analysis

3.1 Energy Budget — Five-Day Target

The KAB-250v4 is documented to sustain up to seven days in Bluetooth standby on a single charge from the LBB-5Sv2 with five 18650 cells. The five-day target is comfortably within this envelope. The table below shows the energy budget for a representative daily use pattern.

3.2 Cell Selection for 5-Day Runtime

37.6 Wh/day × 5 days = 188 Wh required. The LBB-5Sv2 holds five 18650 cells in series for a 21 V nominal output. Usable capacity (80% depth of discharge) determines cell selection:

4. Bill of Materials & Sourcing

All components are available from Amazon US or Parts Express (parts-express.com). AliExpress alternatives are noted where significant cost savings exist.

5. Wiring Guide

5.1 AC Mains Input

• Mount the fused IEC C14 inlet on the rear panel. Drill a 37×17 mm rectangular cutout.
• Connect IEC inlet LIVE (brown) → fuse → adapter LIVE. NEUTRAL (blue) → adapter NEUTRAL. EARTH (green/yellow) → chassis ground stud.
• Use a short internal cable from the adapter’s 2.1×5.5 mm output to the LBB-5Sv2 input jack.

5.2 Battery Board Connections

• Install 18650 cells in LBB-5Sv2 boards, observing polarity markings. 21 V output positive from each board → 1N5822 anode. Both 1N5822 cathodes → common 21 V bus.
• 21 V common bus → KAB-250v4 battery power input (Molex Mini-Fit Jr., red/black harness). Connect all GNDs at a single chassis point.
• Use the BATTERY input port on the KAB-250v4; leave the DC input port unconnected (the LBB handles priority switching).

5.3 Speaker Output — Bridged Mono

The 516F-2 has a single 8 Ω speaker. Bridge the KAB-250v4 for mono output:

• Use the LEFT channel (+) output terminal as the positive speaker leg.
• Use the RIGHT channel (+) output terminal as the negative speaker leg.
• Leave both (−) speaker terminals unconnected in bridge mode.

5.4 Panel-Mount Controls

• Volume knob (KAB-VC) → rear or side panel. Plug into KAB-FC harness J3 connector.
• Bluetooth reset button (included with KAB-250v4) → small hole in rear panel. Press once to enter pairing mode.
• Battery status LEDs (LBB-5CL) → 5-LED cluster on rear panel. Plug into J9 on each LBB-5Sv2 board.
• Main power switch: SPST rocker on AC inlet side — disconnects AC charging, does not interrupt battery operation.

6. Installation Notes — 516F-2 Chassis

The 516F-2 cabinet provides roughly 9.9″ × 7.8″ × 11.5″ of internal space. With the HV power supply removed or inactivated, the chassis floor offers ample room for all electronics.

• Mount KAB-250v4 to the chassis floor using nylon standoffs (M3, 10 mm). Board is 3.6″ × 2.7″.
• Stack two LBB-5Sv2 boards side by side on the chassis floor. Combined footprint: ~9.6″ × 3.6″.
• Route 24 V adapter internally, or mount externally as a wall-wart with DC cable entering through a rear grommet.
• Bluetooth antenna: if the cabinet is all-metal, connect an external antenna (Parts Express #325-128) via SMA or U.FL pigtail through a small rear-panel hole.
• Chassis ventilation: Class D efficiency is 90%. At 10 W average output, dissipation is ~1 W. No forced cooling required.
• Speaker hookup: desolder existing speaker leads and reconnect directly to KAB bridged output terminals using short-run 20 AWG wire.

7. Charging & Runtime Summary

8. Safety Notes

• Do not exceed the LBB-5Sv2 maximum output current of 4.8 A per board.
• Use the fused IEC inlet. A 2 A slow-blow fuse protects the AC feed.
• The KAB-250v4 includes over/under-voltage and thermal protection. Both protection layers are active simultaneously.
• Source 18650 cells from reputable suppliers only (Samsung, LG, Molicel, Lishen). Avoid unbranded cells — capacity and safety ratings are fabricated.

9. Internal Chassis Layout — Top-Down View

The diagram below shows a top-down plan view of the 516F-2 interior with the lid removed, drawn to scale (34 px/in). Amber dashed arrows indicate power flow from mains through to the speaker terminals. Components are positioned to reflect actual physical placement on the chassis floor.

Colour Key

All components mount on nylon M3 standoffs directly to the chassis floor. The KAB-250v4 board (3.6″ × 2.7″) sits nearest the front panel to minimise speaker wire length. The two LBB-5Sv2 boards (4.8″ × 3.6″ each) occupy the central floor area side-by-side. The 24 V DC adapter bridges the rear section, directly above the IEC C14 fused inlet. Total floor footprint of electronics: approximately 9.6″ × 3.6″ — within the available 9.9″ × 7.8″ interior.

10. Ferrite RFI Suppression

10.1 Threat Model — Why This Matters for a Vintage Radio Shack

The 516F-2 conversion introduces three switch-mode noise sources into a shack that almost certainly also contains a Collins R-390A, KWM-2, or S-Line receiver — equipment whose front-end sensitivity spans 500 kHz to 32 MHz. All three noise emitters share a common mechanism: fast-switching transistors generating broadband conducted and radiated interference across that entire range.

10.2 Ferrite Mix Selection

Ferrite mix (material) selection is critical. The wrong mix provides little suppression in the target frequency range. The three mixes used in this build are all available from Fair-Rite (manufacturer) and Amidon (distributor). Do not substitute generic “ferrite” cores from unknown vendors — permeability and loss characteristics vary widely.

10.3 Suppression Points — Location Map

Seven ferrite suppression points cover all major conducted and radiated noise paths in this build. Locations are referenced to the chassis layout diagram in Section 9.

10.4 Winding Instructions

All toroids in this build use the same basic technique. The goal is a common-mode choke: both conductors of a pair wound together so differential-mode current (the audio or DC signal you want) sees effectively zero inductance, while common-mode current (RF noise on both conductors in the same direction) sees a high impedance.

• Bifilar winding: Twist the two conductors loosely (1–2 twists per cm), then wind both together as a single unit through the core. Each pass through the hole counts as one turn.
• Turn direction: Does not matter for a common-mode choke — the winding direction cancels the differential inductance regardless of which way you go through the core.
• Wire gauge: Use a minimum of 20 AWG for the 21V and 24V DC leads (F2, F3, F4) to handle up to 3A without heating. Speaker leads can be 20–18 AWG. Control wires (F7) can be 24–26 AWG as current is negligible.
• Insulation: Strip the minimum needed at the termination end. Use PTFE-sleeved wire or standard hook-up wire. Do not use bare copper — short circuits on the DC rail are a Li-Ion fire risk.
• Core gap: Leave a small gap between the last turn and the first turn where the lead enters the core to prevent voltage breakdown. This matters on the 24V rail with any transient overshoot.
• Securing the choke: Secure the finished toroid with a cable tie to a nylon standoff or dab of RTV silicone to prevent vibration-induced chafing of the winding insulation.

10.5 Ferrite Bill of Materials

All parts are available from Mouser Electronics, Digi-Key, or directly from Amidon Associates (amidon-inductive.com). Fair-Rite part numbers are listed; Amidon equivalents are shown in parentheses.

10.6 Verification — Did It Work?

The most practical verification method available to an amateur operator is the receiver itself. With the 516F-2 Bluetooth system powered and an HF receiver on a typical antenna:

• Before ferrite treatment: Tune across 3.5–14 MHz on the R-390A or KWM-2 with the BFO on. Note the noise floor and any periodic hash or heterodynes attributable to the switching supplies.
• After installing F1 and F2 (AC mains and 24V DC lead): the dominant SMPS noise contribution should drop significantly. Most operators report a 15–25 dB reduction in the S-meter noise floor.
• After installing F4 and F5 (21V rail and speaker leads): any residual TPA3116 PWM hash should disappear. The Class D amp is typically the louder source when driven with audio.
• If noise persists on 160m or 80m after all chokes are installed, add the optional F3 (Mix 77, FT114-77) in series with F2 on the 24V lead.
• For waterfall verification, a before/after waterfall screenshot across 0–30 MHz on any connected receiver or transceiver provides a definitive record of suppression achieved.