Yamaha CR-840 protection circuit issue

EPI-Center

EPI-Center

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Subscriber
hey all,

I finally got to the yamaha that I aquired a few weeks ago. It has had no problems until yesterday, when I turn it on...awaiting the click that comes from the protection relay, it makes a sort of distorted thump and immediately shuts right back off. It then never comes back on. I can get it to come back by shutting off the speakers and turning the volume nearly all the way up, at which point I hear a bunch of wild clicking and then I turn the volume back down and sometimes I have it, sometimes I don't.

I've done the deoxit thing to the best of my knowledge, I know the clicking business is near the power switch...but have no idea how to proceed.
 
yes... the first thing you need to do is measure the DC offset and bias. Don't contune palying the amp in this condition. If there are some marginal or failing components you risk blowing the outputs sections. Turning the volume all the way up risks your speakers also. It is time for a fix and the unit should be taken out of services. By a cheap set of yard sale speaker while you are fixing this, don't use your good spekers.
 
I'm afraid you will need an STK type drive IC. Tese are a bit difficult to find. There is also a factory mod that eliminates the DC response of the power amp.
 
megawatt said:
yes... the first thing you need to do is measure the DC offset and bias. Don't contune palying the amp in this condition. If there are some marginal or failing components you risk blowing the outputs sections. Turning the volume all the way up risks your speakers also. It is time for a fix and the unit should be taken out of services. By a cheap set of yard sale speaker while you are fixing this, don't use your good spekers.
Click to expand...

oh I don't turn the volume all the way up with the speakers on...I switch them off
 
Nakdoc said:
I'm afraid you will need an STK type drive IC. Tese are a bit difficult to find. There is also a factory mod that eliminates the DC response of the power amp.
Click to expand...

once it starts playing, it doesn't shut off and is fine. it's only at startup..
 
You are just asking for trouble. At Yamaha, we NEVER let one of those go back out without the DC mods. You may also be looking at needing a replacement STK30xx V-amp. Like everyone else that used those parts, we made the mistake of believing Sanyo when they said, "not need heat sink!"
 
Last edited:
PacificStereo said:
You are just asking for trouble. At Yamaha, we NEVER let one of those go back out without the DC mods. You may also be looking at needing a replacement STK30x2 V-amp. Like everyone else that used those parts, we made the mistake of believing Sanyo when they said, "not need heat sink!"
Click to expand...

so I'm guessing these mods are big money? Was it just the 840's that have this issue? Because I had one in high school and it had the same issue, I figured I was just unlucky.
 
No, the mods are just a couple of 100uF caps. The trouble is that the STK output devices may be squiffy, and the V-amp may also need to be replaced.
 
Squiffy??? !
All the Yamahas of that series needed the mod. If you pop the lid you'll see several 220 ohm resistors in series. Cut between any two and install a 100uf 25volt NP capacitor across the gap. Do the same for the other channel.
What Pacific Stereo is saying is that the drive IC is already damaged and is no longer reliable.
 
I'd check the DC offset before worrying about these other possibilities. The fact that it plays if you can get past the startup problem suggests (to me at least, FWIW) that you don't have an expensive problem in the output section.

BTW, I recently bought a CR-640 that had a problem sort of similar to yours. It turned out to be a 99 cent transistor in the DC offset detection circuit.
 
C'mon! You dun like "squiffy?" ;)

I don't have a manual for an 840, but I remember that those resistors in series went to ground, a-la the R-700. You don't need a nonpolar part. Lift the last resistor that goes to ground at the STK3056, and series a standard electrolytic cap in with its negative to ground.


Nakdoc said:
Squiffy??? !
All the Yamahas of that series needed the mod. If you pop the lid you'll see several 220 ohm resistors in series. Cut between any two and install a 100uf 25volt NP capacitor across the gap. Do the same for the other channel.
What Pacific Stereo is saying is that the drive IC is already damaged and is no longer reliable.
Click to expand...
 
PacificStereo said:
C'mon! You dun like "squiffy?" ;)

I don't have a manual for an 840, but I remember that those resistors in series went to ground, a-la the R-700. You don't need a nonpolar part. Lift the last resistor that goes to ground at the STK3056, and series a standard electrolytic cap in with its negative to ground.
Click to expand...

I'm not even going to pretend I have a clue what you guys are talking about. I don't even know what an IC is.

It's sounding like you're saying I only need one cap installed....and somehow I would have to get the board out.
 
Don't bother, I am up to my a** in alligators at the moment and that is not likely to change until 2010. I have the PCB on order and the mask ready to see if my "reverse engineered" voltage amp is valid, but that, like my CR-3020, is one of my projects that have been taking a back seat to everything else.
 
actually the last couple times I've turned the 840 on, it makes the goofy sound, and then I wait another 10 seconds and it clicks normally and turns on...it's almost like it's going through 2 cycles and the first time something is not right.
 
PacificStereo said:
C'mon! You dun like "squiffy?" ;)

I don't have a manual for an 840, but I remember that those resistors in series went to ground, a-la the R-700. You don't need a nonpolar part. Lift the last resistor that goes to ground at the STK3056, and series a standard electrolytic cap in with its negative to ground.
Click to expand...

I think I can do this actually....but when you say "lift the last resistor"...is that EE talk for "desolder this guy and remove" ??

so basically, I'm removing the last resistor and installing a cap in his place, with the neg going to the ground that the resistor's ground was going to?
 
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