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Yamaha CR-640 repair

Yamaha CR-640 no sound repair

A local customer purchased this beautiful Yamaha CR-640 receiver from someone locally a few weeks ago. The receiver played beautifully, but some of the front panel bulbs were burned out. They replaced the original incandescent bulbs with LEDs that included current limiting resistors. After this, there was no more audio output and an electrolytic capacitor near the wire connections for the front panel lights was getting very hot to the touch. They replaced the capacitor which unfortunately didn’t correct the issue. Before proceeding any further, they asked us to take a look at it fearing something major had failed.

Yamaha CR-640

After picking up the receiver and talking it over with the customer, we removed the top cover to reveal the nicely engineered PCBs and controls.

Inside of a Yamaha CR-640
The switch rods with the U-joint style connections are a nice touch and they are silky smooth when operated from the front panel.

Since our customer had already done a little troubleshooting, and even kindly marked the capacitor they replaced, we started investigating around the circuit for the front panel illumination. The front panel lighting voltage is supplied by a half-wave rectifier and filter capacitor. This filter capacitor was the one the customer replaced, so this clued us into what the problem could be. We tested the rectifier diode, and it was shorted.

So this explains the front panel lighting, but what about the audio output? This particular half-wave rectifier also plays a role in the protection circuit for the amplifier, so because this was now essentially an unrectified AC signal, the protection circuit wasn’t working and thus wasn’t allowing the output relays to close.

With this information in hand, we talked it over with the customer and they approved the repair. We replaced the diode with an equivalent part, and to our delight the receiver powered up beautifully and the output relays closed. While we were in there, we also replaced another diode of the same type in the power supply.

They both were showing signs of corrosion on the leads, and on the failed diode, it appeared like the corrosion had crept inside the device itself:

Two diodes
Closeup of a diode

Why did it fail at this particular time? It’s possible that there was a transient or stray static discharge on the diode while the customer was installing the new LEDs. Or maybe the LEDs had a different load characteristic that pushed the diode “over the edge” so to speak, even though they draw less steady state current than the original incandescents.

Regardless, it was a treat getting this receiver playing again. Thanks to our customer for trusting us with this repair!

Yamaha CR-640 powered up
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