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Old 13th December 2012, 06:32 PM   #11
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here are few pics I made. It seems to me that I got the wrong ground as the signal is quite noisy, though it's attached to the chasis bolt. Anyway I adjusted the pot to get roughly 1.50vdc on the scope (Vpp value changes very quickly from 1.42 to 1.60 according to the scope's build in functions). The probe is connected to soldered pure silver 0.3mm wire for hi-end audio measurement

It's working fine now, at least doesn't skip tracks that were fine before
thanks for new experience and your advices, Mooly

One last question to clarify, if I get a bit noisy signal, perhaps I should set the pk 2 pk value a bit higher, say some 1.55 average?

P.S. the coupling was set to AC on the scope, is that fine?
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Old 13th December 2012, 07:14 PM   #12
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It doesn't look like it should Maybe its some artifacts of the digital scope and the complex waveform... can't say for sure.

It should be absolutely as like in the picture in the link in the post above.

Going to 1.55 v won't affect things unduly. The reason the signal seems to jump suddenly when the pot is turned is because the point at which laser action begins is sudden. As current increase through a laser diode it functions more like a side emitting LED until suddenly the lasing action occurs and then small changes in current show as dramatic changes in laser output. Thats why the laser current is tightly controlled in a closed loop system.

AC couplings fine. DC coupling just shows the signal sat on top of whatever DC bias is present.
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Old 13th December 2012, 08:03 PM   #13
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..or the probe lying across two transformers with a huge bit of wire...
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Old 14th December 2012, 08:04 AM   #14
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the wire till the probe is about 10cm. Transformers do not affect on the signal noise I assume as I've tried move the probe.

When I stop the CD I get a huge noise still - it's about 0.3v pk 2 pk, when I disconnect the mains, no noise appear on the scope, so I guess I need to play with the grounds. I'll try to solder another wire to 15th pin of the chip to check if that helps...
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Old 17th December 2012, 09:14 AM   #15
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morning!

so I soldered another wire and took the ground directly from the ribbon cable. This helped to lower the oscillations to 140mv pk 2 pk. The signal looks like that now.

Unfortunately, still some discs skip. Also I found out that once you adjust the voltage to ~150mv it could be 130-135mv on the other CD that is more scratched and about 160-1.65mv on almost new CD. Is that ok?
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Old 17th December 2012, 09:37 AM   #16
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It doesn't look right and I don't know why. It should look like this whether it be a 10 portable or a 10,000 player. There's no variation in what you see. It should be rock steady and unvarying.

Maybe the (digital) scope isn't resolving this... I don't know. It wouldn't play if the RF really looked like that.

It's normal to see some variation between discs because the reflectivity varies a little. Take a disc that seems an "average" although most are fairly consistent.
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Old 17th December 2012, 09:57 AM   #17
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well, I'm really confused now...Perhaps I chose wrong TDA's pin, it should be 9th (RFE output)
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Old 17th December 2012, 10:22 AM   #18
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I'm just looking at the data sheet... its very hard measuring on the pickup itself.

Pin 9 (showing the filter components) looks like the RF output.

Its much easier measuring on the chips on the main board. Do you know what chip set it uses ? Is it the SAA7345.
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Old 17th December 2012, 10:31 AM   #19
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You would normally measure here (for example) on pin 8 of the SAA7345 (assuming thats what it uses)

Find the 2K2 resistor and clip the probe to that.
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Old 17th December 2012, 10:41 AM   #20
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thank you very much!

Don't know what is the chip, will take a look later at home and do as you suggested.
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