One Channel Of Defective DAC IC Bypassed In Cambridge Audio Model AZUR-540C
This is the third visit of this CD Player and I request you all to read the earlier articles, as I would be covering only the complaint reported this time with the relevant images required for it.
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https://jestineyong.com/rusted-and-corroded-crystal-oscillators-found-in-cambridge-audio-azur-540c/
The set was brought by a regular customer from a faraway distance and delivered to the hospital where I had taken my wife for follow-up, as he carried it in a suitcase along with another set and travelled in the bus to cut down cost of service. The complaint reported was that only one channel was working. He had kept the player without use for a few months and this was the condition when he tried to play it one day. As per my advice, he removed the top cover and kept it in open space so that sun rays can evaporate condensation and any fungi formation, which are the main causes for malfunctioning of unused electronic devices. But it did not work and that’s how it was brought to me.
I checked the set and confirmed the complaint. The top lid was already open and on inspection of the board, I saw fungi formations in a few places:
I cleaned the board thoroughly using IPA and brushed it and also kept the board open in sun shine later. Then checked again and found that the complaint still remained. As a service manual of this model was not available, I downloaded that was of 640C and studied the circuit. The PS-cum-DAC board looked identical as ICs and board schematic were same. An SMD DAC IC WM8740 was used in the circuit, the two Analogue outputs of which were fed to two separate Op-Amp ICs, NE5532P. I noticed that the input pin of left channel Op-Amp IC was showing dead short to the ground.
After checking all the surrounding components, I replaced the IC using an 8 PIN socket. But the problem was still there.
In order to isolate whether the short was happening due to some issue in the output stage or input stage, I removed the board and combed the entire output stage, sometimes removing the component or lifting one leg of every resistor, capacitor and diodes. Nothing was defective. Then did the same thing in the input stage. Then combed every component similarly in that section and did not find any defects. Then thoroughly checked the resistance shown at pins of both the Op-Amps and noticed that the input pins of left channel were indeed not in order. So, I started tracing the input, by lifting one leg of each stage and once I removed the resistor from the DAC left channel output, the short was not there. Then combed the entire circuit of the DAC in that section and did not find anything defective. So, suspicion was naturally on the DAC IC. I removed the IC:
Though I along with the customer and his yet another friend tried our very best to get a replacement, it was not available. So I studied the datasheet of this IC and noticed:
Two outputs were being fed to op-Amp ICs through two resistors.
Then I re-fixed the IC and ensured that there were no man made shorts because of the closeness of the pins and the size of the IC. Then lifted the resistors (Negative and positive outputs) of the left channel and directly wired the right channel outputs to the other side, which go to the Left Channel Op-Amp.
Then refit the PCB and applied power and was glad to see that the outputs were coming out fairly well on both channels.
The quality might get affected if the recording of CD is such and the signals are processed separately in the digital stage itself. Otherwise, it was ok. Thus the fungi formation killed one side of the DAC requiring this survival bypass!
Though I tried copying the schematics, the images were very poor and nothing could be made out and that is why I had to skip showing it here. Mission accomplished with a lot of learnings done in the process, which got qualified for the satisfaction get entry into the collection bag!
This article was prepared for you by Parasuraman Subramanian from India. He is 74 years old and has more than 30 years’ experience in handling antique equipment like Valve Radio, Amps, Reel Tape Recorders and currently studying latest tech-classes conducted by Kerala State Electronics Technicians’ Association. He has done graduation in BBA degree, private diploma in Radio Engineering and retired as MD of a USA company. Presently working as Consultant to Hospital and other institutions.
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Yogesh Panchal
October 18, 2025 at 5:16 pm
Good job! Sir
IC WM8740 is very costly.
https://www.leedscart.com/index.php?route=product/product&product_id=6877
https://www.farnell.com/datasheets/484663.pdf
sometime PCB internal layer shorting also cause the symptom.
Parasuraman S
October 18, 2025 at 11:29 pm
Ok! Thanks, dear Yogesh Bhai!Shall inform the customer.
Albert van Bemmelen
October 18, 2025 at 6:58 pm
It is sad that a complex WM8740 DAC chip elsewhere normally available can't be bought in your country at present.
After downloading the very detailed 20 page datasheet I noticed it also includes a digitally controllable mute and attenuator function on each channel on pin 25. With as result a Low level for soft mute. And a High level signal for normal operation.
The chip here normally is available for about 5 euro/dollar excluding shipping costs on eBay, Amazon and Aliexpress too!
Parasuraman S
October 18, 2025 at 9:26 pm
Unfortunately, when the import was curbed here, we, technicians are in doldrums! But now things are a bit easing up and we expect to get these spares. Many thanks for your expert comments, dear Albert!
Abdul
October 18, 2025 at 7:54 pm
Good satisfied job,
Best regards
Parasuraman S
October 18, 2025 at 9:26 pm
Many thanks for your comments!
Mark J
October 19, 2025 at 3:42 am
Parasuraman great repair job. Thank you for sharing.
Parasuraman S
October 19, 2025 at 7:59 am
Thanks, dear Mark!
Philip
October 19, 2025 at 12:04 pm
Great determination, unobscured focus and underailed attention constitute to finally hitting a target. Chaseing down for a short circuit may prove to be challenging in cases where tiny SMD components come as a hindrance as you persue the trouble maker. Partially lifting the ICs may altogether present problems during troubleshooting. Its even more challenging when you finally discover that the problem-causing IC has no identificatiom on it and worse still has different pin links layout to other familiar or common IC link pins to the board circuits. Also failure to determine the value of a faulty SMD component is a drawback. I see these lots of times in small gadgets like radioes.
Parasuraman S
October 19, 2025 at 4:08 pm
Many thanks for your involved comments and inputs!