Another Yamaha CR-640

[timothya]

I had purchased a dead CR640 for parts to revive another unit that had a dead right channel.
I kind of thought it might be kind of fun to try to bring this unit back to life too.

I am planning to replace the output modules with my STK-0050II's (I had originally bought them for my other 640 but was able to take a Yamaha module from this one). I'll go ahead and do both just to keep both channels the same.

I think at least one of the pre-drivers is toast though. Here are the voltages (taken with both power modules removed and 1K resistors installed where pins 1,3 and 0,8 would be):

Right Left
1. 0.3 V 0.25V
2. 0 V 0V
3. 0.3V 0.25V
4. -39.5V -39.5V
5. -40.3V -40.4V\
6. -41.1V -41.1V
7. 5.42V 3.03V
8. -0.05V -2.97V
9. 0V 0V
10. -.05V -2.95V
11. 0.6V -2.62V
12. -5.5V -9.8V
13. -5.8V -10.2V
14. 5.8V 3.43V
15. 40.6V 40.6V

I was thinking about replacing one or both pre-drivers with the STK-3082 per this thread:
http://www.canuckaudiomart.com/forum/viewtopic.php?p=134672#134672

What do you think?

 

[Bstable]

I think I wish mine still worked. I do not have the skill to fix equipment, and I will never pay what it would take to fix mine (whatever that is ?)

Good luck with yours. It is a very nice sounding receiver.

 

[ecluser]

I'm happy to see that my suggestion

http://www.audiokarma.org/forums/showthread.php?p=3721313#post3721313

works for testing the voltage amp. I must admit that I have never tried it as I've never worked on a CR-640...

As I can see, the current source in the voltage amp has ~6mA, this is why you have ±6V at pins 14 and 13

Did you try to adjust the offset on the channel with +3.4V at pin 14 and -10.2V at pin 13? I think it is the problem.

The STK3082 has the same function (voltage amp, but is a dual channel) as the original iGO2950, with the exception that all the protection components implemented in the iGO2950 are absent in the STK3082. So many components on your board would be useless.

I think that if something is bad in the second (first/second) voltage amp, it is part of the protection circuit. If you are ready to try a new IC that has no protection circuit, why not removing the protection components on the board, (namely C713, C717, R717, C715, C719 and R719) because the rest of this IC seems good to me. It cost nothing to try this.

But try to adjust the offset first.

Good luck!

 

[ecluser]

For your information, I had seen the internal design of the iGO2950 in this old thread:

http://www.audiokarma.org/forums/showthread.php?t=145244

I printed the schematic but I didn't saved it on my computer and merrylander removed the attachement afterward. It would be interresting if merrylander could upload it again, or give us a link to the schematic. You would understand how the circuit works.

My printing is too bad, sorry...

 

[timothya]

Well, with the 1k resistors in place, I can get the DC offset on the right channel to 1-2 mV, but the left channel with the pot turned all the way counterclockwise, will only get as close as -955 mV. The relay clicks in though. The left pre-driver is probably toast. What do you suppose the STK3082 would do to the DC offset? Perhaps it is taken care of internally?

This unit is basically a junker at this point as it was bought for parts anyway. I am "experimenting" at this point.

 

[ecluser]

So now you have -1V at pin 10, +5V at pin 14 and -8V at pin 13, or something close to these values.

I'm not sure the left pre-driver is toast, it looks good to me. Check R709 and R711, and remove or make sure that C707 is really good.

Did you try to disconnect the protection components on the left channel?

 

[timothya]

You are pretty much spot on with those voltages. R709 and R711 are good, and C707 is at least not obviously bad. It looks to have been replaced with an NTE one along with most of the rest of the caps on the power-supply/amp board. I'm considering popping in my STK's to see what happens.

I have not disconnected anything in the protection circuit yet.