Yamaha C6 Preamplifier Need Service Manual

[Okke 70s]

...so, I replaced the plate cappies, and an electrolytic that didn't look so good. Hooked it up and... same results. Then I hooked up the power amp, disconnected the headphones and.. same results. Well, it appeared to run hot faster actually, but I'm not sure about that. As for the jack: I found nothing unexpected, and nothing different between both channels. Although, the resistance to the grounding is a bit higher on the one channel then on the other. 8,45k ohm vs 7,77k ohm. Is that to be considered alarming, or within tolerance? I'll be ordering the SM any way.

 

[Okke 70s]

Alright! So I went thru the phases:

Replaced some plate cappies + a bad looking elco in the power supply - no change
plugged in a power amp (headphones out) - no change
Plugged out all sound sources and destination - no change (those 2watt resistors still run critically hot in about a minute)

- sighted -

Bought the SM and started measuring. (see here)
The output voltages of the tranny are 6% higher as a result of the 230v net voltage we have now, instead of the 220 then. I replaced the main caps with low ESR types, but that should not have much of an effect right?

However the negative output voltage of the rectifier is higher than that of the positive output. The negative is directly attached to the plagued resistors, which are then attached to the collectors of transistors 431 and 432 respectively. Causing(?) a deviation at the base of these transistors that is small in absolute voltage, but a 233% percent off.. And then I noticed a voltage across diodes 407 and 408 that make no sense to me. Diode 408 seems to maintain a higher voltage at the anode then at the cathode. Does that mean the diode is bad for sure, and perhaps the cause of all this? It checks out fine on the (simple) diode tester of my multimeter..

So what do you think?


ps. At one point I figured it might be just a bad calibration. Power supply voltages were just on-and-about within spec, although on the high side. So I adjusted (with VR 405). However, the positive pole is still a smaller voltage (24.8v) then the negative (-25.1v). DC offset of the EQ amp was tight, with less then 2mv on both sides. For measuring the flat amp the volume has to be at max. Well, I'm a little afraid that this might lead to spec-tacular failure, so I skipped that one.

 

[Okke 70s]

And yesss! replacing the diode (or varistor) solved the burning resistors problem. The VD1212 is not obtainable anymore, but I found some great info on how to resolve that right here.

However, the asymmetry in the power supply remains (higher voltage on the pull then on the push side). And the right channel has a crack, like as caused by 'clipping' when I turn up the volume even just a bit. Perhaps tr 430 and 432 got damaged by the heat? Or must the source problem be somewhere in the power supply? Those heatsinks get quite hot.

Any input guy's please, I'm starting to feel like I'm talking to myself ;-)

 

[avionic]

However the negative output voltage of the rectifier is higher than that of the positive output. The negative is directly attached to the plagued resistors, which are then attached to the collectors of transistors 431 and 432 respectively. Causing(?) a deviation at the base of these transistors that is small in absolute voltage, but a 233% percent off.. And then I noticed a voltage across diodes 407 and 408 that make no sense to me. Diode 408 seems to maintain a higher voltage at the anode then at the cathode. Does that mean the diode is bad for sure, and perhaps the cause of all this? It checks out fine on the (simple) diode tester of my multimeter..

Typo's in the schematic ( minus signs missing). D407 is suspect.
What voltage are you seeing on both ends of R531? What's the measured resistance of R531? Should be 2.2KΩ. Whats the measured resistance of R537? Should be 15Ω.

 

[avionic]

...so, I replaced the plate cappies,

Ceramic disk capacitor ?

 

[Okke 70s]

Yes, D407 was bad and with a make-shift replacement in place there is no more burning of resistors here..

The 'clipping' like cracking sound is also resolved now. It turned out that some damaged tracks of the PCB needed bypassing. The amp sounds good now.

But I still got about 0.5 volt difference between positive-to-ground and negative-to-ground output of the rectifier. I had indeed replaced two blackened Ceramic disk capacitors. I am now replacing all electrolitics and type A844 transistors, like I intended to do in the first place. So I can not power the amp up now. When the replacements are complete I will measure again, including R531, thanks for the tip!. Resistance on R531 and R537 checks out fine.

 

[Okke 70s]

R531 measures 2,47 and 0,45 volt. It's counterpart (R532) 2,45 and 0,45 volt. While it should be 1,5 and 0,6 volt for both right? Please don't tell me it's the IC..

 

[avionic]

R531 measures 2,47 and 0,45 volt. It's counterpart (R532) 2,45 and 0,45 volt. While it should be 1,5 and 0,6 volt for both right? Please don't tell me it's the IC..

If it was the IC .It would have to be both. 409 and 410.