CED VideoDisc and Player Discussion

[toasterking]

My SKT400 plays audio from mono discs at about the same level as my other entertainment components, but when a stereo disc is loaded (the "STEREO" indicator lights), I have to turn my amplifier's volume all the way up for the audio level to even approach comfortable (and it's still not quite loud enough). My VCR and tape deck measure the peak line level audio from the CED player at about -30 dB while in stereo mode. This can't be right!

On the PCB diagram inside the player, I see two audio level adjustments: R4127, which is "LEVEL / L PLUS R" and is described here, and R4128, which is "LEVEL / L MINUS R". Would I be correct in assuming that the "L MINUS R" adjustment is for stereo audio and is what I want to adjust, and the "L PLUS R" adjustment is for mono audio only? Otherwise, is there another adjustment I'm missing?

Thanks

[Guest]

If you connect the player to a TV set via the RF output (channel 3 or 4), does the audio level of a stereo disc sound comparable to a mono disc?

Matt

[toasterking]

Wow, a very good question -- I hadn't even tried that. But putting it to practice, yes, the sound level of a stereo disc is about the same as a mono disc through the RF output.

[Smoky Pond]

Stereo audio is "compacted", so a decoder circuit is used to restore the audio to its original levels.

Since mono discs seem to have the correct output at the R/L line-out RCA jacks, there is probably nothing wrong with U4600, the output amp for the audio line-out jacks. Also, since the audio level of a stereo disc sounds right when played through the RF output, there is probably nothing wrong with the circuitry prior to the audio decoder, since the sound sent to the RF modulator from a stereo disc is the sum of the L and R channels PRIOR to audio or compaction decoding. That of course leaves the audio decoder itself.

A fair number of components make up the audio decoder. Depending on your technical abilities, you can try troubleshooting using the SJT-400 service manual available at http://www.dragons-lair-project.com/tech/pages/nfl.asp. Or you can simply replace U4400 (decoder rectifier op-amp) and U4500 (decoder control op-amp) and see what happens. U4500 is a standard LM324 Quad Op-amp. However, there is no industry standard replacement for U4400. You can check with CEDatum to see if they have a replacement. Email them at CEDatum@ccrtc.com

Matt

[CEDatum]

CEDatum stocks all the original RCA IC's for the J/k stereo audio section of the player including the U4400. Follow Smoky Ponds advice on diagosing the problem. Contact John at CEDatum for the part or parts you want. It's that easy !

[toasterking]

Smoky Pond, thank you very much for the hints about the audio circuitry and for telling me what to look for. The tips should certainly help. And most of all thank you for the link to the J400 service manual! There should be a link to this elsewhere on CED Magic (hear that, Tom?), as this will help others with numerous other repairs. When I get some more free time I'll probe the components in the audio decoder section and see what I find. If/when I repair it, I'll post the results to serve as a "fault tip" for others' repairs.

Thanks also John for offering to help with the parts!

[toasterking]

As much as I love to tinker with devices such as my SKT400, I've been tight on free time lately so I decided to just replace U4400 and U4500 per Smoky's suggestion. Lo and behold, it worked -- easy repair! I'm measuring the audio output level on a stereo disc with a peak now of -1 dB -- much more reasonable! Thank you Smoky for going to the trouble of reading the schematic and making the recommendation. U4400 was an RCA 1421754-002 and U4500 was actually an RCA 1421754-001. I replaced them both with 1421754-003. John at CEDatum was very helpful as always.