How can I get my new receiver to decode the AC-3 s

The previous two posts have correctly identified the problem as being the RF modulator. If your receiver does not have an RF-modulator input jack for laser disk players (and most new units these days won't, since laser disks are "old technology"), then you will have to buy one and insert it between the laser disk player and your receiver. Laser disks used an analog recording format for the audio signal, not Dolby Digital (or other digital surround medium), which is why the audio signal must be converted to a radio frequency that can be utilized by the receiver or surround processor.

many of the newer model pioneer elite receivers still have built-in rf demodulators. i use the vsx-37tx in my HT system, and i know for sure it has one, since i use it for DD 5.1 from my LD. . .

check the features on the website, it'll say: ac-3 rf demodulator or something similar. or you can purchase an external one, as the others suggested.

sdcampbell, just fyi: you are incorrect in stating that ld's do not use dolby digital or other digital formats - it depends solely on the disc. many older ld's have an uncompressed pcm track as well as an analog-only track. and many newer ones *do* have DD and DTS tracks, along with a pcm and an analog track. they (the DD and DTS tracks) are indeed RF modulated and need to be demodulated in order for a receiver to recognize the signal, but that is because the commonly accepted technology changed for sending compressed digital data from rf modulated to using a digital cable with the popularization of dvd.
the standard analog tracks are not rf modulated, because they don't need to be. the go out via rca jacks like anything else. the pcm tracks as well would be decoded by the player's internal DAC and then sent out the rca's.

Get it modified by MSB
www.msbtech.com

I am back... My friend sais you will need the RF adapter. He has a Kenwood 990D player that has a coaxial output like a DVD player (also the RF). So he does not need the adapter. Pioneer made a few later players this way also. If you don't want to spend the $100 for the RF adapter you could look on eBay for another machine.

Also Pioneer currently makes a DVD/LD combination player. The regular DVD coax output works for laserdiscs also.