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?
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?
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
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
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
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.
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.
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
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
Hi lexontario,
Wonderful repair job. Leaking capacitors have trashed many circuit boards in the past. Multilayer PCBs can be more difficult to clear holes of solder, but surface mount components are generally about the same as a two layer board to work on.
You should be able to see where the traces were. You can use kynar wire (for wire wrapping) for signal leads, cut component leads for supply and common. Just be patient and work carefully to avoid damaging the PCB. The most important component is the PCB, the most expensive too.
-Chris
Wonderful repair job. Leaking capacitors have trashed many circuit boards in the past. Multilayer PCBs can be more difficult to clear holes of solder, but surface mount components are generally about the same as a two layer board to work on.
You should be able to see where the traces were. You can use kynar wire (for wire wrapping) for signal leads, cut component leads for supply and common. Just be patient and work carefully to avoid damaging the PCB. The most important component is the PCB, the most expensive too.
-Chris
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.