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Mark Levinson No. 39 DAC repair
Mark Levinson No. 39 DAC repair
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Old 10th March 2017, 03:06 PM   #1
astouffer is offline astouffer  United States
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Default Mark Levinson No. 39 DAC repair

I'm in the process of repairing a Mark Levinson No. 39 DAC and getting stumped.

The DAC is a dual mono layout so I have one good section to compare with. Anyhow Something on the 5 volt bus is reading 50 ohms and causing the regulator to run hot. I removed all the electrolytic caps and the regulator but still read a short. I can't imagine one of the film caps going bad so that leaves me with three chips, DS34C86, PCM1702, and DAC7800.

Also the relays seem to switch randomly while playing. I'm guessing that is a symptom of this malfunction?
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Old 11th March 2017, 05:06 AM   #2
canvas is offline canvas  Taiwan
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Any tantalum caps? Look for these. They tend to short after years of operating. Power up and use a thermal image camera to find the hottest component.

Last edited by canvas; 11th March 2017 at 05:10 AM.
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Old 13th March 2017, 03:45 PM   #3
astouffer is offline astouffer  United States
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Quote:
Originally Posted by canvas View Post
Any tantalum caps? Look for these. They tend to short after years of operating. Power up and use a thermal image camera to find the hottest component.
No tantalum caps, only lots of Wima film caps and a few ceramic. Checking around the board with a good ohm meter I'm leaning towards the DS34C86 being bad. Although a friend of mine does have a good thermal camera. I may give that a shot.

Here is a picture of the board for the curious. http://i.imgur.com/aVekq0g.jpg
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Old 13th March 2017, 04:38 PM   #4
anatech is offline anatech  Canada
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Mark Levinson No. 39 DAC repair
Hi astouffer,
Firstly, film capacitors can become very leaky or even short. I've even seen polystyrene capacitors short and pull down a power supply. Don't count anything out!

To many capacitors to measure on that. You know what you should do? It can be heavy handed if you aren't careful, but what we do is supply the power buss using an external supply (pull the regulator if you want). Then power up and allow the circuit to "cook". The offending part(s) will become warm to hot. Don't burn off any traces, you don't need full supply voltage if it is dragged down. This will save you hours in labour time.

-Chris
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Old 14th March 2017, 03:49 AM   #5
Extreme_Boky is offline Extreme_Boky
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Default Get the lab power supply

Connect an external power supply (laboratory power supply with a separate dials for voltage and current). Connect it to the offending bus inside the CD player, crank up the current until the offending cap/IC/resistor/whatever... explodes. Replace the part.
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Old 14th March 2017, 03:55 AM   #6
RonSSNova is offline RonSSNova
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You can also hose it down good with freeze spray. Then see where it warms up first.
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Old 14th March 2017, 03:53 PM   #7
astouffer is offline astouffer  United States
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Default Solved!

Haha all the suggestions for the brute force method. I've used it before at work to find shorted tantalums. I was hesitant to try that method because this DAC doesn't belong to me, I'm fixing it up for a friend to sell.

Anyhow after taking off the film and ceramic caps I was left with three chips. I desoldered one leg of the DS34C86 and found it to be the problem. It reads 52 ohms across the power pins.
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Old 14th March 2017, 04:14 PM   #8
canvas is offline canvas  Taiwan
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Congrates! I had similar problem with my LCD TV before. I replaced all the caps. There were more than a hundred of them, and the worst part is that they were all SMD caps. It took me 2 days to fix it.
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Old 14th March 2017, 06:51 PM   #9
anatech is offline anatech  Canada
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Mark Levinson No. 39 DAC repair
Hi astouffer,
Well, good for you on finding that.

The method I suggested doesn't involve exceeding the normal supply voltage. The idea was to make the defective part warm without damage to surrounding components or the PCB. Obviously, if the current is very high, you would not increase it to normal levels.

This method works well with recording studio equipment and logic boards in particular where you have a huge amount of parts hanging off the power rails. You probably risked damage to the board more by removing all those parts to be honest with you. Never mind all that extra work for yourself. Have faith in the methods you've used before. I wouldn't hesitate to follow this course of action in similar circumstances. I have never caused any extra damage to the customer's equipment, but I have burned my finger on more than one occasion when a component got excessively (as in beyond my intention) hot. Those were usually capacitors when I expected an IC to be the root cause.

Best, Chris
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Old 7th December 2018, 05:26 AM   #10
lexontario is offline lexontario
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Hi In need of a wiring schematic for ML #39
I have some caps that leaked and the traces on board are damaged/missing
Another option, any pics of the top and bottom of the dac board
of the area by the caps that sit under the shielded dac .
Thanks
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