Author Topic: Yamaha C-6 pre-amp frying components  (Read 232 times)

Offline Okke

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Yamaha C-6 pre-amp frying components
« on: December 24, 2018, 11:15:15 AM »
All I want for Christmas.. Is a service manual. Or the golden tip of course, that might even be better.

The situation is as follows. I got this C-6 on the table, with a mobile phone as sound source and a headphone plugged in. It plays fine for a few minutes. Then suddenly two 2watt resistors and attached transistors get crazy hot! To the point where smoke starts coming off, weldings melt and even the PCB gets damaged. Then I have to turn it off.

Measuring resistances (powered down), I can not find any difference between the left en right channel. I assumed one of the melted weldings was bad before, causing the heat that later damaged the rest of it. So I replaced the resistors and re welded the transistors. But it just starts smoking again after a while. Transistors measure the same with diode tester as the good ones on the other channel. Voltages on both sides of the resistors (powered up) is also the same on both channels: about 43-42 and 41-40. All I found for a service manual is the pdf below. The resistors and transistors in case are marked in the "detail" picture. It is the bottom right of the schematic (pdf file in neaxt post).

Does anyone either have a proper service manual of the Yamaha C-6, or an idea what might cause this problem? I'd be very greatfull!

Offline Okke

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Re: Yamaha C-6 pre-amp frying components
« Reply #1 on: December 24, 2018, 11:27:58 AM »
..the best I can do within the file size limit.

Online MacGeek

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Re: Yamaha C-6 pre-amp frying components
« Reply #2 on: December 25, 2018, 06:58:54 AM »
Try hifiengine.com.  It appears they have both an owners manual and a schematic
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Offline Okke

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Re: Yamaha C-6 pre-amp frying components
« Reply #3 on: December 30, 2018, 05:47:34 PM »
Thanks, but that's the one I got.. There is a plate cappie that measures zero resistance in circuit. I'm severing the hope that replacing it will resolve all problems. Well, that will be next year!

Offline Okke

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Re: Yamaha C-6 pre-amp frying components
« Reply #4 on: March 24, 2019, 11:50:12 AM »
So To give you the conclusion: It turned out that the variactor or "double diode" right in the middle of the detail picture was the cause of all the trouble. Apparently these are failure prone components, and they are no longer produced. You can simulate them however, by putting two zener diodes in series. The amp is working alright now. Although it still runs a bit hot on the heat sinks, and there is a small difference between the positive and negative power supply voltage. All within spec tolerances though. Well, I'm happy I finally got this thing working! It hasn't really stolen my heart though..   

Offline Sir Thrift-a-Lot

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Re: Yamaha C-6 pre-amp frying components
« Reply #5 on: March 24, 2019, 12:52:34 PM »
Thanks for letting us know.   Congrats on getting it all sorted out.

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Re: Yamaha C-6 pre-amp frying components
« Reply #6 on: March 24, 2019, 04:18:15 PM »
So how does it sound?
Mac stuff, Sony HDR-F1HD AM/FM/HD tuner, Denon DRS 810 cassette, Denon CDR-W1500 CD recorder, Music Hall MMF-9 w/B&O MMC2, Tandberg 10X