Left Channel Output Found Charred In QUED A240CD Amplifier
This amplifier was brought to me by a regular customer of mine with the complaint that the left channel became dead and he also smelt something burning. I opened the set and cleaned the inside thoroughly and inspected the extent of damage to the Amp. Word cannot explain to the extent a picture can. So, just look at a few pictures that I took:
The output transistors BD911 and BD912 were dead short. The high watt resistors were burnt, so also many other components on the left channel were burnt/short.
I removed all the components of the output section from the left channel.
You can see many burnt resistors and transistors in the pre-amp stage too. So, I removed all the components from the left channel. Fortunately, I got the circuit diagram of it from the web. Even otherwise, I could just copy what was in the right channel. On checking a few more transistors and diodes were found short. So, I picked up only those components that were unaffected and in good working condition from this lot. In the meanwhile I ordered for BD911 and BD912 transistors to two different suppliers. Why? Because now-a-days a lot of duplicate components have flooded the market and we need to be cautious. For example, when I checked the transistors received from a supplier, both BD911 and BD912 were NPN transistors, which meant that the numbers are just printed on any similar looking transistors and sold! Fraudsters make money this way and the dealers are either hand-in-glove with the suppliers or totally ignorant.
Anyhow, I assembled the components that were in the charred area of the PCB, in a small piece of strip board. I also cleaned the burnt area to the extent possible and isolated it from the tracks of preamp. Then populated the preamp section with all components, some new and some old, like how it was connected. Checked and rechecked to ensure that there were no man-made mistakes. Then wired the plus and minus rails as well as the other connections that come to the output section.
On further inspection, I also found one Zener diode burst in the preamp section and I replaced it:
I was really shocked to see the extent of damages that had taken place in this Amp and was wondering what caused it! Anyhow, after rechecking everything, I applied power and found that the output transistors were getting extremely hot, but only the right channel was working and the left channel was still silent. When I checked the signals using my new 100 Mhz oscilloscope, the signals were ok upto the base of the output transistor. This indicated that the so called BD911/912 were once again fake. Well I bought these transistors from even reputed suppliers, who are dealing internationally. Results were same. As the output transistors were getting extremely hot, I could not do the tests for more than five to ten minutes, fearing another failure. In between these tests on various days, once the replaced transistors became dead short! So, I decided not to connect the output transistors and did comparison of voltages at both stages present in the left and working right channel. I found these to be exactly same. I did a cold test of resistance measurements, after discharging all caps and found these also to be matching. After making several attempts, I replaced all the active components in the left channel and also the potentiometer and adjusted the bias current as specified in the service manual. (The schematics and descriptions were manuscripts!)
Then after doing matching of the voltages and resistance with the right channel, I connected the output transistor board and got the same result; i.e., no sound output. So, even the new BD911 and BD912 transistors were fakes! Alas! We, technicians have to take the beating always! Then fetched fresh pairs from another source. By this time, the Strip board had become unusable, upon which I struck with the idea of providing heat shrink sleeves on all components that are to be fit in the charred area, so as to ensure that they do not contact the PCB. Well, this was something very innovative and I felt confident that if I am lucky with the new pairs of Transistors, it would work very well.
Well, to cut a long story short, my guess was correct and the Amp worked very well on check-up. Moreover, the power transistors did not get heated up! It was only normal warmth that we find in Power Amps. Well, I just fought my impulse to jump with joy like a baby or run like Archimedes (of-course, with the dresses on (LOL))! I tried the Amp for several hours before deciding that the ordeal was over! I made some special connectors for enabling connections of the speakers. In the Amp, separate sockets are provided for left and right channel with options for switched and un-switched. In switched output, the external speakers would be cut off, when headphones are inserted. I called and told the customer about this achievement and he was so happy to get back his favorite Amp, which got saved from a sure dump. At one stage I had almost convinced him that I would remove all components from both channels and provide a stereo IC such as STK series that I had in stock. Because the vexation had reached such a stage! Anyhow, the climax was comedy and we could bring the much awaited result!
Mission accomplished after a lot of struggle and turmoil, adding satisfaction in its maximum intensity to the collection bag!
This article was prepared for you by Parasuraman Subramanian from India. He is 72 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.
Please give a support by clicking on the social buttons below. Your feedback on the post is welcome. Please leave it in the comments.
P.S-If you enjoyed reading this, click here to subscribe to my blog (free subscription). That way, you’ll never miss a post. You can also forward this website link to your friends and colleagues-thanks!
You may check on his previous article on Track Leak Under SMD Resistor Found In SHARP CRT TV MODEL 21FX300