PLL doesn't lock in Tentlabs DAC

[jpfastre]

The PLL doesn't lock in the Tentlabs DAC (Tube DAC). I changed already U11, U12, U13, U14, U15, U17 without result. I checked the resistors and capacitors of the PLL circuit. The bicolor LED lights red at the start and lights than alternate green or red.
The output voltage at pin 6 of the TLC251 is a unstable voltage around about 2.5 V. I use the Tentlabs VCXO.
Please help.

[janneman]

Have you asked Guido Tent? I bet he will be happy to help you out.

Jan Didden

[Joseph K]

jpfastre,

Before changing parts did You try the Dac with different sources?
This Dac is really sensitive for the right [absolute] frequency value of the CD drive unit's clock source. It's not enough to suppose that Your drive is good / expensive so it's OK. For example I have a Rotel RCD 990 which was totally off, I had to adjust it. On the other hand some CD roms were just right [with their ceramic resonator..]

I would say it's a defect of the PLL if You were not able to find not even one drive unit with which it would work.

Ciao, George

Ps.: By the way it's a TLC271 which was originally designed in..
Not a big difference, but maybe some offset problems could come up without adjustment?

[jpfastre]

Have you asked Guido Tent? I bet he will be happy to help you out.

I'll try it .
Thanks

Before changing parts did You try the Dac with different sources?

I tried it with two CD players and one DCC recorder with the same result.

Ps.: By the way it's a TLC271 which was originally designed in..

My mistake, it was a TLC271


Any suggestions?

[Joseph K]

Next idea: are You sure that the Cs8412 itself gets locked right in the first place?

Because from Your description it seems that the dc levels / circuit behaviour is OK for a free running system.. So, does things change when You plug/unplug the input SPDIF signal?

Though it will be hard to do smthing without at least a scope..

Ciao, George

[jpfastre]

Next idea: are You sure that the Cs8412 itself gets locked right

Hallo George
When I remove signal:

The LED lights yellow= signals no SPDIF
pin 25 of CS8412 goes high = ERF
pin 6 of TLC271 goes to zero Volts

I have a simple old one channel scope.

What can I do?
Regards
JP:

[Joseph K]

Jp,

So it seems to work, the 8412. And also the phase comparator section works in some way. Hm.

With the scope You should observe signals similar to these:

http://www.diyaudio.com/forums/showt...521#post175521

see also here:

http://www.diyaudio.com/forums/showt...296#post185296

If You checked:
all power supplies, with the scope, and it's 5V DC with absolutely no signs of oscillation, also it's 6V Dc on the TLC271, and no osc. also there, [Always suspect those LM317's.. aproposito, what kind of cap did you apply on the reg's output? 10uF solid polimer - might be a problem]
And there they are both the ~ 700 KHz 5v square wave signals on the output of the two counters/dividers;
and there it is the sawtooth signal as described in above link on the "pll sound" output;

Then still there is the possibility that your external sources are so unfortunate [or the VCXO is not up to tolerance..] that You can not get into pull-in range; this problem can be checked by a high precision freq. counter; [if you can organize one]
Also there might be a problem with the filters time constants, so your PLL loop is not stable. This should not be a problem if all the main filter caps are film as prescribed; all the resistors are really that value that prescribed.

My guess would be these last two possibilities + or a possible PS problem, as from your description the main functionality seems to be there..

[jpfastre]

Hi Georges,
Thank you for your help, I feel we are close to diagnostic it.

all power supplies, with the scope, and it's 5V DC with absolutely no signs of oscillation, also

Voltages OK and no trace of oscillations.

what kind of cap did you apply on the reg's output? 10uF solid polimer - might be a problem]

Vishay solid aluminium.

And there they are both the ~ 700 KHz 5v square wave signals on the output of the two counters/dividers;

pin 3 U13A and pin 3 U14A (picture) near identical.
pin 5 and 6 of the same IC's: block cyclic changing in width. Is that normal?
pin 3 U15: picture
Pll sound output gives the sawthoot but mixed with another wave.

Also there might be a problem with the filters time constants, so your PLL loop is not stable. This should not be a problem if all the main filter caps

I'll check it again.

JP

[jpfastre]

pin 3 U15: picture

[jpfastre]

PLL sound output