My modification of a Pioneer CLD-D703 laserdisc player for

(AC-3) Output

by Eric Koester
last changed January 26, 2001

Click on pictures to see a larger image.

This modification is to be done AT YOUR OWN RISK and there is no guarantee that the modification will work for you. Failure to correctly install this design may result in damage to you and/or your laserdisc player. This web site takes no responsibility for any modification you may attempt.

It was the summer of 1995 and I had just purchased my Pioneer CLD-D703 laserdisc player.

In 1995, laserdisc players were the highest quality of video and audio you could get for home theater.  In 1995, Dolby Pro Logic was still considered the state of the art in surround sound formats.  Only one year later, my player was rendered obsolete by the next generation of laserdisc players that offered the coveted Dolby AC-3 output - also called Dolby Digital by the marketing department of Dolby Labs.  According to the Chronology of Dolby Laboratories page, by September 1996, the 100th laser disc with 5.1-channel AC-3 (Dolby Digital) audio, Twister, was already released.   I had just shelled out $850 for the D703 so that I could have all the cool bells and whistles (like freeze and slow motion on CLV discs) and now I was already missing out.  Not only would I have to buy a new laserdisc player, but a new receiver with a Dolby Digital decoder in it as well!  I was not ready for a complete system replacement.  I decided to ignore Dolby Digital as long as I could.  Within months of the Dolby Digital videodisc player introduction, DTS started encoding their digital surround sound tracks onto laserdiscs.  With DTS, the D703 would work fine with no modification!  That sounded great!  However, a new receiver with a DTS decoder in it would have to be purchased...damn!  There was still no way to win.  There was no way to economically have either Dolby Digital or DTS without buying a new laserdisc player and/or a new receiver.  I decided to wait some more.

One day in the fall of 1997, I was browsing the internet for laserdisc sites and I came across a few laserdisc/AC-3 modification pages such as:
Kevin Nakano's Do It Yourself AC-3 page
Pioneer CLD-D703 Dolby Digital Modification page
Laserdiscs and AC-3
After seeing these www pages, I realized that I could relatively easily modify my laserdisc player myself and have Dolby Digital output without buying a new laserdisc player!  I had no idea people were doing this.  By the look of the pages, hundreds of people around the world had done it!  For a while, I considered buying a mod kit from Precision Laserdisc.  In the fall of 1997, their upgrade kit was $75 plus $5 for shipping.  Before I ordered one, I decided that I should open my laserdisc player and see if I could predict how hard it was going to be to install the modification board without their $80 worth of parts and instructions.  I opened up the player (now well out of warranty) and the audio board on the D703 was the very top board in the case!  I didn't have to remove any boards to get to the audio board.  The points where I would have to attach the coax cable and power wires were very obvious!  All the points I needed to find were clearly labeled on the pcb silk-screen!  Cool!  I felt confident that I could save myself $80, make the board myself, figure out where my connection points were and install it myself!

From what I have gathered, the circuit on the modification board is a copy of the circuitry that Pioneer added to models that came after the D703 to give them an RF Dolby Digital output.  I used circuit number 1 from Kevin Nakano's www page.

I layed out a small (1.5 inch x 3 inch) circuit board using surface mount components and got the parts together for the board.
1 custom circuit board
1 Q1, switching PNP transistor SMT SOT-23 package - DigiKey # FMMT2907ACT-ND
1 Q2, general PNP transistor SMT SOT-23 package - DigiKey # FMMT3906CT-ND
4 0.1uF capacitors SMT 1206 package - DigiKey # PCC104BCT-ND
2 22k Ohm resistor SMT 1206 package - DigiKey # P22KECT-ND
1 100 Ohm resistor SMT 1206 package - DigiKey # P100ECT-ND
1 75 Ohm resistor SMT 1206 package - DigiKey # P75ECT-ND
1 100k Ohm resistor SMT 1206 package - DigiKey # P100KECT-ND
1 10k Ohm resistor SMT 1206 package - DigiKey # P10KECT-ND
1 680 Ohm resistor SMT 1206 package - DigiKey # P680ECT-ND
I got all my parts from a mail-order electronics parts catalog called DigiKey.  I had no luck cross referencing the transistor types used for Q1 or Q2 and so I had to make my best guess at replacement transistors.  I figured that a general purpose RF transistor would work for Q1 since it was simply a buffer amplifier for the RF signal that was being tapped from the audio board.  That worked fine.  Q2 is simply used as a switch to turn on a voltage level shifting network formed by R3 and R4.  I thought I would try to use a general purpose PNP transistor such as a 3906 for Q2.  That didn't work perfectly.  While it did correctly shift the DC level that the RF signal passing through the mod. board was riding on, it adversely affected the analog audio signals on the D703's main audio board.  I later found out what was going wrong and fixed the problem.
The transistor originally specified in the modification schematic (a DTA124EK) is a PNP transistor that has bias resistors built-in.  If you you can't find one and use a regular PNP as a substitute (like I did), you have to add your own bias resistors outside of Q2.  Most of the modification pages recommend that you use 22k Ohm resistors external to Q2.  One 22k resistor is placed in series with the base of Q2 and another 22k resistor connects the base and emitter pins.  The pages in my little link list above never tell you why you need these resistors.  I found out and can tell you.

The D703 uses an ACTIVE HIGH MUTE signal.  The MUTE signal is a TTL signal which is either 5V when you push the MUTE button and 0V when the player is in PLAY mode.  This is called an ACTIVE HIGH signal.  Q2 acts as a switch.  It is turned on and off by the MUTE signal that is connected with Q2's base.  As the MUTE signal turns off and
goes to 0V, it acts like a drain and draws current from the base of Q2.  This has the effect of pulling the 0V MUTE signal up to nearly 4.3V.  This is bad because the rest of the D703's audio board also looks at the MUTE signal to decide when to mute the analog audio outputs.  With the MUTE signal now alternating between 4.3V and 5V, the audio board sees the MUTE signal as on all the time and always stays muted.  With the audio board muted all the time, you don't get much (if any) audio from the analog audio outputs of your player.  So, by putting the 22k Ohm resistors in your circuit, you isolate the MUTE signal from the transistor base and keep current from flowing from Q2's base into the 0V MUTE signal.  Now, as far as the rest of the audio board knows, there is no modification board attached.  The analog audio works as before and the AC-3 output works great too!

When you take the cover off the D703 and look at the player with the player's back facing you, the audio board is left-most horizontal pc board.  The audio board measures roughly 5 inches wide by 15 inches tall.


The RF signal (AFM signal) is labeled "RF" in white printing on the pc board.  To see the printing right-side-up, you now need to rotate the player so you are looking at the front.  When looking at the audio board from the front of the player, the row of pins that has the RF pin is in the lower right hand corner of the board.  The RF_GND pin is right next to it.  I used 75 OHM mini coax for the RF signal into the mod board and out of the RF board.  I recommend connecting the RF signal with coax since it is good engineering practice to use it and I know this method works well.  Those are the two pins you will need to attach your coax cable that goes to the RF input of mod board.  The center conductor of the coax goes to the RF pin and the shield of the coax goes to the GND pin.


The next signal along the edge of the audio board is the MUTE signal.  I attached a wire from the MUTE pad on the mod board and ran it to the pin labeled MUTE on the audio board.  It was about 2 inches away from the RF tap point.


The next signals along the edge of the audio board were the power pins.  The mod board +5V and -5V.  I found them very clearly labeled about 3 inches away from the MUTE pin along the edge of the board.  The pins are right next to each other but I connected the +5V wire to the +5V copper track (which connects to the +5V pin) a little way away from the -5V pin just to make sure there wouldn't be any chance of a short between +5V and -5V.


The next point along the edge of the audio board that needs to be connected to the mod board is ground.  There are many points on the board that are labeled GND.  I chose one of the closest connection points to the mod board.  I then soldered a wire from the GND pad on the mod board to a pad that was electrically connected to the audio board ground.  It happened to be solder pads under one of the chassis screws.  In this picture, you can also see the RF output coax running to the output RCA connector on the back panel of the player.


The RF output of the mod board is then connected to the output RCA connector using 75 OHM mini coax.   I tipped the player up on it's side so that if metal particles from the drilling fell, they would not lodge on any player circuit boards.  The output RCA connector should be insulated from the chassis of the laserdisc player with spacers made of plastic or other insulating materials.  I had to remove the player's audio board in order to connect the coax to the output RCA connector.


I ordered a set of RCA connectors and their matching plastic washers from DigiKey.  They were part numbers:

After the output RCA connector had been installed in the rear panel of the player and all the wires had been connected to the mod board, it was time to secure the mod board in the player.  I chose to use Velcro to do this.


It was at this point that I got a laserdisc to test the signal levels going in and out of the mod board during operation.  With the lid of the player still off, I plugged in the player's power cord and loaded a laserdisc.  I pressed play and probed the RF OUT pad on the mod board with my oscilloscope probe.  I watched the signal on the scope as the disc played and as I hit the PAUSE button.  The RF signal was indeed riding on a dc level.  The dc level of the signal went to about +4.5V when the PAUSE button was hit and back to +2.5V when the player was put back into PLAY mode.  The levels matched the pictures and values found on Kevin Nakano's www page.  All looked good.  It was time to hook the player's new AC-3 output to my AC-3 decoder.


The first AC-3 disc I tried was Star Trek:  First Contact.  I turned on the decoder and pressed play on the player.  After the disc spun up to running speed, the AC-3 light on the decoder came on.  The output of the speakers was drop dead silent and then sound emerged from the speakers!  It worked!  The first thing I noticed with AC-3 sound was how well separated the channels were!  The dialog of the movie stayed in the center channel and did not bleed into the front right and front left channels as it does with Dolby Pro Logic Surround.

It was just about time to put the lid on and finish the job.  Since I installed the mod board with the wires and components facing the inside surface of the player's metal lid, I decided to put black electrical tape on the inside surface of the lid just to make sure that the electrical connections on the mod board never touch the lid.


I then put the lid back on the player and tried more laserdiscs.  I was very pleased with the results of the project.

I hope you found this page helpful and enlightening.  My desire in putting this page up was to put forth the information that I learned and experience that I had with this project.  By doing this, I hope to save other people some time in their journey to the AC-3 finish line!

If you have questions or comments, send an e-mail to and I'll do my best to respond within a week.

Nope, I sold them all.


Kevin Nakano's Do It Yourself AC-3 page
Pioneer CLD-D703 Dolby Digital Modification page
Laserdiscs and AC-3
Laserdiscs and AC-3 - Pioneer 2950 Specific Information
Samsung LaserDisc Player DV500K modification
Modification of a Pioneer CLD-1090
Modification of a Pioneer CLD-A100
Pioneer CLD-1450 Mods
DIY Project # 3 - An AC-3 RF Output Upgrade
CLD-97 AC-3 RF Output Modification
Dolby Digital (AC-3) Modifications for Laserdisc Players
Panasonic LX-900 AC-3 Upgrade Page
Modify the Pioneer CLD-S104 Laser Disc player for AC-3 RF output
MSB Technology

Precision Laserdisc