McGohan 10 Watt Amplifier Model MS 103 - Public Address (PA) Amplifier |
I was given two McGohan MS-103 10 Watt Public Address (PA) Amplifiers by an old friend (RIP). They were both pretty dirty, but one of them worked. The other one was partially dis-assembled and assumed to be non-working. The working one was cleaned up and put to work with my amateur radio equipment. This amplifier is what you would consider "Old School" in that it doesn't have inputs that newer amplifier systems might have. For example, newer systems might support dynamic, condenser, and wireless microphones or AC/Battery power or a Line output for making recordings. But, for my purposes, it had everything I needed. Anything it didn't have, that I might require in the future, I can easily add.
Most of the system is pretty easy to figure out, but there were some details that I wanted to know. I did a lot of searching on the internet, but other than some pictures of the unit, I could not find any manuals or schematics.
Contained on this page is a Owner's Manual. This is not the "real" Owner's Manual, nor are the schematics original. I made up the manual contents useing other Public Address (PA) Amplifier manuals as a template. The manuals from other manufacturers gave me a template of information, that the owner might require. And some of it, I just made up. As for the schematics, I have a fully operational unit and a parts (non-working) unit. So the schematics are reverse engineered using both units. Having more information on the unit might increase the value of systems that are currently for sale.
I do not used this unit for PA applications, but do use it as a common audio driver for multiple radios. The multiple input capability and precedence capability allows me to have several separate radios connected at the same time.
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Notes |
Note: The McGohan MS-103 PA Amplifier
is all solid state. So the
Octal (8 Pin) sockets that are mounted internally on the chassis,
are NOT for tubes of any kind. The Octal
sockets are intended for Impedance Matching Transforms
that have Octal plugs at their base. The transformers are mounted this way
in order to reduce any electromagnetic pickup (30-60dB shielding). These transformers are
intended for use when certain microphone types are employed. The image on the right is and example
of what the transformers look like.
Note: While I was analyzing the circuit, I noticed that there are at least two different versions (revisions) of the MS-103. The layout of the Pre-Amp circuit board is different. It appears that the main difference is with the Mic. Precedence circuitry. On the non-working unit (SN 03930), the Mic. Precedence circuitry uses a single transistor, with a couple of resistors and capacitors. However, on the working unit (SN 07743) the Mic. Precedence circuitry uses two transistors.
For the most part, I could not find any external indication of the difference. However, after carefully matching the two units, I found that the newer unit (SN 07743) had a "G" (ground) printed over one of the Mic. Precedence connections. The older unit (SN 03930) did not have the "G" because the Mic. Precedence connections are above ground.
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McGohan 10 Watt Amplifier Model MS 103 Unofficial Owner's Manual |
Description
The McGohan model MS-103 solid-state public address amplifier is rated at 10 watts.
The unit accommodates most microphone types, offering the capability for both high impedance (Hi-Z) and low impedance (Lo-Z) microphones. Either a Balanced (3-wire) or a Un-Balanced (2-wire) microphone connection can be used through the PAGE (microphone) input terminal connections. Hi-Z or Lo-Z impedances are selectable via a switch on the bottom of the unit..
The front panel provides independent volume controls for the Page, Aux 1, and Aux 2 inputs. Bass and Treble controls, for adjustment of tonal balance, are also available on the front panel.
Outputs are provided for 8Ω speakers, and 25V/70V constant-voltage systems. A terminal strip with screw terminals allows convenient connection of speakers.
Power and Grounding
The MS-103 amplifier operates from a 120 volt, 60 Hz source and have a miximum power consumption of 25 watts. A thermostat, enclosed in the power transformer, protects the unit against overloads.
The AC line cord has a three-prong plug which should be plugged into a three wire grounded, 120 volt, 60 Hz outlet. Since it is important to ground the amplifier, where a three-wire outlet is not available, use an adapter (e.g., Leviton No. 5017) and connect the grounding pigtail to the screw securing the wall plate. If the wall plate screw is not grounded, connect a wire from the GND terminal of the amplifier to a water or steam pipe. The GND terminal associated with the 8 Ω speaker output can be used for this purpose.
Technical Specifications
| Power Output: | 10 Watts | Front Panel Controls: | PAGE (Mic), AUX 1, AUX2, |
|---|---|---|---|
| Frequency Response: | 80 to 10,000 Hz +/-2dB | BASS, and TREBLE Control | |
| Hum and Noise: | MIC Input 55dB below rated output | (AUX 2 Music Limit on rear panel) | |
| AUX Input 70dB below rated output | Bottom Panel Switches: | Mic Selector (Hi-Z | Lo-Z) | |
| Sensitivity: | MIC Hi-Z Input: 3mV | Music | Speech (PAGE) | |
| MIC Lo-Z Input: 300µV | Tone Control Action: | Treble: 10KHz +/-11dB | |
| AUX Input: 0.3 to 2.0 V | Bass: 500Hz +/-11dB | ||
| Input Connections: | PAGE (3 Screw-type terminals), | Power Requirements: | 120 VAC 60 Hz, 25 Watts |
| AUX 1 (phono), AUX 2 (phono), | Semiconductors: | All Silicon - 9 Transistors, 6 Diodes | |
| LINE (2 Screw-type terminals) | Dimensions: | 8-5/8" W x 3-1/2" H x 8-1/2" D | |
| Mic Precedence (2 Screw-type terminals) | (21.9 x 8.9 x 21.6 cm) | ||
| Outputs: | 8Ω, 25V (62Ω), 70V (500Ω) | Output Connections: | Screw-type terminals for |
| Weight: | 5 lbs (2.27 kg) | 8Ω, 25V, and 70V tap. |
Auxiliary Power
The Auxiliary Power receptical is a three-wire grounded outlet. Any associated equipment (not to exceed 200 Watts) connected to it by means of a three-prong line cord will be grounded, providing the amplifier line cord has been properly grounded.
Description of the Rear Panel Connections
PAGE (Microphone)
A low or high impedance, Balanced or Unbalanced, microphone can be connected to the round terminal area labeled PAGE, on the rear panel. The wiring is in standard XLR format where Terminal 1 is Ground, Terminal 2 is Mic+, and Terminal 3 (unmarked) is Mic-.
The diagram shows how a Balanced microphone (Fig. 1) or Un-Balanced microphone (Fig. 2) can be connected.
For ease in connecting and disconnecting a microphone cable, it would be best to wire in a standard XLR connector.
AUX 1/LINE
Either the AUX 1 input, or the LINE input, can be used. But not at the same time.
The AUX 1 input may also be used for any high impedance signal source having a high level (0.35-2.0V) output, such as a AM/FM Tuner or turntable (with a crystal/ceramic cartridge). To connect a turntable (with crystal/ceramic cartridge) to the AUX 1 jack, use a single-conductor shielded cable terminated in a standard RCA phono plug. It is recommended that a separate ground wire be connected between the turntable base and the amplifier Ground terminal to minimize hum pick-up.
A turntable with a magnetic cartridge can also be used, but a external Pre-Amp will be needed to bring up the output voltage level.
The LINE input terminals provide you with a 600 Ω balanced input. This input, after passing through a matching transformer (600Ω:15,000Ω), connects directly to the AUX 1. The details for this matching transformer (LT-2) are below. The intent of the LINE input is for use with a telephone line or paging system interface.
AUX 2/Microphone Precedence
The Aux 2 input can be used in the same manner as the Aux 1.
A built-in circuit provides the capability of muting of the Aux 2 input channel. This is achieved by connecting a customer-supplied SPST normally-open switch to the Mic Precedence screw terminals, on the rear panel. If shielded cable is used to provide the short on the Mic Precedence screw terminals, tie the shield to the Mic Precedence screw terminal that is labeled G.
The diagram (Fig. 3) shows the connection necessary for muting. When the Push-To-Talk (PTT) switch is pressed, one part of the switch connects the microphone. The other part of the switch shorts out the Mic Precedence screw terminals. This will then mute any audio that is being fed into the AUX 2 input.
Phono
Connect a phonograph (with ceramic or crystal cartridge) to either the Aux 1 or Aux 2, using a single-conductor shielded cable terminated in a standard phono plug. It is recommended that a separate ground be connected between the phono player base and the amplifer GND terminal to minimize hum pick-up.
Speakers
Connect the speaker system directly to the speaker output terminals on the rear of the amplifier. Most speaker systems or speaker boxes have a specific impedance and are polarized. An example might be 8Ω or 4Ω impedance with one terminal labeled"+" (Positive) and the other terminal labeled "-" (Negative). For 8Ω speaker systems connect one lead (-) to the terminal labeled GND. The other lead (+) to the terminal labeled 8Ω.
Multiple speakers can be used, as long as their total impedance is equal to 8Ω. Fig. 4 shows and example of connecting one 8Ω speaker OR two 4Ω speakers, in series. Note the polarity markings (+/-) on the speakers.
The 70V/25V constant-voltage outputs can be used if the speakers are going to be located a fair distance from the amplifier, say, in the ceiling or walls. Or, you might want to put speakers in separate rooms, with more power in one room than another and the speakers vary in power and impedance. The 70V/25V constant-voltage outputs would allow you to customize your setup to your needs. Briefly, with this system, each speaker has a transformer that has taps on the primary and secondary for selecting power input and impedance output. However, since this is not a tutorial on constant-voltage systems, I would refer you to Elliott Sound Products discussion on High Voltage Audio Systems.
The constant-voltage outputs are considered balanced, in that, the outputs are not referenced directly to ground. This can help in reducing hum and noise pickup when your using long lines. If you are using shielded cable and a ground is required in your system, put a jumper between the COM and GND output terminals and connect the shield to that jumper.
Operation
Power: The front panel on/off switch applies power to the amplifier. It has no effect upon equipment connected to the auxiliary receptacle on the rear panel. The switch illuminates to indicate that power is on.
From the codes (EIA) stamped into the power and audio transformers, it was determined that the transformers were made by Mohawk (EIA# 883). The power and output transformers are dated 1958 | 1968 | 1978 | 1988 (both 41st week).
MIC 1, 2: The MIC2/AUX selector switch should be in the MIC2 mode. The MIC controls are used to adjust the volume of the microphone inputs. Rotate the control clockwise to increase volume or counterclockwise to decrease volume. When microphone is not inuse, be certain to turn control to minimum position.
AUX: Set the MIC2/AUX selector switch to AUX. The AUX control is used to adjust the volume of the auxiliary input. Rotate the control clockwise to increase volume and counterclockwise to decrease volume. Rotate the control to the minimum position when auxilieary input is not used.
Microphone Precedence: A built-in circuit provides muting over the Aux channel. Connections are made from an external switch to the AUX MUTE terminals. An SPST normally-open switch is required for this function. When contacts are closed, the AUX channel is muted.
Bass and Treble: Use the Bass and Treble controls to adjust the tonal balance of the amplifier output. The Treble control can be used to enhance or suppress high frequencies (10KHz) by +/-11dB. The Bass control can be used to enhance or suppress low frequencies (500Hz) by +/-11dB. To start with a flat frequency response, rotate both control to their "12 O'Clock" position.
Input Transformers
One transformer used in the unit is referenced as a MC-2. The MC-2 Microphone Transformer is intended for tube mic-preamps, moving coil preamps, and low impedance dynamic microphones. In the MS-103, this transformer is connected to the three-terminal balanced microphone input terminals, on the rear of the unit. Manufactured by Altec-Peerless for McGohan. They are an exact version to the famous 4722 Altec transformer except the input impedance is a fixed 200 ohms.
The other transformer is a LT-2 which is the same as the Altec 15095 Line Transformer. The LT-2 provides balanced load for amplifiers, has 30 dB electromagnetic shielding, and a frequency response of +/-1dB, 20-20,000 Hz, with a maximum operating level of +23 dBm (0.001 watt ref.). Electostatic shield and windings balanced to provide 80 dB attenuation for longitudinal currents when used in balances circuits. 3-1/8" H x 1-1/2 D.
Note that these transformers are becoming very rare and very expensive. If you thinking of buying a McGohan MS-103 PA Amplifier, make sure you ask the seller if the transformers are included. I have seen a limited amount of MC-2 transformers offered for $160.00 each. Without the transformers the Page (microphone) input and the Line will not be usable. If you need that functionality, and don't have the transformers, you might want to consider using a one or two transistor interface that does the appropriate impedance matching, to replace the transformers.
MC-2
| MC-2 Specifications | |
|---|---|
| Primary: | 200 ohms |
| Secondary: | 50k ohms |
| Frequency Response: | +/- 1db 30hz to 15khz |
| Has built-in 60db electro-magnetic shielding. | |
| Octal plug-in design. | |
| Altec 4722 Microphone Specifications | |
| Primary: | 30/50 or 120/200 (with c.t.) |
| Secondary: | 40K/65K ohms |
| Frequency Response: | +/- 1db 30hz to 15khz |
| Has built-in 60db electro-magnetic shielding. Maximum operating level is 0dBm. 1-5/16" D x 1-13/16" H, Octal plug-in design. | |
The MC-2 Microphone Transformer is intended for tube mic-preamps and moving coil preamps. Manufactured by Altec-Peerless for McGohan. They are an exact version to the famous Altec 4722 Microphone Transformer except the input impedance is a fixed 200 ohms. The table on the right shows the specifications for the McGohan MC-2 and the Altec 4722, for comparison purposes.
A schematic diagram of the McGohan MC-2/Altec 4722 is shown on the left. This is a view from the bottom of the transformer can, or from the bottom of the octal socket.
LT-2
| Altec 15095 Line Transformer Specifications | |
|---|---|
| Primary: | 125/150 or 500/600 (with c.t.) |
| Secondary: | 15K ohms |
| Frequency Response: | +/- 1db 30Hz to 20,000 Hz |
| Built-in 60db electro-magnetic shielding. Maximum operating level is +15 dBm above 30 Hz. +18 dBm above 40 Hz. 1-5/16" D x 1-13/16" H, Octal plug-in design. | |
The LT-2 Transformer provides 125/150 Ω or 500/600 Ω (with c.t.) Ohm input from microphone pre-amplifier or other low impedance source. They are an exact version to the Altec 15095 Line Transformer. Line Input is from pin 1-6. For 150 Ω input strap 1-4 and 3-6. For 600 Ω input strap 3-4. The table on the right shows the specifications for the Altec 15095, which should be the same as the McGohan LT-2.
A schematic diagram of the McGohan LT-2/Altec 15095 is shown on the left. This is a view from the bottom of the transformer can, or from the bottom of the octal socket.
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McGohan MS-103 10 Watt PA Amplifier Schematic Diagram |
Below is a four page schematic of the McGohan MS-103, 10 Watt PA Amplifier. These drawings are not original McGohan drawings. The drawings were created by myself, when I reverse engineered the McGohan amplifier. You will probably find more detail on my drawings than the originale drawings. Since the parts in the unit do not have Reference Designators listed anywhere, and, since I do not have a original schematic, I made up my own.
For my purposes, the Reference Designators are used to indicate the location of the parts. Each Reference Designator starts with one or two alpha characters (e.g. R, C, VR, Q, etc.). The alpha characters describe the part type (e.g. R = Resistor, C = Capacitor, etc.). This is followed by a number that is one to three digits in length (e.g. R1, VR103, etc.). The number describes the physical location, as shown in the following list.
- 1 - 99 denote Chassis mounting - These might be capacitors that are mounted on input connectors or the transformers/diodes used in the power supply.
- 100 - 199 denote Pre-Amp Board - This board contains several pieces of circuitry. Mic. Precedence, Microphone Pre-Amp, and Audio Driver.
- 200 - 299 denote Power Amp Board - This board contains the Power Amplifier, with the exception of the output transistors. The output transistors are mounted on the rear panel of the unit. Power, from the power supply is distributed from the Power Amplifier board.
- 300 - 399 denote Capacitor Board - This board only contains two capacitors. C301 which is the filter capacitor for the power supply (page 4) and C302 which is the output coupling capacitor for the Power Amp (page 3).
The symbols labeled IOxxx are used to indicate wiring. As an example, on page one, the AUX 1 connector center conductor (pink wire) is shown going to IO101 and R101. The AUX 1 connector is mounted on the rear chassis and is wired to the Pre-Amp board to pick up R101. The other side of R101 then connects to VR1 via a Orange wire connected to IO103.
These are some adjustments that I need to make to the schematics, when I get time. Each page of the schematic takes about 500 lines of Javascript and are fairly complicated. Sometimes, something as simple as a changing a Reference Designator can have a devastating ripple effect on the rest of the drawing.
- Page 2 - The Bass (VR5) and Treble (VR6) control wiring does not indicate that these controls are on the front panel and are wired to the Pre-Amp board, where the rest of the tone control parts are. Drawing space may be an issue.
- Page 1 and 2 - The variable resistors do not have any values. That's because I don't know the values. When I get time I will determine the values.
McGohan MS-103, Page 1 - Aux 1, Aux 2, Line, and Mic. Precedence inputs.
McGohan MS-103, Page 2 - Microphone Input (PAGE), Microphone Amplifier, System Pre-Amp.
McGohan MS-103, Page 3 - Aux 1, Aux 2, Line, and Mic. Precedence inputs.
McGohan MS-103, Page 4 - Power Supply and Speaer Output Transformer.
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