My electrical engineer, Dan Kuechle, recently acquired a Decware DAC free of charge from a forlorn user. Among other things, Dan likes to refurbish old audio equipment. He has everything from original Phase Linear 400 amps to original Dyna 120 amps up and running, and much more.
This DAC became an interesting fun project for him and he got it working perfectly. It turns out it is a really nice DAC once debugged.
I thought his description of how he cured it mind be of interest to some of you so here is Dan's story about it.
Regards,
Frank Van Alstine
This note concerns the ZDAC1 DAC sold by Decware. Sales were discontinued in September of 2011 with the availability of parts in the wake of the tsunami that hit Japan cited as the reason. All parts that I looked up were available at Digikey (
www.digikey.com).
When I received my ailing ZDAC it was not working at all. It’s past history was that it worked when new, failed within 2 weeks, started working again on its own, failed, was sent back to the factory and repaired, failed again after working for 2 weeks, intermittently worked / didn’t work for a month or so, and finally stopped working all together with it’s owner so frustrated that he gave the DAC away (lucky me!).
Working without schematics, there were several things that I stumbled upon that didn’t seem right. I fixed them and the ZDAC1 hasn’t missed a beat since. If you are trying to follow this note to fix an ailing ZDAC1, you may not need all the fixes. I guess it will depend on how lucky you are. I will present the items in the order that I found them. This is not the recommended order I’d apply them to another unit.
1) First thing I looked at was the logic levels on pins 10 and 11 of the CS8416 receiver chip. These signals switch with the toggle switches on the front panel to select the source. The logic one was low, slightly below 2.0 volts. I fixed it one way, then it went bad again, then I fixed it another way and finally found the problem. The power on LED was miswired (I think) so it was being powered with the logic one source voltage and was limiting that voltage to the forward voltage drop across the LED, which in this case was 1.9v. When the LED was unplugged, like when the unit was taken apart, everything worked fine. Simply unplug the LED to fix this problem. Once the ZDAC1 is fully working it’s a simple matter to wire up the power on LED so it won’t interfere with anything else (or simply leave it unplugged – but I’d clip the wire so some other unfortunate person doesn’t plug it back in).
2) There was nothing coming out of my CS8421 up-sampler daughter board. Signals going in, nothing coming out when a song was played through the ZDAC1. The way the daughter board connects and plugs into the ZDAC1 makes me think the daughter board can be bypassed with a few jumpers. I did not try this. Whether my daughter board was really bad – I’m pretty sure it was. Here’s what I found: I/O power to the CS8421 is 3.3v. Core power should be 2.5v, but is 3.3v in this design. The data sheet for the CS8421 states that 3.3v on the core power pin will not damage the chip, but the chip is not guaranteed to work under these extreme conditions. It is a straightforward fix to cut a trace to the core power pin, add a 1n400x diode to drop 0.7v, and connect things back up again. However, the trace that has to be cut is under the CS8416 chip. Luckily, I know a guy who can replace surface mount chips like the CS8416, so I bought a new one from Digikey (for $10.78 plus shipping) and did the fix. I also added a 1k to ground after the diode to ensure minimal current flow and added a .1uf cap to ensure the new 2.6v core power was clean. Once completed, I have never seen the same symptoms of inputs to the up-sampler with no outputs. I think it might be possible to lift the core power pin (3) and solder a wire to the floating pin rather than remove the chip, cut the trace, and replace the chip. But I did not do this.
3) At this point the ZDAC1 was working on my bench. But, as soon as I moved it to my workshop (Minnesota, late Fall, workshop only heated when occupied) it refused to work until the workshop got up to about 60 degrees F. After a lot of playing around with power on resets and capacitor values, I found the big 10uf power on reset electrolytic cap on the up-sampler board did not measure 10uf, it measured 1uf. I replaced it with a 4.7uf. I then added 4.7uf to the cap that already existed on pin 9 of the CS8416 receiver chip/board (use pin 22 for gnd). This resulted in nice –POR signals that have 3.3 v power coming up first, followed by the receiver chip’s reset being released, followed by the up-sampler’s reset being released. It is stressed in the data sheets that power and clocks must be stable before reset is released. Once these changes were made, the ZDAC1 has been working perfectly (at all temperatures).
