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Protect Mode In Yamaha C4 Pre-Amp Solved

By on December 8, 2015
yamaha pre-amp repair








One Yamaha C4 Preamp was brought back to me after a gap of almost about one year and 7 months.   This has a history. This set had visited my service center three times in the past with the complaint that it becomes dead after working for a few minutes. But when I had put it on at my place and tested it for hours and days together, it never failed even once. So, I had returned it back to the customer, after completing a general check up and cleaning.


Since servicing this unit was very difficult, as the different boards were interconnected with loop wires, and are on one top of another, I told the customer to bring it when it gives a complaint. Thus, when it was brought back to me in Januay, 2014 with the complaint that ‘one channel was weak’, it was serviced by me thoroughly, by combing the entire boards, replacing entire electrolytic capacitors (around 80 nos.), retouching all solder joints on the board (it took me almost seven days of sitting for all these!). Did a thorough cleaning. Had downloaded its service manual from, and checked all circuits for proper voltages. It was fine. Customer took it back from me in March, 2014 and reported that it was working as good as original.

This time, the complaint was ‘it is on protect mode’. So, I removed the covers, did a thorough cleaning job, and started my probing into the problems. In order to remove the front panel, we need to remove the knobs using Allen keys and some of these have two screws on them.

yamaha amp repair

Since accessing the boards are obstructed by the front end boards, I removed them by desoldering array of jumper wires, as you can see from the two pictures above. These are the boards removed.


Then rechecked all the components on these boards by cold test, using ESR Meter, Multimeter and removing some of the suspected components to test on Peak Atlas DCA. Found a couple of double-diodes VD1212 short. These were connected on the front end amp board and were responsible for controlling the protection relay. Since neither data nor replacement was possible, after combing the web, I came to know that two IN4148 diodes connected in series will do the job and are better. This set has a headphone amplifier board, which also houses a portion of the DC control circuit.

The smoothed DC from the power supply board (located next to the transformer, on the bottom row in the first two pictures) goes through the tone control board and reaches the headphone amp-cum-power supply regulator board. The dual supply of 42V DC is controlled by Transistors to regulate at  +/- 30V for functioning of all the boards. Separate tapped sources are for illuminating the 12V DC lamps of Power, Tone Bypass and Audio mute, which are located right under the square knobs which have transparent covers. The colour is obtained by a thin film covered on the bulbs. After completing the cold tests, I decided to interconnect the boards by providing lengthy external wires. I used 25 inter connecting wires. This was a laborious job!   I had to cut different colored wires, remove sleeves from both ends, scrape to make it shining, twist it and touch it with flux and solder, which means I had to do this on 50 tips of the wires! Then soldered these on the power supply board, then to other boards as you can see from the pictures below:



Then switched it on. It was working fine. What made me do this crazy thing? If I had chosen to connect these boards back, I had to put these boards on one top of another in its places, preventing access for troubling shooting, in case of a failure. Fortunately, there was no other defect and I ran the set for a few minutes. It worked fine. So, reconnected the boards in its place by removing the temporary connecting wires provided by me and switched it on. It worked fine! Incidentally, this set comes on only after a few seconds of delay, as there is an al-round self check performed by circuits (not by any micro controller ICs).


I ran this for hours together. It worked fine. So, fit the covers back. Again tested it for several hours continuously. Continued to check whether there was any malfunction by switching it on and off for a few days. It worked very well.


An audio clipping is attached to listen to the quality of this preamp!  The recording is from the headphones, by placing my mobile phone between the two phones! So, another laborious job found its result satisfyingly! Another feather in the cap?



This article was prepared for you by Parasuraman Subramanian from India. He is 65 years old and has more than 30 years’ experience in handling antiques equipment Valve Radio, Amps, Reel Tape Recorders and currently studying latest techs classes conduct by Kerala State Electronics Technicians’ Association. He was a BBA graduate, retired as MD of a USA company and presently working as Consultant Manager, Purchase & IT, in Irinjalakuda Cooperative Hospital.

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You may check on his previous repair article below:




  1. Gerald

    December 8, 2015 at 2:14 pm

    Hello Parasuraman,

    Thanks for sharing this interesting repair. You must be the most patient man in the planet 🙂

    I never came across this VD1212 before so I did some research on the Internet. Most seem to write that this component is a common source of trouble and yes all replace it with two 1N4148 connected in series. This is new to me, always good to learn something, thanks...

  2. Yogesh Panchal

    December 8, 2015 at 3:12 pm

    Good job!! Sir,

  3. Albert van Bemmelen

    December 8, 2015 at 4:30 pm

    You keep doing the almost impossible Mr. Parasuraman! Not many engineers will take that time to repair such a vigorous job. And thanks for the VD1212 info by replacing them by 2 diodes 1n4148. Since you repair a lot of unbelievable audio defects and sure could use more datasheets and service manuals, have a look on these links:
    It also provided me on about 111 GB of very interesting schematics and datasheets on all sorts of devices and components including books ! Cheers.

  4. Gerald Millward

    December 8, 2015 at 4:54 pm


    Your friend is fortunate indeed to have someone able to devote so much time, patience and effort to repairing his pre-amp. I stand back in admiration.


  5. Suranga Electronics

    December 8, 2015 at 4:57 pm

    good Repair...

  6. Dave

    December 8, 2015 at 5:52 pm

    Amazing, what patience - were you charging for your time? if so it probably costs more than the Amp is worth.
    I might be inclined to refuse to look at it in the future...
    I'm all for a challenge, but this one is a bit too much.

  7. Parasuraman S

    December 8, 2015 at 9:33 pm

    I thank all my Technician/Engineer brothers/sisters across the Globe for the support and encouragement expressed. It is this push that keeps me doing such almost impossible jobs! May God bless you all! My special thanks to Jestine Yong for making this possible! My thanks are also due to my wife, daughter-in-law and grand-children, who never bothers me while I am at work, though I am in the habit of waking up as early as 3 AM to do my work! Some interesting work has made me get up even at 2.30 AM!

  8. Andre Gopee

    December 8, 2015 at 10:13 pm

    Wow! what patience, I admire you for that. It is always great to read your articles because I myself like to repair audio equipment and you share lot of information that I may not know... such like web-sites for manuals etc. Thanks for sharing and good luck for future repairs.

    • Parasuraman S

      December 9, 2015 at 12:10 am

      Thank you!

  9. Bernie Scott

    December 9, 2015 at 12:23 am

    Fantastic Job sir...I commend you you for your efforts.....And thank you for the tip on the diodes.....I never knew that either......

  10. Robert Calk

    December 9, 2015 at 1:36 am

    Wow! Nice job, Parasuraman. No doubt you have the tenacity of a tiger!
    Rather than scrape wire to prepare for soldering, I use sandpaper if I can.

    • Parasuraman S

      December 9, 2015 at 4:09 pm

      I too use sand paper, wherever possible. I used multi-strand wires for this job, and that cannot be properly cleaned by sand paper. So I use a sharp pen knife kept on my table for this purpose. It gives a shining surface and never cuts the strands. With the knife you can force in slightly, whereas with a sandpaper, you have limitations. Moreover, it will leave its own dust on the contact!

      • Robert Calk

        December 9, 2015 at 9:09 pm

        I clean the dust off with alcohol. A knife will get the job done.

  11. Ulises Aguilar Pazzani

    December 9, 2015 at 9:52 am

    Mr Subramanian, a well job done . always read Your articles take care Sir

  12. Parasuraman S

    December 9, 2015 at 4:10 pm

    I thank one and all for your very encouraging comments!

  13. Tyrone

    December 9, 2015 at 8:51 pm

    Thanks Parasuraman for this post which is very enlightening God bless you.

  14. kamal

    December 13, 2015 at 3:49 pm

    hi sir,ur doing fantastic job.

  15. Seppo Ruuskanen

    July 12, 2016 at 5:54 am

    Yes, thank You a lot. I too have a C4 where the sound cracles up after it begins to warm up. So far I have resoldered the uppger boards and expect to resolder the lower ones during the week. I also plan to replace as many of the old elcaps as I can to modern better ones.


  16. Daniel Fiorda

    February 4, 2017 at 5:14 am

    Great Job Sir! Your work encouraged me to open my equipment and to repair it. Unfortunately my C4 didn't get better changing such diodes solely. I had to remake all solderings and now is working properly.

  17. louis maggio

    July 6, 2017 at 12:15 am

    Dear Sir
    I am an original owner of Yamaha components "M4 Amp, C4 Pre amp, T2 Tuner and YPD10 turntable.

    Several years back I purchased a Mac Amp to replace M4 which I still own.

    My C4 is having problems with the Right and Left channels!

    My question is I need to have this unit repaired and I am at a loss of who to send it to ? Can you please give me a contact name of who I can call to discuss having this unit repaired. Best Regards

    • Parasuraman S

      September 13, 2017 at 7:11 am

      I do not know where you are located. Perhaps Jestine Yong may know from his contact list and email or get it from you to reply you.


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