SPDIF coax no signal lock, optical works fine, need help

[ridikas]

I have an old unbranded DAC from the early 90's (possibly DIY), where the SPDIF has stopped working. No signal lock. Optical however works perfect and DAC sounds great. I would like to get SPDIF going again. Any help would be appreciated.

The coax RCA jack (ground floating, not touching chassis) on the DAC is connected to the board via a 5" Belden RG179 75 Ohm cable. The center conductor goes directly to the primary of a 1:1 pulse transformer (Coilcraft WB1010). The shield is connected to the same transformer, but through a 10uF (electrolytic) and 0.1uF (ceramic in parallel) capacitors.

The secondary of the pulse transformer is connected/shunted by a 75 Ohm resistor. I measured the DCR of the primary and secondary windings with an Ohm meter and they were 0.4 Ohm, similar to the 0.3 Ohm listed in the datasheet.

I also replaced the 10uF electrolytic capacitor, as well as all electrolytic capacitors in the DAC with no change.

Passed the 75 Ohm resistor, the digital signal goes to pins 1 and 2 of a DS34C86N quad line receiver and then to a PC74HC86P Philips microchip.

I have this suspicion that possibly the quad line receiver has failed. But I'm not sure on how to test it. It's being fed by a present 5VDC. Is this something that's likely, or should I be looking elsewhere?

Any ideas, or suggestions are greatly appreciated. Also, if anyone has these chips as spares and is willing to sell them I would be interested in trying.

 

[Chris Daly]

A regular fault is the RCA ( a bad connector if ever there was one for SPDIF, BNC much better )
at the player end losing contact with its circuit board track.

As plugs are pushed on and pulled out , they tend to break the solder joint.
Takes about 10 mins to fix by ideally using solder wick, then re-soldering
the connector to the circuit board. .

Worthwhile checking.

Cheers / Chris

 

[Dr Jim]

Silly question:

According to the datasheet here:

DS34C86N Datasheet | DatasheetLib.com

Pin 4 is the data enable for inputs A (pins 1 + 2) and input C (pins 6 + 7) - what is its state?

My hunch is there's a SPDT switch upstream which either pulls it or the other input pair enable pin (12) high - switching between Optical or Coax S/PDIF.

Suspect the switch and its +VDC source - you can test by jumping the from the chip's VCC (pin 16) to pin 4 and see if the coax input works.

It looks like the S/PDIF data stream is fed to pin 14 of the PC74HC86P which converts it a parallel format - with a scope you should be able to easily see the S/PDIF clocks from either the optical or coax inputs and trace them through the circuit.

If the switch circuit is working correctly - then suspect that something in the coax input may be wonky - see how the clocks look at the cable & then follow them through the input xformer and up to the DS34C86N and see if they get munged.

Good luck

Jim

 

[jean-paul]

I measured the DCR of the primary and secondary windings with an Ohm meter and they were 0.4 Ohm, similar to the 0.3 Ohm listed in the datasheet.

That was wrong to do. You have magnetized the very small core of the transformer as you have put a DC current through the winding. SPDIF pulse transformers are very sensitive to DC current. For this reason they are often protected by means of a DC blocking capacitor.

You will need to demagnetize the pulse transformer ! So an AC voltage on the winding slowly rising to the point where the core saturates and then slowly back to 0V.

BTW I have a hunch that the DS34C86N is defective but as said by Jim you should be able to see that with an oscilloscope. Tip: if you order one better make it 2 pieces for possible future repair. These ICs will be hard to find when time passes.

 

[ridikas]

Silly question:

According to the datasheet here:

DS34C86N Datasheet | DatasheetLib.com

Pin 4 is the data enable for inputs A (pins 1 + 2) and input C (pins 6 + 7) - what is its state?

My hunch is there's a SPDT switch upstream which either pulls it or the other input pair enable pin (12) high - switching between Optical or Coax S/PDIF.

Suspect the switch and its +VDC source - you can test by jumping the from the chip's VCC (pin 16) to pin 4 and see if the coax input works.

It looks like the S/PDIF data stream is fed to pin 14 of the PC74HC86P which converts it a parallel format - with a scope you should be able to easily see the S/PDIF clocks from either the optical or coax inputs and trace them through the circuit.

If the switch circuit is working correctly - then suspect that something in the coax input may be wonky - see how the clocks look at the cable & then follow them through the input xformer and up to the DS34C86N and see if they get munged.

Good luck

Jim

Thanks for the suggestions. Pin 4 on the quad line receiver shows 5VDC. Pin 16 also shows 5VDC. Jumping pins 4 and 16 did not produce a signal lock. Pin 3 (output A) shows 1.8VDC and is going into the PC74HC86P chip. Inputs C and D are both shorted to ground. The quad line receiver is only used as a dual line.

There's nothing else upstream. I don't have access to a scope at the moment. Any other suggestions? Thanks!

 

[ridikas]

There's also a TL7705ACP reset/supply voltage generator which traces back to those two chips.

 

[wwenze]

Clip on two wires onto the 75ohm resistor and connect that to the SPDIF source and what happens?

 

[ridikas]

Clip on two wires onto the 75ohm resistor and connect that to the SPDIF source and what happens?

Tried just that, no signal lock.

 

[infinia]

is the line receiver biased at 1/2 *Vcc with 2 equal resistors / blocking cap.? if yes its being used in a linear fashion with gain E.g a Schmidt trigger. You can test by forcing it to logic 0 or 1 and checking outputs.
It might be bad data at sending end too. ( you should see some change with a DMM turning the data on/off.)
another way see Fig 5 http://sound.westhost.com/project85.htm

 

[ridikas]

Update:

Thanks for all of the help. It was the quad line receiver! I replaced it with a new readily available/inexpensive part from TI, MC3486N, and everything works flawlessly. This was a rather uncommon problem, but I guess it happens.

 

[infinia]