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Old 25th September 2004, 09:03 PM   #1
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Unhappy Broken CD HELP!

Please help newby!

I recently decided to go into adventure of tweaking my old Philips 721 CD Player with no electr. knowledge at all. I was reading from net a lot and decided to give it a try. I changed some caps, op-amp, going just for better quality, and replacing 1N 4002 diodes for SB160. Everything was OK for about 10 days when some brushing noise begin in the left channel. On the next day all I can hear from left channel is loud brumm. Now I measure 2.1V on OP 1 out and 4.6V on OP 2 out! I was trying to figure it out alone for 3 days now but I'm real dummy when it comes to electronics. I have no el. schematics but I can provide photos.
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Old 26th September 2004, 12:47 AM   #2
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Sounds like you may have made an error and installed one or more replacement electrolytic capacitors backwards. They have now failed and you will have to replace them with new ones and carefully observing proper polarity.
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Old 26th September 2004, 01:45 AM   #3
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No it is not the case, I double checked before installing and again and again. But I did use pair of non-polar rubicons instead of previously installed standard Elna. Could that be the problem?
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Old 26th September 2004, 08:29 AM   #4
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Quote:
Originally posted by Bassivus
No it is not the case, I double checked before installing and again and again. But I did use pair of non-polar rubicons instead of previously installed standard Elna. Could that be the problem?
Yes I think so.
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Old 26th September 2004, 11:02 PM   #5
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Default Re: Broken CD HELP!

Quote:
Originally posted by Bassivus
Please help newby!

... Now I measure 2.1V on OP 1 out and 4.6V on OP 2 out!...

Ups sorrrrry! Today after a good night sleep I did some measuring again and found that I gave the wrong information about Voltages! Now triple checked voltages measured on Op-amp are:
Stand by
1. pin 0
2. pin 0
3. pin 0
4. pin-0.8
5. pin 0
6. pin 0.05
7. pin-0.2
8. pin-0.8

On
1. pin 2.1
2. pin 3.27
3. pin 3.27
4. pin-13.5
5. pin 3.27
6. pin 4.7
7. pin-12
8. pin 12
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Old 28th September 2004, 02:22 AM   #6
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Today I changed bipolar caps back to old, but the problem still remains! I have prepared photo of both sides of the audio section so if anyone has some suggestions feel free!
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Old 28th September 2004, 11:53 PM   #7
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Default Still trying...

Today I did some more measuring and here are the results. I have measured DAC L and R out without OPA are the same 4.6V. Measuring resistance on L and R OPA out gives the symmetrical results. Check out the pin 7 in these 3 diferent conditions


No OPA:

1. 4.6
2. 4.7
3. 3.28
4. -13.8
5. 3.28
6. 4.7
7. 0
8. 11.9

OPA with V+- pins disconnected:
1. 4.65
2. 4.66
3. 3.29
4. -13.75
5. 3.29
6. 4.66
7. 3.68
8. 11.8

OPA on:

1. 2.1
2. 3.27
3. 3.27
4. -13.5
5. 3.27
6. 4.67
7. -12.4
8. 11.9
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Old 29th September 2004, 07:59 AM   #8
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Wild guesses but still.

What DAC does it have? It would seem that you have something wrong in the DAC. The output of the IV stage, which i am guessing the opamp is for, is at -12.4 volts( pin7) trying to get pin 6 down to 3.27volts, same as pin 5. The value of pin 6 near 5v indicates to me that there might be a short between the +5 of the DAC and the pin on the DAC driving pin 6 on the opamp.

Or it could be that the resistors between 6 and 7 are broken/disconnected. Measure the resistance with the power off.

As for capacitors i do not see why there would be any problem with non polar caps.
Low max voltage, a too low ESR which could cause oscillations in the powersupply, not too likely, or polar caps turned the wrong way with low max voltage are more of a concern.
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Old 29th September 2004, 10:46 PM   #9
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Talking CD FIXED!!!

The reason of all trouble was broken copper lead on PCB, right in line with solder joint, leading out of pin 6!!! Of course I have read about poor quality of PCB in CD721 and of course, first things I have checked were solder joints. But when I measured connections and resistance I must have made just enough pressure for it to connect and to give normal results. Any ways it is now fixed and SOUNDS GOOD. Thanx to all of you who tried or even considered helping me.
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