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The innards of a Baldwin HiFi Power Amplifier
All of the Baldwin HiFi Power Amplifiers are built on the principle where the Amplifier circuitry section is split away from the Power Supply module in a seperate case. This ensures that the unit will produce NO HUM!

The 2 enclosures are connected together by a 7 core (10 Amp each core) umbillical cable with heavy duty 7 pin connectors.
2 x L10 Deluxe Mono Block Amplifiers are shown below:

Mono Block L10s

The Power Supply (PSU)
There are 3 main "Direct Current" [DC] Rails within each Baldwin HiFi Power Supply [PSU].
The main amplifier modules require Plus and Minus DC Rails (related to a common Ground [or Earth]). This dual rail PSU system can be asked by the Amplifier Circuitry to provide lots of current [Amps] - so a large Torroidal and high powered Full Bridge Rectifier is employed.

Separately, the Amplifier Circuits need control Voltages too so another regulated DC Supply is also provided within the Main PSU box.

The image below shows fusing (all fuses are standard 20mm size) and circuitry to be found in any Baldwin HiFi Amplifier PSU case:
PSU Fusing and circuitry
Where are the fuses located?
Note: please remove the IEC Kettle Power Lead before opening the Unit or changing Fuses.

Fuse 1 is hiding in the pull-out tray within the IEC Mains inlet socket.
This fuse delivers all AC 240V power to the unit.

Fuse 2 is within a holder on the mini low Voltage regulator board.
This Fuse delivers AC 240V to the Low Power Circuitry.

Fuse 3 & 4 are within Holders on the larger high Voltage regulator board.
These fuses protect the large Torroidal Transformer should there be a fault further on in the High Voltage Circuitry.

Fuse 5 (+52VDC Rail), Fuse 6 (-52V Rail) and Fuse 7 (+16VDC) on the rear:
Fuse locations
Where are the PSU Boards & adjustments located?
Note: please remove the IEC Kettle Power Lead before opening the Unit or changing Fuses.

To an engineer - this should be self explanatory

The High Voltage DC Protection Board is a Baldwin HiFi safety feature.
It has 2 Relays - one for +52VDC and the other for the -52VDC Rails. If either of these Primary PSU rails fail then both Relays will disconnect so that the Amplifier unit will not receive only one rail without the other (this would blow up a good amplifier circuit).
The Blue LED on the front of the PSU unit will light if both Rails are Good.

The Low Voltage DC Regulator delivers 16VDC out to the Power Amplifier umbillical cable through pin 7 on the connector. The bottom rear fuse (1 Amp QB) protects the control equipment within the Power Amplifier Case. It is important to ensure that the Voltage measured at the bottom fuse is set to at least 16VDC (measured across to Earth) adjusted by the mini blue potentiometer. Any Voltage lower than this will not deliver enough DCV to the control equipment and the Speaker Protection board may well not work properly.

Board locations
The Power Amplifiers
Note: please remove the IEC Kettle Power Lead before opening the Unit or changing Fuses.

Representation of the wiring - Meter Drives / other drives not shown
Board locations
The 405c Power Amplifiers
Note: please remove the IEC Kettle Power Lead before opening the Unit or changing Fuses.

The Standard 405c Amplifier Case has 4 circuit boards.
2 x Amplifier Boards with their own dual 4 Amp QB protection fuses. There are no adjusters on the Amplifier boards.
The rear mini board is the Speaker Protection Soft Start / Soft Close system. Some models have the red LED indicator soldered to the board, other models have this indicator on the front panel. On start-up, the Red LED will come on to indicate the speakers are disconnected. The Circuitry measured the outputs of each Amplifier Board to ensure each is working properly. After around 3 seconds, assuming both Amplifier modules are measured good, the Relays will click in to enable the Loudspeaker connections (and the Red LED will go out). On power down the opposite happens so that there are no loud thumps through the Loudspeakers.
Should the Red LED come on during normal operation then the system will disconnect the Loudspeakers as it has measured a fault within an Amplifier module.
The forward upright board on the left is the Amplifier Output Monitor meter adjustment unit. 2 x mini potentiometers allow you to adjust the meters as you wish. These pots are setup at time of manufacture and should normally not need adjustment.

Shown below is the Deluxe version of the 405c which has the additional Input Level Monitor Board which (via a switch on the front panel) will allow you to view the PreAmp output levels on the meters. The adjustments are, again, set at the time of manufacture using full scale readings for an analogue sine wave input set to 0dB (0.775V).
Amp Board locations
The L10 Power Amplifiers
Note: please remove the IEC Kettle Power Lead before opening the Unit or changing Fuses.

The Standard L10 Stereo Amplifier Case has 4 circuit boards.
Please read the 405c description above for what the boards do.
Shown below is the Deluxe version of the L10 which has the additional Input Level Monitor Board.
The L10 Amplifier boards do not have internal protection fuses - so rely on the rear PSU Case +VE and -VE fuses for protection.
Amp Board locations
The L10 Power Amplifier setup
This procedure should only be undertaken by competant engineers with the correct equipment.

The L10 Amplifier module is setup at the time of manufacture. It has a vernier multiturn Potentiometer to enable careful adjustment.
Setup process: No input connected. No Speaker connected.
You will need a Meter set to milliVolts DC.
For optimum performance the unit should be set so that a DC Millivolt Meter reads 0.15mV with the meter probes connected as shown.
L10 Setup L10 Setup
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