Hi All,
I'm modding a Philips CD753 for a friend - I've done one before with no issues. I've removed the opamp section and will replace it with a lampizator valve stage. At the moment the voltage out DAC is directly connected to my amp. All I have done so far is to put separate 7805 regs to the three sections of the DAC, replace the rectifier diodes with fast recovery and the smoothing caps with better quality ones of the same value. The regulators are working correctly and there is no fluctuation in supply when playing. In the video on the link, you can hear the first 35 seconds of sound are fine then the drums come in and the scope flatlines. All previous testing was done with a folky album so the problem didn't show.
http://picklethedog.com/cd.mp4
Thanks guys
I'm modding a Philips CD753 for a friend - I've done one before with no issues. I've removed the opamp section and will replace it with a lampizator valve stage. At the moment the voltage out DAC is directly connected to my amp. All I have done so far is to put separate 7805 regs to the three sections of the DAC, replace the rectifier diodes with fast recovery and the smoothing caps with better quality ones of the same value. The regulators are working correctly and there is no fluctuation in supply when playing. In the video on the link, you can hear the first 35 seconds of sound are fine then the drums come in and the scope flatlines. All previous testing was done with a folky album so the problem didn't show.
http://picklethedog.com/cd.mp4
Thanks guys
Sounds like time to replace the laser assembly.
Mechanical isolation can also help, I've used a 50mm thick concrete paver sitting on a partially inflated bicycle inner tube.
Mechanical isolation can also help, I've used a 50mm thick concrete paver sitting on a partially inflated bicycle inner tube.
Was it working properly before you started? Are the output muting transistors still present?
Are they triggered when the audio is killed? Post the schematics.
Are they triggered when the audio is killed? Post the schematics.
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What is the scope connected to?
Are you sure it isn't the amplifier randomly cutting out in the presence of the noise and hash that will be riding along with the audio?
Are you sure it isn't the amplifier randomly cutting out in the presence of the noise and hash that will be riding along with the audio?
I think it was working ok but maybe I never played a loud CD. The o/p transistors are gone. Here's the schematic. The only difference is that I have taken the 10v from the rectifier and fed it to three separate 7805s that now power the three power inputs to the DAC. I've checked these as OK.
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That's very strange then because I can't see how a change in the audio data content on a disc could suddenly cause the DAC to not output anything.
Have you looked with the scope directly on the DAC output or as close as you can get?
Have you looked with the scope directly on the DAC output or as close as you can get?
Are you using the DAC output coupling capacitors? The DAC will output +2.5VDC without them.
If your amplifier lacks input coupling capacitors, this would not work properly.
If your amplifier lacks input coupling capacitors, this would not work properly.
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The scope is connected directly to the dac output (apart from a bit of wire) I don't even have a coupling cap.
Yes I know the ones you mean. Yes they are still in place (I learnt that the hard way modding my own one) 😉
Well that's a new one on me 🙂
All I can suggest you do is to go around ALL the pins on the DAC with a scope and look for what else (if anything) that changes as the trace flat lines.
All I can suggest you do is to go around ALL the pins on the DAC with a scope and look for what else (if anything) that changes as the trace flat lines.
Maybe it's the decency blocker kicking in? Can you say "Fat bottomed girls" these days?
I'd monitor the clock, word select and data
Did you remove the muting transistors completely from the PCB? Does KILL signal go high when the audio cuts-out?
Are the Vref on DAC pin 7 and 4 stable; do you still have the capacitors 2508 and 2507 (47uF) on PCB?
Laser diodes'amplifier, summing, and limiting - may be malfunctioning for some reason.... check if you lose the eye-pattern signal when audio cuts-out...
Good luck.
Did you remove the muting transistors completely from the PCB? Does KILL signal go high when the audio cuts-out?
Are the Vref on DAC pin 7 and 4 stable; do you still have the capacitors 2508 and 2507 (47uF) on PCB?
Laser diodes'amplifier, summing, and limiting - may be malfunctioning for some reason.... check if you lose the eye-pattern signal when audio cuts-out...
Good luck.