MMSR77

Thank you for your purchase of a Boscoe Music MMSR77 Kit!

The MMSR77 is a reproduction of the 2-band preamp used in late-1970s Music Man StingRay basses. Each preamp is individually tested before shipping and is designed to help recreate the response and character of the original StingRay electronics.

The kit includes prewired controls, output jack, and battery connections. Because the MMSR77 uses screw terminals, most installations can be completed without soldering.

Kit Contents

Your MMSR77 Kit includes:

• MMSR77 preamp

• Prewired volume potentiometer

• Prewired treble potentiometer

• Prewired bass potentiometer

• Prewired output jack

• Prewired battery snap with WAGO connector

• Self-adhesive neoprene insulating pad

• Mini screwdriver

• Three push-on chrome knobs

The kit is available with two potentiometer shaft lengths:

15mm shafts — intended primarily for basses using a metal control plate.

18mm shafts — intended for rear-routed or through-body installations where the potentiometer shafts must pass through the wood of the instrument.

The 18mm through-body kit also includes a preassembled potentiometer grounding harness with ring terminals. Installation of this harness is described below.

Pickup Selection

The MMSR77 will work with many pickups and wiring configurations. For the closest approximation of the original late-1970s StingRay sound, however, a vintage-style Music Man humbucker is recommended.

The original circuit was designed around an Alnico V humbucker with its two coils wired in parallel, with approximately 1750–1950Ω DC resistance and approximately 700mH inductance. Early StingRay pickups varied somewhat in winding count and specifications.

Examples of vintage-style replacement pickups include:

• Aguilar AG 4M and AG 5M

• Nordstrand MM4.2 and MM5.2

• Seymour Duncan SMB-4A and SMB-5A

A pickup wired in series can still be used with the MMSR77, but its higher output, resistance, and inductance will produce a different response from the original parallel-wired StingRay pickup.

For additional models and tested wiring configurations, see the Pickup Wiring Reference.

Power

The MMSR77 is designed for normal operation at 9V and is safe to operate at up to 18V. Higher voltage may provide additional headroom, but does not necessarily improve the sound.

The supplied battery snap is intended for normal 9V operation. An appropriate two-battery series harness is required if you choose to operate the preamp at 18V.

The output jack switches battery power automatically. Battery negative connects to the Ring terminal of the supplied TRS output jack. When a standard mono instrument cable is inserted, the plug connects Ring to Sleeve and completes the battery circuit.

Unplug the instrument cable when the bass is not being used. Leaving a cable inserted keeps the preamp powered and will discharge the battery.

Before You Begin

Before disconnecting the original electronics, take several clear photographs of the existing wiring. These can be extremely helpful if you need to identify the bridge ground, pickup wiring, battery wiring, or original control arrangement later.

Disconnect the battery before beginning installation, and please complete all wiring before reconnecting the battery.

Before mounting the components permanently, test-fit the preamp, potentiometers, output jack, and wiring in the control cavity. The MMSR77 can be a close fit in some StingRay-style basses with small crescent-shaped control cavities.

Arrange the components so that:

• the wires are not pulled tight,

• the preamp is not forced against the potentiometers or output jack,

• the screw terminals remain accessible,

• and the underside of the preamp cannot contact conductive shielding or metal.

Do not shorten any wires until you are satisfied with the component layout.

Installing the Kit

1. Mount the Potentiometers and Output Jack

The three potentiometers are supplied prewired. No soldering to the potentiometer terminals should normally be necessary.

The traditional control functions are:

Volume — 25K Audio or Linear Taper (A25K, B25K)
Treble — 1M Reverse Audio Taper (C1M)
Bass — 100K Audio Taper (A100K)

Do not fully tighten the mounting nuts until you have confirmed the position and orientation of all components and routed the wiring.  The order of components, from the neck moving towards the bridge, is traditionally:

Volume → Treble → Bass → Output Jack

The small anti-rotation tab on a potentiometer may be removed with pliers if the instrument does not have a corresponding locating hole and the tab prevents the potentiometer from sitting correctly.

15mm kits for control plate installation

The 15mm-shaft potentiometers are intended primarily for installation through a metal control plate.

Install the potentiometers from underneath the control plate and secure them with the supplied mounting hardware. Use only the washers and spacers necessary for the particular instrument.

A metal control plate will normally provide electrical continuity between the potentiometer bodies. Once installation is complete, verify with a multimeter that the potentiometer bodies are connected to circuit ground.

18mm kits for through-body installation

The 18mm-shaft potentiometers are intended for rear-routed instruments where the shafts pass through the wood of the body.

Before tightening the potentiometers, make sure enough shaft extends above the body for the knobs to install correctly while still leaving adequate engagement for the mounting nuts.

Because the potentiometers are separated by wood rather than a common metal control plate, the included potentiometer grounding harness must be installed to prevent unwanted noise in the circuit.

2. Install the Through-Body Potentiometer Grounding Harness

This section applies to 18mm through-body kits.

The supplied grounding harness electrically connects all three potentiometer bodies to the MMSR77 circuit ground.

The harness runs:

Volume → Treble → Bass → MMSR77 GROUND

Place the first ring terminal over the mounting bushing of the Volume (A25K or B25K) potentiometer inside the control cavity.

Route the wire to the Treble (C1M) potentiometer and place the next ring terminal over its mounting bushing.

Continue to the Bass (A100K) potentiometer and install the final ring terminal over its mounting bushing.

The bare wire extending from the Bass end of the harness connects to either GROUND terminal on the MMSR77 preamp.

Make sure each ring terminal makes good electrical contact with the metal potentiometer hardware and is not isolated from it by a nonconductive spacer.

After installation, a multimeter can be used to verify continuity between all three potentiometer bodies and the MMSR77 GROUND terminal.

3. Position and Insulate the Preamp

The solder joints and component leads on the underside of the MMSR77 must not contact conductive material.

Conductive shielding paint, copper foil, metal control plates, potentiometer bodies, or output-jack hardware can cause a short if they touch the underside of the preamp.

Position the supplied self-adhesive neoprene pad beneath the preamp to provide an insulating barrier between the circuit board and the instrument.

Make sure the entire solder side of the preamp is protected from accidental contact with conductive surfaces.

Do not force the preamp into a cavity where it is being pressed against other components.

4. Connect the Preamp

The current MMSR77 provides two GROUND screw terminals, one on each side of the preamp. Both are provided to make installation and wire routing easier.

Use either or both GROUND terminals as convenient for:

• pickup ground and shield

• bridge ground

• output jack Sleeve

• potentiometer grounding

• cavity shielding ground

More than one wire may be connected to a GROUND terminal when necessary, but the second terminal makes it easier to distribute the ground connections without crowding a single screw terminal.

Use the supplied mini screwdriver to open and tighten the screw terminals.

Insert each conductor fully into the terminal and tighten the screw until the wire is secure. The connection should be snug, but excessive force is unnecessary.

After tightening each terminal, give the wire a gentle pull to confirm that it is securely clamped.

Make sure no stray strands of wire are exposed where they could contact an adjacent terminal.

Pickup Wiring

Connect the pickup output lead or leads to PICKUP INPUT.

Connect the pickup ground, shield, and any bare shield wire to either GROUND terminal.

For the Sterling by Music Man H-1 pickup configuration tested by Boscoe Music, the coils are connected in parallel as follows:

Red + White → PICKUP INPUT

Black + Green + Bare → GROUND

Sterling by Music Man basses have been produced with several different pickup and wiring configurations, so do not rely on wire color alone.

If you are unsure of your pickup wiring, identify the coils with a multimeter before making connections.

Many Sterling basses are wired with their humbucker coils in series from the factory. The original StingRay used its pickup coils in parallel, so changing a compatible four-conductor pickup to parallel wiring will generally provide a closer match to the vintage configuration.

Some vintage-style pickups may require additional attention to grounding and shielding. For example, vintage-style construction may leave the pole pieces electrically isolated from ground.

For tested pickup models, coil identification, polarity information, and series/parallel wiring examples, see the Pickup Wiring Reference.

Bridge Ground

The bass should have a ground wire connecting the bridge to the electronics.

Connect the bridge ground wire to either MMSR77 GROUND terminal.

A missing bridge ground is a common cause of hum that changes when you touch the strings or bridge.

If your instrument has conductive cavity shielding, make sure the shielding is also connected to circuit ground.

Potentiometer Connections

The potentiometers supplied with the kit are already wired. The following information identifies where each wire connects to the MMSR77.

When referring to potentiometer terminals:

• CCW (lug 1) — counterclockwise lug; on the left when viewed from the bottom

• Wiper (lug 2) — center lug

• CW (lug 3) — clockwise lug; on the right when viewed from the bottom

Volume Potentiometer — A25K or B25K

CW (lug 3), purple wire → VOL CW

Wiper (lug 2), white wire → VOL WIPER

CCW (lug 1), black wire → GROUND

Either MMSR77 GROUND terminal may be used.

Treble Potentiometer — C1M

CW (lug 3), purple wire → TREBLE CW

Wiper (lug 2), green wire → TREBLE WIPER

CCW (lug 1), yellow wire → TREBLE CCW

Bass Potentiometer — A100K

CW (lug 3) → Not connected

Wiper (lug 2), green wire → BASS WIPER

CCW (lug 1), yellow wire → BASS CCW

Output Jack and Battery Wiring

The supplied output jack is a TRS jack with Tip, Ring, and Sleeve connections.

Output Jack

Tip, white wire → OUTPUT JACK

Ring, black wire → Battery negative through the supplied WAGO connector

Sleeve, black wire → GROUND

Battery

Positive, red wire → 9V+

Negative, black wire → Output jack Ring through the supplied WAGO connector

The WAGO connector allows the battery snap and output-jack Ring leads to be joined without soldering.

To use the connector, raise the lever, fully insert the prepared conductor, and close the lever. Give the wire a gentle pull afterward to verify that it is secure.

Depending on how your kit was packaged, part of this connection may already be assembled.

Why Battery Negative Connects to Ring

This connection is intentional.

When a standard mono instrument cable is inserted into the TRS jack, the Sleeve of the plug electrically connects the jack's Ring contact to ground. This completes the battery circuit and powers the preamp.

Removing the instrument cable disconnects battery negative and switches the preamp off.

For this reason, do not permanently connect battery negative directly to GROUND, and unplug the instrument when it is not being used.

Grounding and Shielding

Good grounding is important in any active bass.

The MMSR77's two GROUND terminals provide convenient connection points throughout the installation. All of the following should ultimately be connected to circuit ground:

• pickup ground/shield

• bridge ground

• output jack Sleeve

• potentiometer bodies

• conductive cavity shielding, when present

For a 15mm control-plate installation, the metal plate will normally bond the potentiometer bodies together. Verify continuity between the control plate/potentiometer bodies and MMSR77 GROUND after installation.

For an 18mm through-body installation, use the supplied ring-terminal grounding harness to connect the Volume, Treble, and Bass potentiometer bodies to MMSR77 GROUND.

If you encounter excessive hum, check these ground connections before troubleshooting the preamp itself.

Copper foil or conductive shielding paint can reduce noise, although additional shielding is not always necessary.

Remember that any conductive shielding near the preamp also creates a possible short-circuit surface, so always use the supplied insulating pad beneath the PCB.

Output Protection

The MMSR77 includes the 1kΩ output protection resistor that Music Man incorporated into later versions of the StingRay preamp.

This resistor helps protect the increasingly scarce LM4250CN opamp if the instrument output is accidentally shorted.

The protection resistor is part of the normal MMSR77 circuit and should remain in place.

Final Installation

Once all electrical connections have been made:

1. Check every screw terminal against the labels on the MMSR77.

2. Verify that no loose wire strands are exposed.

3. Verify that the preamp is insulated from conductive shielding and metal.

4. Verify that the bridge is grounded.

5. For a 15mm control-plate installation, verify continuity from the potentiometer bodies to GROUND.

6. For an 18mm through-body installation, verify the ring-terminal grounding harness is installed and connected to GROUND.

7. Verify battery positive goes only to 9V+.

8. Verify battery negative connects to output jack Ring, not Sleeve.

9. Leave the instrument cable unplugged and connect the battery.

10. Insert an instrument cable and test the bass before closing the cavity or fully tightening the hardware.

Testing the instrument before final assembly makes it much easier to correct a loose or misplaced wire.

Installing the Controls and Knobs

Once the circuit has been tested successfully, finish tightening the potentiometers and output jack.

Do not overtighten the potentiometer nuts. Tighten them only enough to keep the controls securely in position.

Excessive force can damage the potentiometers.

The supplied chrome knobs fit the 6mm, 18-tooth splined potentiometer shafts and are push-on.

Align each knob carefully with the shaft and press it down using gentle, even pressure.

When removing a knob, lift evenly from opposite sides rather than applying excessive force to one side of the shaft.

Complete Wiring Diagram

The diagram below shows a complete MMSR77 installation using a single humbucker wired in parallel.

The pickup wire colors shown in the diagram should represent only a specifically identified pickup configuration.

For the Sterling by Music Man H-1 configuration tested by Boscoe Music:

Red + White → PICKUP INPUT

Black + Green + Bare → GROUND

Pickup wire colors can vary by manufacturer, model, and production run. Always verify the pickup before making connections.

For additional pickup models, coil identification, magnetic polarity, and series/parallel wiring information, see the Pickup Wiring Reference.

Troubleshooting

No Output

First check that a good instrument cable is fully inserted into the output jack. The cable completes the battery-negative circuit, so the MMSR77 will not normally power up without a plug inserted.

Verify:

Battery positive → 9V+

Battery negative → output jack Ring

Output jack Sleeve → GROUND

Output jack Tip → OUTPUT JACK

Also check that the pickup is connected to PICKUP INPUT and GROUND and that every screw-terminal connection is secure.  Note that it is possible to accidentally insert the wire under the box clamp inside of the terminal - when this happens even though the screw will be tight the wire will be loose. 

It's a good idea to gently tug on each wire to ensure that they're actually clamped down.  If you have a multimeter you can also check continuity between the screw at the top of the terminal and the lug at the other end of each wire to ensure a good connection.

Excessive Hum or Noise

Check the bridge ground first.

Verify that all potentiometer bodies are grounded. On a through-body installation, check the complete ring-terminal grounding harness from Volume through Treble and Bass to MMSR77 GROUND.

Verify that cavity shielding, if present, is grounded.

Make sure the underside of the preamp cannot touch conductive shielding, a potentiometer body, the output jack, or other metal.

Pickup Sounds Unusually Loud or Dark

If a four-conductor humbucker is wired in series, it will generally have greater output, resistance, and inductance than the parallel configuration used in the original StingRay.

Verify the pickup's coil wiring and compare it with the Pickup Wiring Reference.

A Control Does Not Operate Correctly

Verify the appropriate CW, WIPER, and CCW wires against both the potentiometer diagram and the labels printed on the preamp.

Because the controls are supplied prewired, first check the screw-terminal end of each wire before changing any connections at the potentiometer.

Battery Drains Quickly

Make sure the instrument cable is unplugged when the bass is not being used.

Verify that battery negative connects to the output jack Ring and has not been permanently connected to GROUND or Sleeve.

If you have any questions about installation, pickup compatibility, or troubleshooting, please contact us!