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Servicing Yamaha Amp AX-530
This amplifier was brought to me with a complaint that it was having sound distortion problem and was lying with a service technician waiting for an SMD IC. Since he could not procure one, it was brought to me. I opened the set and after doing simple cleaning work, looked inside to see the present condition of the set, did dry solder touch up on the main board and dismantled the set, and carried out dry solder on the other boards as well.
Replaced all the electrolytic capacitors as many had the ESR either at brim or out of range.
Procured the SMD IC 4558 from UT Source and fixed it in the missing place. The clumsy removal of the SMD IC by the previous technician has caused a tear on the PCB! I patched it up.
Extended the wire of Power Transformer, reassembled the set on the table itself for testing purposes. Checked and found that the distortion was still there. Checked the signal fed from my Signal Generator at various stages using oscilloscope. Found that the left channel is the one having distortion. Removed the output transistors and checked them using Peak Atlas and found these to be alright, though the transistors used on the left channel and right channel had different numbers; probably someone had changed it due to failures that would have happened in the past. The distortion was still there after the preamp stage. Even though the customer had provided me a schematic of the Amplifier, tracking the path or locating was very difficult as many of the components had no reference number printed on the board. In my opinion, this is a bad design as far as servicing the board is concerned. If a service technician is not able to locate the part in a path with reference to the schematic, it is useless. So, I located the parts myself and marked it with sketch pen! This amplifier used a complementary symmetry circuit, with feedback protection, as a result of which, if one of the components is removed, that entire section will not work and no signal will pass through. So, after the preamp stage, up to the amplifier output, we have to just grope in dark and confine ourselves to only cold testing!
Let me be honest, in this amplifier case, I was just groping in dark as I could not quickly locate the reason for distortion. I had already exhausted all the trouble shooting methods known to me from my years of experience! I had removed all the active components and tested. I had removed one leg of all the passive components and tested. I checked and compared voltages before that with the other channel. I transplanted every component, including the SMDs (yes, I mean, ‘every’ component) with the other working channel! But same result! I cleaned the board several times, did look for any hairline crack or strained track. Nothing at all! I realized then, why the set was lying for months together with the other technician.
Same thing happened with me too! I will make an attempt, leave it aside and again take it up after a few weeks! It went on and on. I think I would have made around 12 attempts! Now have a look at the connections I had made and other related pictures, to know how difficult it is to remove these from the table and keep it back:
In the schematic, you can see pencil marks done by me after checking all the components and transplanting it with the other channel! In fact, I had done many other cross way checkings, but forgot most of it because of the prolonged work. I almost gave up and told the customer about my inability to find out the cause and informed him that I will be doing a final attempt, after which, he has to take it back in ‘as is where is condition’, which he kindly agreed! Then, one fine morning, after a series of very successful repairs, in good mood and with full of recharge and vigour, I took this set out and looked for any track cuts, that would have happened while checking the components or transplanting, and resulted in distortion by a sheer co-incidence!
Yes, it was there! The track was very, very minutely (no camera capture possible) cut near the transistor that controls the relay in the protection circuit. It was just a sheer chance, which could have come out of my desperate prayers! It is my assumption that by taking out the components and doing transplantation, the board would have got a new life and the earlier problem could have been due to any deep dry solder or bad joints. That would have got solved, but the cut track recreated it! What an irony! In conclusion, I may have to say that it was due to some mental block that I could not locate the fault earlier, probably fed in because of the fact that it was lying with another technician for a very long time (that too a known person to me!). (I had consulted him too and learnt what were the things he did.)
So, reassembled the set and checked. Except the input of CD, all the other inputs were working. So removed the rear input board once again and found that the SMD IC 4558 used there was defective. No output signal and it was loading too. Replaced it with 4570 that I had with me and refixed it.
On testing, I could see that there was a very slight variation in the quality of signal when fed from CD input. But there was no other go, as otherwise I may have to order this SMD IC again and wait for it to come. I did this only after taking consent of the customer.
Tested the set for many hours and days. No issues! Reassembled the set and I keep testing it every now and then, until the customer picks up.
Finally, here is the toll:
A job completed satisfactorily and to my great relief!
Update: The customer picked up the set subsequently, and has not given me a feedback, though I tried to call him several times. I have no idea why!
This article was prepared for you by Parasuraman Subramanian from India. He is 68 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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