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Help with Yamaha CR-640?

T

timothya

New Member
Just picked this up. It had been in storage for 10 years. Tried it out on some cheap plastic mini-speakers at low volume and it does seem to work. I started to disassemble it for a good cleaning and found the dried up remains of a small spider where it had fried itself between the leads of one of the 0.47 ohm emitter resistors. The resistor measures 30k, so it's open. It also smoked a 390 ohm 1/4w resistor in series with it, but that's all the damage I can find so far. I am wondering if I can replace the 5W 0.47 ohm emitter resistor with a regular 5W sandblock type. Is there anything else I should replace while I'm at it?
 
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If those resistors are damaged, so are the output transistors and possibly the drivers. I'd pull the outputs and check them for shorts.
 
Well, the outputs are hybrid modules, and I don't know how to check these. I *think* they are ok though. There are no dark or burned traces near them. My DC offset on the channel with the open emitter resistor is -121mV. It does make sense that it would be way out of wack with one resistor open. I just wonder if I can replace the .47 ohm 5w with a regular wirewound ceramic sandblock, or do I have to use a metal film type as I can't find the exact replacement.

BTW, I also checked the voltages on the pre-driver module per another thread and they seem to be ok.
 
Well, I put in the new resistors, fired it up and heard "sizzle", smelled smoke, and the 4A fuse blew. Can't find what made the smoke smell, but I compared some resistance readings between the IG02980 on the bad right channel with the left ( I don't know why I didn't think of this before ). The output is, in fact, shorted after all. I understand that the IG02980 can be replaced with an STK-0040II or STK-0050II. Where is a good source for these? I work for an electronic store, but we only handle NTE, and these STKs don't cross.
 
You may also have taken out the 0.47 emitter resistor again, the ceramic cases don't blow but the cement they are sealed with will show dark marks.
 
Thanks, merrylander, I will be replacing that again too. The original had a blackened spot on it and it measured 30k. I am concerned about the pre-driver though since the 390 1/4w resistor was a crispy critter. Before I started replacing anything, pins 13 & 14 had 3.66v on them. After I pulled the almost open 0.47 ohm emitter resistor, pins 13 & 14 had appx 13V on them, and after I removed the output module, they have full supply rail voltage (39v) on them. The voltage on those pins seems to depend on what kind of load they are attached to. WIsh I had the service manual instead of just a schematic.
 
I bought the sevice manual.If you want it I'll be glad to mail it to you.I'm currently working some seriously long hours including thru the weekend as well.If interested,I might be able to send it before the end of the week.
 
With that much voltage on 13 and 14 I fear the pre driver is toast. Those pins are the output to the power amp module and if it is disconnected they see no outside load.
 
timothya said:
Thanks, merrylander, I will be replacing that again too. The original had a blackened spot on it and it measured 30k. I am concerned about the pre-driver though since the 390 1/4w resistor was a crispy critter. Before I started replacing anything, pins 13 & 14 had 3.66v on them. After I pulled the almost open 0.47 ohm emitter resistor, pins 13 & 14 had appx 13V on them, and after I removed the output module, they have full supply rail voltage (39v) on them. The voltage on those pins seems to depend on what kind of load they are attached to. WIsh I had the service manual instead of just a schematic.
Click to expand...

It looks to me that your voltage amps are correct. The pin 14 is the output from the current source. Pin 13 is the collector of the voltage gain transistor.

When the output module and the 0.47 Ohm resistors are good, you should have approximately +1.7V at pin 7 of the power IC, a little more at pin 0, which is connected to pin 14 of the voltage amp IC. Also, you should have approximately -1.7V at pin 4 of the power IC, a little more (negative) at pin 1, which is connected to pin 13 of the voltage amp IC.

When the 0.47 Ohm resistors are removed, the emitters of the output transistors, inside the power IC, are not connected. The voltage across the single emitter resistor for the last driver stage (R6 in the internal schematic for the STK 0050II) will rise and, consequently, the voltages at pins 7 and 4 of the power IC.

Finally, when there is no power amplifier module, there is no load for the current source. In fact your meter is the load, and this is normal to have approximately the full suply voltage between pins 14 (positive) and 13 (negative).

Now, I will make a suggestion to test your voltage amplifier modules (IG02980) without power amplifier module connected to it. The 0.47 Ohm resistors must be good and installed. Connect a 1k resistor from hole 0 (of the power IC) to hole 8, and another 1k resistor from hole 1 to hole 3. Now you should have a relatively low voltage between pins 13 and 14 of the voltage amplifier module. It's hard for me to predict this voltage with exactitude since I don't know the magnitude of the current source inside the voltage amplifier module, but I think you should have something near ±4V.

Your output offset voltage should be low if the voltage amplifier module are good.

The 390 Ohms resistors should have a higher power rating than 1/4W. The schematic shows 2W for the one connected between pins 8 of each module... Anyways, if this resistor becomes open you will lose the protection circuit but the amplifier should work.
 
ecluser - Thanks, I'll try that when my new 0.47's get here. Also, yes, the 390 ohm resistor connected to pin 8 is 2W (it's fine). It's the one connected to pin 11 that burned up (it's only a 1/4W). I think the spider shorted out the emitter resistor causing the 390 ohm 1/4W in series with it to burn up (it measured only 24 ohms) causing a cascade failure of the emitter resistor.

Thats my theory anyway :)
 
I think the 390 Ohms resistor connected between pin 11 of the voltage amplifier module and pin 3 of the power amplifier module must have the same power rating as the other 390 Ohms. They have the same function in the protection circuit, one for the positive part of the signal and the other for the negative part of the signal. If one of those resistors becomes open, you have less protection. If one becomes much lower, like you have, the output current from the power amplifier module will be very limited, thus the distortion with a normal load at normal listening level.
 
Yes, I thought that odd too, but they look original, and both channels are the same. Additionally, I just (an hour ago!) picked up another dead CR640 from which I can scavenge the original 0.47's and *maybe* at least one final. It has 1/4W resistors in the same location.
 
Hmm... can't really tell what is wrong with this dead unit I just picked up. Lots of bad solder joints and a few lifted pads. Not from heat though. It looks like somebody was prying around on some of the ceramic disks and pulled the pads loose. A resistance check indicates that the final outputs aren't obviously bad though and the emitter resistors have never been touched so I might have some good parts here. Probably won't get to pull them until the weekend though...
 
Well, I had some time tonight so I replaced the output module and .47 ohm resistor with the ones from the donor unit. It works (woo-hoo!). The DC offset seems high, though not enough to cause a problem for the protection circuit: Left: 108mv, Right (the one replaced): 60mv.
The volume level seems to be balanced and the left channel sounds good, but the right channel sounds gurgly around the edges.

Do you suppose I should replace the pre-driver with one of the ones from the donor unit?
 
There is adjustment for the DC offset. If you look beside the protection relay you should see two test points TL and TR. Near the pre-drivers are variable resistors, one for left one for right. Using mini grabbers (Don't even think of using alligator clips) measure between a test point and the chassis. turn the variable resistor (slowly they can be twitchy as hell) until the offset is zero or as close as you can get.

Note: Volume control at zero and input selector to an empty input.
 
merrylander - I was going to wait to adjust the dc-offset after the other repair is complete, but if there is value to doing it now, I certainly could.

ecluser - I'll have to wait until I get home from work tonight, but I'll post it when I have it.

I'm thinking about finding my o-scope and running a test tone through the thing to find out where the distortion is coming from (it's finding where I put the probes that will be a challenge!).
 
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