Trouble Shooting Lite DAC 60 - Static in One Channel

TomW16 · « **on:** 21 Mar 2008, 07:28 pm »

I bought a used Lite DAC 60 off of eBay recently that I knew had a trouble with one of the outputs; the right channel has "static". I am hoping that a little sweat equity will provide me with a good deal. It was modified by Pacific Valve and appears to have some capacitors changed including Mcap "White Jacket" at the outputs. With the lid off, it appears that a solder pad for one of the output caps is lifted, which is the right channel. The cap is solidly in place, however.

Doing the easy trouble shooting first, I swapped the tubes but the problem persists in the same channel so it's not a tube issue. A wooden stick poking around didn't identify any loose parts that affected the problem. With the unit unplugged, I'll let the caps drain for a good half hour and then take a look at the bottom of the circuit board.

The "static" appears to correlate with the output. Soft passages do not have any "static" or distortion, however, bass or loud passages has this static, which leads me to think that something might be saturating. I might be up a creek if it turns out to be a surface mount chip

Any advice or words of wisdom on troubleshooting this unit would be appreciated.

Thanks in advance.

Tom

JoshK · « **Reply #1 on:** 21 Mar 2008, 07:32 pm »

Quote from: TomW16 on 21 Mar 2008, 07:28 pm

With the lid off, it appears that a solder pad for one of the output caps is lifted, which is the right channel. The cap is solidly in place, however.

That most likely is your problem. It sounds like a bad solder joint and I was going to mention to check the output cap solder joint, but then saw this. That is most likely your issue. It basically probably needs some fixing.

TomW16 · « **Reply #2 on:** 21 Mar 2008, 07:37 pm »

Thanks for the quick reply Josh. I'll keep my fingers crossed that the output cap solder joint is the culprit. I'll post back once I've done the minor surgery on the cap and let you know whether there is more to it or not.

Cheers,
Tom

TomW16 · « **Reply #3 on:** 21 Mar 2008, 09:22 pm »

I resoldered the suspect output cap and even added a copper wire jumper to ensure that the lifted solder pad had a good connection but the distortion remains. I'm open to suggestions at this point.

Thanks.

Tom

tanchiro58 · « **Reply #4 on:** 21 Mar 2008, 10:11 pm »

Quote from: TomW16 on 21 Mar 2008, 09:22 pm

I resoldered the suspect output cap and even added a copper wire jumper to ensure that the lifted solder pad had a good connection but the distortion remains. I'm open to suggestions at this point.

Thanks.

Tom

It seems you need to clean the tube sockets (both for sure) and pins of the output tubes with Deoxit spray D5 (http://www.parts-express.com/pe/showdetl.cfm?Partnumber=341-200). This problem always occurs when people are trying to roll tubes.

Good luck,
Tan

TomW16 · « **Reply #5 on:** 22 Mar 2008, 12:05 am »

Thanks for the tip Tan. I'll take a close look at the tube sockets and maybe reflow some of the upgraded caps to see if it is a weak solder joint.

Thanks.
Tom

TomW16 · « **Reply #6 on:** 22 Mar 2008, 02:46 am »

Thought I had it...I found a solder bridge on a transistor but when I removed the bridge, no sound at all, nada, zilch from either channel. Re-established the bridge and back to the original distortion issue out of the right channel only.

I might need to send this in for some professional TLC.

Tom

markC · « **Reply #7 on:** 22 Mar 2008, 01:34 pm »

Have the tube power supply caps been replaced as well? Might want to check those too.

JoshK · « **Reply #8 on:** 22 Mar 2008, 04:04 pm »

You wouldn't happen to have a Scope do you?

I'd take a voltmeter and probe around (*carefully*) and measure the voltages, comparing the left channel to the right and see where they might differ. This may or may not shed light but isn't hard to do.

I really wouldn't guess it would be the power supply caps or both channels would distort equally, unless it is the local B+ bypass caps. Check to see if the right channel's bypass is soldered well. This should not be hard to find if you know the pinout of the tube, you can see the cap closest to the anode just above the anode resistor (plate load).

It could also be the components in the I/V stage which are passive. If they aren't soldered well, it will cause the DAC to distort and the tube will amplify it. It could be the DAC itself, but I wouldn't begin to know how to determine that without a scope. I'd start with the easy things and one by one eliminate them as a possibility.

TomW16 · « **Reply #9 on:** 22 Mar 2008, 05:28 pm »

Unfortunately, I do not have a scope. I have used my volt meter to verify continuity of the circuit but haven't measured the circuit live due to the high tube voltages. I will eventually get mini alligator clips so that I can place them where I would like a measurement then turn on the equipment and verify the readings without both hands inside the box. With a Harbor Freight store in town and volt meters that sell for $5 each, I can buy a few and make multiple reading simultaneously while outside the box. I am just a weekend warrior who likes to tinker and learn about electronics so a little knowledge (and false comfort) can be dangerous, especially with tube voltages.

I swallowed my pride this morning and took the DAC into the local electronics repair shop and they think it is likely a transistor that might be going bad since there is no distortion with no or low input signal. I am now "investing" $30 to obtain a professional opinion and, hopefully, a simple repair.

I'll let everyone know what the final part or parts are that need replacing.

Thanks.
Tom

TomW16 · « **Reply #10 on:** 25 Mar 2008, 01:58 am »

So here is the latest in the saga. I called Pacific Valve technical support on the weekend to find out whether they have encountered this "static" problem before and Joe called me today. Joe mentioned that it is typically a transport issue when this happens since the DACs either work or they don't. I mentioned that I had tried two different transports (one with coaxial and one with toslink) and the static problem was definitely in the DAC.

Joe was very friendly and mentioned that since I could solder, I could return the defective circuit board and Pacific Valve would replace it with a new one for under $100 with shipping. There would be no warranty since I would install the board myself, which I am comfortable doing with little risk of damage.

So I'm up from $30 for a possible fix to $100 for a probable fix with a new board. In any case, I thought that the customer service from Pacific Valve was pretty good to allow me to conduct some of the labor to help keep costs down.

I'll let everyone know how the new board works in a couple of weeks after it arrives.

Cheers,
Tom

markC · « **Reply #11 on:** 25 Mar 2008, 02:03 am »

A new populated board for under a hundred bucks sounds like the way to go. Almost makes me want to order one for back-up. And I could tweak the hell out of it too!

TomW16 · « **Reply #12 on:** 25 Mar 2008, 02:15 am »

Unfortunately, I do have to return the old board in order to get the new one so no back ups.

markC · « **Reply #13 on:** 25 Mar 2008, 02:17 am »

Damn! It would be nice to be able to experiment.

TomW16 · « **Reply #14 on:** 10 Jul 2008, 05:43 am »

So the long story short is that there was no replacement board and my board was sent on a slow boat to China for repair. The good news is that it arrived yesterday (after more than 3 months) and when I re soldered the power lines to the board this evening it worked perfectly. NO STATIC.

I thought the DAC-60 sounded good but not as transparent as my old AVA DAC. I'll reserve judgement until I wire up a buffer for my passive preamp, which might even the playing field for the DAC-60. The DAC-60 has some JJ Tesla tubes so I'll also try a pair of Amperex 6922s that I have to see how it changes the sound.

Cheers,
Tom

markC · « **Reply #15 on:** 11 Jul 2008, 04:17 am »

So you got the same board back with repairs? Can you tell what was repaired?
I find my modded Dac-60 very easy to listen to, but as you state, "not transparent". I've done quite a few mods to mine and it is, IMO, much better than stock, but lacks refined detail.
I went head to head with my friends DAC last weekend, and preferred my 60 for bass and vocals, but the detail of brush on symbol and the steel sound of a plucked string fell short. I'm not sure why the detail or "transparency" is lacking or what to try to improve it. Surely the dac chip is capable, isn't it?

TomW16 · « **Reply #16 on:** 11 Jul 2008, 05:05 am »

Hi Mark,

I looked at the board when it came back and I couldn't see what was replaced but my guess would be a transistor somewhere was going bad. Pacific Valve didn't tell me what was repaired. I guess all that matters is that the board works properly now.

I just spent hours swapping between the AVA DAC and the DAC-60 this evening and I think I'm going to keep the modified DAC-60 in the main system. There is something about tubes that are seductive. Yes, the AVA DAC is likely more accurate but there appears to be more body to sounds with the DAC 60. It could simply be tube distortion but it sounds pleasant.

I'll have to re-read the AC posts on modifying the DAC 60 again to see what improvements are possible. I'll stay away from the surface mount modifications though.

Cheers,
Tom

markC · « **Reply #17 on:** 12 Jul 2008, 03:52 am »

I don't know how involved you want to get, but for me the best mods are:
good couping caps
good tubes
replace the I/V resistors
re-do the low pass filter
replace the B+ caps

For about $300 worth of parts, you can take this DAC much farther than stock.
I have not heard a factory modded unit, (which costs less than my stock + parts).

JoshK · « **Reply #18 on:** 12 Jul 2008, 04:27 pm »

mark, do you have any details on what you did to the low pass filter.

TomW16 · « **Reply #19 on:** 13 Jul 2008, 03:49 am »

Thanks Mark. I feel comfortable replacing parts as long as they are not surface mount. I am pretty much a kit guy since I don't have the technical background to design things from scratch. That is why I am very grateful for AudioCircle where people with much greater knowledge than myself share their experiences so that others can benefit. The Felix AC filter is a great example. Thanks again for that Josh!

According to the Pacific Valve website, the following was done to the DAC 60 already "we replaced some critical capacitors with WIMA Black Box Metallized Polypropylene, Mcap "White Jacket" across the outputs and Elna caps for audio. I'm sure there is still room for improvement.

Cheers,
Tom