1541A-Based Player - Static Modulating Distorted Noise - diyAudio
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Old 22nd May 2010, 07:17 PM   #1
dtm1962 is offline dtm1962  Canada
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Join Date: Apr 2003
Location: Sarnia, Ontario, Canada
Default 1541A-Based Player - Static Modulating Distorted Noise

I have a Rotel 855 that has (after some mods) have a static modulating distorte noise overlaying the music (when playing only). I changed it to NOS prior to doing the following mods so I don't believe it is the NOS is a factor.

I have separate +/-5VDC for the TDA1541A; separate +5VDC for the SAA7220
& as well one for the SAA7210.
Separate +/-15VDC for the analog output stage & same +15VDC for the clock.
A new Tent-Labs based clock was added as well. I of course remove the 470pF caps & original crystal & as well as the resistor

So the player was working fine prior to above mods.

What I have done so far is to check the actual voltages at each of loads and they are all arounf +4.75VDC for the SAA7210 (Pin 40) & SAA7220 (Pin 24). They are reading +/-4.75VDC for the TDA1541A (Pin278 & 26) & +/-14.75 for the Tent Labs Based clock & analog output stage. The new power supplies are 317/337based and are totally isolated from the existing power supplies. ( I checked and double checked and the impedance between the new & old power supplies are >1Kohm or higher.

I did not upgrade the -15VDC for the TDA1541A and the original PS reads -14.25V (@Pin 15)

I have set up the 400mVDC for the focus offset & 50mVDC for the laser current for the drive unit.

I changed out to a new separate CDM4/19 & set up the appropriate voltages as indicated above.
I changed out the opamps in the output stage with a brand new set of opamps.
I changed out the TDA1541A with another brand new 1541A.
I changed out the clock with another Tent-Labs-based clock.

I checked the DEM (Pin 16 & 17) & just used a 100pF shorting out the pins to see if it makes a difference. The SMD mounted 470pF has not been checked yet to see if it shorted out but I can't see why that would be affected.

As I said I check & rechecked the voltages and they are consistly the same.

The music is listenable but the noisy overlay of modulated distortion in really noticeable. I would say the high frequencies >5Khz and above are affected the most. (Kind of like a slur - when you had too much to drink lol!)

So is it sound like the NOS? new clock issue? or separate PS? or ?????

Any help would be appreciated.

Thanks in advance,
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