AC-3 Laserdisc Player Upgrading

This excerpt was originally found at Colin Hunter's AC-3 website which was hosted at AOL and can only be accessed via wayback machine. It is courtesy of R Larsson who modified the article originally found in the Norwegian AudioVideo magazine.

Location of the AFM (Analog Audio) Signal:

Find the place where the AFM-signal splits into two channels. Tap the signal before it splits. (If you find the chip for the “Audio Demodulator & CX”, follow the signal back to the band pass filters (possibly marked BPF) of 2.3MHz (ch 1) and 2.8MHz (ch 2). Follow it further back before the signals split, and hook the wire up there. NB: the wire should be as short as possible - maximum approximately 25cm/10 inches).

Location of the Mute Signal:

To find the mute-signal locate the Audio-Out section, and look around there. Alternatively, if your player has a “Sub Code Out” jack on the back, the mute-signal is on it, or find the “Mechanism Control” chip where the mute-signal originates.

The inverter Q3 is only needed if the MUTE signal is active “LOW” (i.e. has a positive voltage (+5V during play, as it is in the Pioneer CLD-2950 PAL/NTSC player for which this circuit was originally designed). If the mute signal is active “HIGH”, Q3 must be deleted from the circuit. (This can be measured with a volt meter). Typically, for many NTSC-only players, there is no need for Q3.

(Strictly speaking, you don't need the mute-signal at all - you can just drop transistors Q2 and Q3, and wire resistor R2 to +5V. I believe all that happens is you will get some unwanted sounds when the player is not in “PLAY”.)

Mounting of the RCA Jack:

The jack should not be in direct contact with the chassis. The ground-connection comes through the shielding of the 75 ohm coaxial cable.

C2 is connected to the chassis (Rear panel GND.)

Testing:

A simple test of the function is to measure the AC-3 output with a voltmeter. It should be 3.6V in “PLAY”, OV in “PAUSE”. Parts List:

R1 680 ohm, 1/4 watt
R2 10K ohm, 1/4 watt
R3 100K ohm, 1/4 watt
R4 75 ohm, 1/4 watt
C1,C2 100nF, ceramic
Q1 2SC1740 or MPS3904, Transistor NPN
Q2 DTC124ES, Digital transistor NPN, 22K+22K
Q3 DTA124ES, Digital transistor PNP, 22K+22K
RCA jack for mounting on the back, not in direct contact with the chassis 75 ohm impedance coaxial-cable

(Transistors DTC124ES and DTC124ES can be replaced respectively with type numbers MPS2222A and MPS2907, with 22K resistors at the base and across the base/emitter junctions as shown in the circuit diagram)

Last Word:

This information was originally published in the Norwegian magazine, AudioVideo. I have made only small changes to the circuit-diagram and to the above description. If your player explodes in a bolt of green flame, well, that's too bad, but you did it at your own risk. :-)

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