Servicing AUDIOSOURCE Amplifier
This sleek Amplifier had come to me from a faraway place for service in January, 2014 with the complaint that it was not working on its peak sound, after getting the Toroidal Transformer rewound, and the customer suspected the output voltages could be less.
But he could not go back to the person who did the rewinding to get it rectified, as he died soon after that. So, the details such as what were its original voltage and ampere etc. had fallen into oblivion. I traced the company’s web address in Portland, wrote an email to them, who promptly sent me the schematic.
As it did not contain voltage details, I wrote back to them, but they regretted as this model was discontinued long back and some genuine reasons for their inability to provide the details asked for (I do not remember exactly now, but I think it was some server crash or company taking over or something). Looking through the capacitor rating, I guessed it could be around +/- 40V DC. In order to know the secondary voltage of the Transformer, I got my friend make a 24-0-24V, 5A standard square Transformer. Looking at the 6A Fuse in the secondary, I assumed it could be enough. Fit it and found that it was not enough and the real power rated in their user manual was not coming out. Then I gave the Toroidal Transformer to my friend and asked him to adjust the secondary to get at-least 32-0-32. In the meantime replaced all the electrolytic capacitors on board and did all that was necessary as preventive. I changed the 10000uF/50V Tank Caps to 100V. As my friend at Mapranam (about 12 Kms from my home) fell sick followed by his wife, both of whom had to get hospitalized, there was delay in getting the Tx back (What this has to do with a repair article? You will soon know when you complete reading this article!). When the readjusted Tx was received and fit, it worked very well. The DC output was +/- 43V. The Amp was taken back in April, 2014.
It made the revisit in August, 2015, with AC fuse blown, transformer primary in open condition! Customer told that they probably had a voltage surge. But the problem was that there was no one around our area who undertook rewinding of Toroidal Transformer as it required special tools and machines to do it, and there was not much demand for it. So, I requested the customer to locate one himself, as he was more or less having wide-spread contacts due to his hobby of dealing with antique audio equipment. After a lot of efforts finally he located someone near his place and took the transformer there. But they were doing some professional amplifier assembly and in selling business, I think, and regretted their inability to rewind it to produce 32-0-32V with a 5A Rating. They offered a new Transformer with a voltage of 28-0-28V with 5A, which was accepted as there was no other go. During this process, many months passed by! I got the new Tx along with the old one and fit it in the Amplifier and tried. But the protection relay was not getting on. I checked for any DC voltage coming out from the Audio output that can trigger protection mode. There was nothing. I rechecked all the components in the protection circuit and even replaced the semiconductors and other components. But in vain. But, while doing this, I noticed that the relay got on when I touched at the base of Transistor Q003, where the R009, R010 and D005 were joining. So, immediately it struck me that it was because of lack of adequate voltage. You will remember that the output of the Transformer was less. But, this was overlooked by me! A stupid mistake indeed! I just added one capacitor at the junction to boost the voltage a bit. That’s all, the relay worked.
While working on this circuit, I noticed that the relay contacts were dark. Since I remember that the customer had indeed told me when he brought the set for the first time about relay chattering, I replaced it.
Here are some more snaps after I finished the work:
Everything seemed to be working fine and so I left it working and went around my house for other works. Suddenly the sound stopped after a few minutes, and when I rushed in to see what went wrong, I got burning smell and saw smoke all-around in the service room. I switched off the set, and investigated what went wrong. The AC fuse was blown and the primary of transformer was open. Apart from this, there was damage to my isolation transformer, as it was very hot and its fuse too had blown. I checked and found that its primary had got shorted! So, removed and sent it for rewinding. But my friend told me that the cores are damaged and made a new one for me with better cores, better rated wires, in which I also added an MCB in place of the fuse. I once again reconnected the external 24-0-24V Transformer and checked whether the Amp was ok. It was fine, except for the fact that the output was not good enough. So, we were back to square one, i.e., Toroidal Tx! I spoke and chatted with the supplier and he said that the person who did Toroidal Tx for them left and they did not have anyone who could do it. But he agreed to refund the cost. That was very nice of him.
It took several months again for all these to happen, and we did not know two years passed by then! I combed with my Technical Group to find out someone who would rewind the original transformer for 5A or more with a 32-0-32V AC output. Finally, after much persuasion, one of my friends who handle high end Theatre Amplifiers, agreed to do it. I dropped it at his place in my car, as it was raining heavily. Again weeks passed, as he was down with Fever, followed by his son, both of whom got hospitalize one after the other! Looked like this was a ‘jinxed’ Transformer. (Remember that the first person, who rewound this died immediately after that! Second guy who took it had series of hospitalization. This third person also got hospitalized followed by his son. Whole thing looked like a dramatic horror story! I too had personal losses in my workshop, though not caused by this Tx. But it was lying around, bringing bad luck for me! (LOL)) I went to my friends shop in two wheeler and collected the TX. His name is Santosh and runs a shop at Nattika, around 18Kms from my house. Here is his snap with the Tx manually rewound (by hand) by him.
I returned and connected the TX and switched the set on. It was giving +/-43V DC once again. The transformer was redone very nicely and professionally, using superior quality wires and coverings, with very strong leads.
As the set continued to work properly, I fit it in next day. My friend visited me to see how the Transformer was, and took snaps of my working on this Amp (May be a solid proof that I am doing these services myself (LOL).
I did the so called ‘soak’ testing or ‘ageing’ of the set for several hours and was relieved to see it worked very well.
Here is the picture of all that I replaced in this set, during this visit, which includes preventive replacement of fixed filter capacitors around the Rectifier diodes.
I had replaced the AC fuse with 2A Fuse as advised my friend, in place of 3.15A it had when the set was brought to me. While replacing, I noticed that the calculations done by my friend was accurate (that is the calculation he is seen doing in the picture given), as there indeed was as an indication at the back to use 2A Fuse! (Again overlooked by me until now!)
So, the curtain falls on this long story, I think worth, as a lot of practical experience and stupid mistakes are shared, with an amusing touch of horror novel!(LOL)
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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