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Protect Mode In Yamaha C4 Pre-Amp Solved
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 http://elektrotanya.com/yamaha_c-4_sch.pdf/download.html, 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.
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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