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Pioneer SM-700 Amplifier Repaired

By on May 26, 2014
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I have been involved in Automotive Electrical repairs for 8 years. I have always been interested in electronics, but didn’t really know much about the area until about 18 months ago when I decided to take it up as a hobby.

During my research I found Jestine Yong’s ‘Testing Electronic Components’ and after purchasing the E-book, studied it very carefully.

I was keen to put what I had learnt into practice and so when a workmate said that he had an amp that wasn’t working, I offered to help.

FAULT

Pioneers SM-700 Amplifier only worked on one channel. My workmate said that he still used it because it had such an excellent sound quality that he didn’t want to get rid of it.

pioneer amplifier repair

DIAGNOSIS

I was able to download a schematic of the amp and study the electrical path. I used an oscilloscope to find outputs at different areas. I found a blown fuse and so right away thought I had found the problem. But, I had only found a symptom.

So I further disassembled the amp so that I could isolate the channels and give easier access.

pioneer amplifier repairs

By placing a globe across the fuse, the high current draw made the light glow brightly and I could quickly see that I had a short to ground. I did some voltage tests at the outputs of each of the transistors and found that Q10 was not reading correctly.

I unsoldered the transistor and after testing, found it to be faulty.

pioneer amplifier repairings

pioneer amplifierrepair

The schematic diagram of Pioneer SM-700

I installed another 2SC793 and retested across the fuse with a globe – no short this time!

I reassembled the amp and tested and gladly found that I had successfully repaired the amp.

This was my first major electronics repair and it gave me the confidence to continue to learn, study and experiment with components that people have asked me to look at for them.

I am not qualified enough to do this professionally, but enjoy it as a hobby. I encourage others who are interesting in the field to get involved by studying E-books, such as the ones that Jestine produces, YouTube videos and forums that specialize in these areas – it is really worth the effort for the satisfaction you receive!

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This article was prepared for you by Mark Rabone from Australia.

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2 Comments

  1. Andre Gopee

    October 26, 2015 at 9:07 pm

    Hi Mark, nice job... Where did you find the schematic diagram? i love to repair these type of equipment but I cannot locate diagrams, so maybe yous site can be of some sort of assistance in the future. Thanks again.

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  2. ayjay

    May 16, 2016 at 12:31 pm

    Hi there.. Great job on finding the fault..

    I'm currently repairing an amp too.. But still couldn't find the problem.

    Why do you need to disassemble the transistor to test?.. I thought you can just measure using diode function on the multimeter?.. How did you test the transistor?

    Maybe you can show how did you test voltages at output of the transistors. How do you know the voltage reading at output is not correct?

    Also, how you check using oscilloscope. What is the setting and which point you need to check?

    I'm currently studying the functional blocks of the amplifier.

    I'm still trying to understand the input blocks, adjusters, and finally the power amplifier circuit.

    I've an amplifier and I've change all the faulty parts such as thermistor, thermostat, diodes. I have checked randomly the capacitors. And I've checked the transistors with digital multimeter. Still It's still in protection mode (fault still not cleared).

    Appreciate for any advise, thanx.

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