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Old 26th April 2009, 09:23 PM   #1
arupg is offline arupg  Latvia
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Default Problems with Theta Digital TLC

I have the Theta TLC for years that I recall & its quite a wonderful little piece of device. Oflate, it is giving trouble. While the CD Transport is locked to the dac via this Theta TLC, it loses the lock status and upon toggling the push-button switch on the back of the Theta that toggles the input between Toslink or coax, the "lock" resumes.

Further, even in locked status, the LED lock on the dac flickers and I can faintly hear high freq distortions in the background of the music playing.

I'm unable to figure out what may have gone wrong in the Theta TLC. I did remove the Theta from the setup to see if my DAC was creating the problem but its fine. The symtoms resumes back upon bringing the Theta back to the setup. Hence, its the Theta.

I'm quite a newbie in this. Can fellow members kindly guide me as to how to go about solving the high freq distortions that I hear bleeding into my music and also the sudden unlocking of the DAC.

I desperately need your help. I dont have schematics or any circuit diag for the Theta Digital TLC.

Will appreciate if someone can come forward.

Thanks in advance!!
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Old 27th April 2009, 01:33 AM   #2
kevinkr is offline kevinkr  United States
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Theta Digital apparently has no information available for the TLC, I looked on their site. This has been a problem with a lot of products that were made by the previous incarnation of this company both under the Theta label and as an OEM for others. Hopefully someone out there might be able to help you.

http://www.thetadigital.com/faq.shtml#time_linque

I was faced with similar issues with my PSA Lambda Drive about 4 yrs ago (this was made by Theta) and ended up selling that and the matching Ultralink II. I was pretty unhappy at the time, but there is much better sounding gear out there today.
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Old 27th April 2009, 08:43 PM   #3
arupg is offline arupg  Latvia
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@kevinkr Thanks for your post. I really need to get hold of the schematic. Is there anyone who could help??
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Old 29th April 2009, 05:33 PM   #4
arupg is offline arupg  Latvia
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[IMG]]http://i44.tinypic.com/149o4jm.jpg[/IMG]
Click the image to open in full size.
Click the image to open in full size.

The TLC uses a free running PLL within the CS8412 interface for clock recovery and jitter suppression without a second crystal-based lock (as used by Audio Alchemy, for example). The digital output is re-clocked via a Dual D-type flip-flop MC74ACT74N with extra signal conditioning provided by a high-speed hex unbuffered inverter MC74HCU04N—hence the TLC's high output level, clean waveshape and fast risetime. There's also a Quad-2 ch multiplexer 74HC157AP.

This output is transformer coupled via a Pulse Transformer but, because Theta uses a 'stereo' dual-RCA phono socket (a cost-saving, presumably), the grounds of both coaxial input and outputs remain connected!

I am experiencing "tiny high freq chirpings" in my music when playing (just like small scraches in an analog LP) and suspect its a semiconductor device slowly malfunctioning. I have now removed the PCB from its chassis to track which of the ICs it could be.

With multitude experience of fellow members, I would appreciate if you would help me/ guide me as to how I may be able to rectify the snag I am experiencing.

Many Thanks in advance !!
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Old 29th April 2009, 06:35 PM   #5
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The only component I see on there that I know to be unreliable is the CS8412. I've seen a number of these fail over the past few years. The symptom each time was failure to lock onto the signal, but I could see it causing your symptom in early stages of dying.
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Old 29th April 2009, 09:03 PM   #6
arupg is offline arupg  Latvia
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@ stephensank
Many thanks for pointing me to the right direction as its getting increasingly difficult to get hold of CS8412 ICs these days. Ebay has some leads so I reckon I better hurry...
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Old 29th April 2009, 09:47 PM   #7
arupg is offline arupg  Latvia
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They also say that this Theta TLC uses a stereo RCA female set for the coax IN and OUT - having a common Gnd pin. And a chance for interference - hence higher jitter.

Please see pict marked in :
GREEN +ve OUT
PINK +ve IN
RED Common GND



I want to install separate high Quality Female RCAs. Could I use the GNDs marked with X ??Click the image to open in full size.
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Old 29th April 2009, 10:01 PM   #8
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Don't know the TLC circuit, but if it is feasible, I would definitely suggest isolating the output ground, which would require jack replacements. Always a good idea to replace those crappy jacks, anyway, of course. I do think it is marginally possible that you simply have a ground loop problem caused by the grounded transormer output.
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Old 30th April 2009, 01:41 AM   #9
kevinkr is offline kevinkr  United States
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What is that switch used for? Given the age of this unit, if it has anything to do with bypassing the circuitry in the TLC you might want to clean it. (Contact oxidization)

Edit: I see now that it appears that switch selects toslink device or RCA, presumably an input. Cleaning would be a good idea - eliminating that switch altogether if you only use the RCA would be better.

Another area of concern is all of those tantalums, long term they aren't that reliable and replacing them with Sanyo Oscons could be a good move.

If the PLL in the CS8412 is loosing lock due to deteriorating caps in its loop filter the symptom you describe would be exactly the result. I would check or change those caps first thing. Finally I would consider replacing the CS8412, OTOH you could eliminate the need for it by reclocking the spdif input internal to the DAC.

I'm gonna be mean, but I'm pretty sure a WM8804 spdif I/O module from Twisted Pear Audio will blow the TLC right out of the water if you use it for reclocking, and not cost you a lot in the bargain. Link here: http://www.twistedpearaudio.com/digital/wm8804.aspx - it's temporarily out of stock as these guys products are quite popular. (I use this as a receiver in my home brew dac, but it receives and transmits spdif and can do so simultaneously.)

IMO RCA jacks are NEVER the right choice for 75 ohm spdif audio, I recommend you find the right BNC and cables - really helps with reflections at the connection interface point. (My homebrew dac and transport use "true" 75 ohm cable and connectors.)
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Old 30th April 2009, 05:31 PM   #10
arupg is offline arupg  Latvia
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@ stephensank

You may be just spot-on with the Gnd loop thing that you point out. In fact over last 24 hrs, the circuit PCB is totally out of its chassis, sitting on a wooden platform all hooked up and very happily singing to glory. Haven't had those high freq "chirpings in my music" and apart from once or twice not locking straight-away, its been pretty good in locking transport to DAC otherwise.

Meanwhile, I heatsinked the CS-8412-CP hoping if it had some minor problems it would be better.

I am working at removal of those crappy RCA female sockets with better chassis mount ones but Theta isn't kind in giving sufficient space for those - hence will have to mod the chassis back plate.

I'll be "watching" the PCB behavior over time now and will keep you posted. Thanks once again for the great help. Much appreciated!!

------------------------------------------------------

@kevinkr

The switch is there to select Toslink as 1 or Coax RCA as 2. Anyway, I use only only coax so plan to pluck out that Toslink connector & the switch.
Also, am building a 2amp powersupply with good regulation to power up the Theta TLC. Hope all this would solve my hassles with this otherwise quite a wonderful device.
Also, many thanks for your help.
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