Waveform of the Day, Part 2

[Ray Gianelli]

Yamaha M-70​

Here's another waveform that, when analyzed, helped pinpoint the area in need of repair.

I purchased this amplifier for repair from a member here. He disclosed that there was distortion in the left channel. Putting the scope in dual channel mode, it was obvious where the trouble was. Here's the waveform; the top trace is the left channel, bottom trace is the right;



Well, the seller spoke truth! Severe clipping on positive going portion of the left channel sine wave. This points up to the NPN transistors in the left channel. Why? The B+ is common to both channels, so the issue has to be in the left channel circuitry. Outputs usually short, and the amp wasn't going into protection. So time to check some voltages before the outputs.

It didn't take long to find the trouble. See partial schematic below:

TR127 had an open emitter-base junction, as did TR101. Replacing these 2 transistors restored the amp to normal. No other issues were found, and the amp worked flawlessly in my rig for a solid month before sale.

The oscilloscope, and more importantly the interpretation of the waveform pointed me in the right direction. It pretty much ruled out 75% of the amplifier as the cause of the malfunction, and pinpointed the area that soon yielded a successful repair.

 

[Scott75s1974]

Thanks for these. Very helpful.

 

[Billy Oxygen]

Question. When you say "outputs usually short", are the "outputs" just the physical output terminals structures or is there an component(s) known as outputs? It may seem like a stupid question but it may help me understand. Outputs shorting in my nascent mind is not a component(s) failure but a physical structural circuit failure causing the current to go places it is not designed to go.

 

[Scott75s1974]

Question. When you say "outputs usually short", are the "outputs" just the physical output terminals structures or is there an component(s) known as outputs? It may seem like a stupid question but it may help me understand. Outputs shorting in my nascent mind is not a component(s) failure but a physical structural circuit failure causing the current to go places it is not designed to go.

"Outputs" are the final power transistors in the amplifier. Usually the big high-power transistors on the heatsinks. The penultimate transistors are called "Drivers" which feed the Outputs.

 

[Billy Oxygen]

Thank you that pushed me off high center.