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Yamaha CR 1040 protection mode question

Bigears

Bigears

Manager of my own day
Just picked up a Yamaha CR 1040 at a garage sale. Nice overall condition but usually will not come out of protection mode. I can turn it on and it will play through headphones out of one channel, but if I crank up the volume it goes into protection mode. Sometimes it goes right into protection mode as soon as it’s turned on.

I don’t think I can check DC offset because of this. I know this unit uses unobtainable IC’s preamp and main output. A quick search here and I have a little more info, but I’m looking for a way to tell if any of these IC’s are fried so I can plan my next move.

TIA
 
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TIA, you will have to remove the bottom cover to check. Pin 15 of the driver chip should have about 48V DC on it. Pin 5 is the one with no connectiion, so that is how you tell the pin layout. Now pins 13 and 14 should have very low voltage on them, if they are close to the Pin 15 voltage they are toast.

Flip it over, assuming the driver chips are OK. Near the driver chips are two test points and two variable resistors. Measure between the test point and the chassis and adjust the associated VR for 0 volts, or as close as you can get.

In that general area are four 390 ohm resistors (orange, white, brown) if any of them are fried it could be the power amp chip that is feeding high DC back to the driver chip, fortunately these are available STK0080 II

Rob
 
Rob, thanks for the troublshooting info. Here is what I found: I had 50 vdc at pin 15 on both driver chips. I had 3.2 vdc on pins 13 and 14 on the rear most chip and .06 vdc on pins 13 and 14 on the chip closest to the faceplate.

On the topside, I got the rear most VR and test pin down to .001 vdc and the other, closest to the faceplate test pin was stuck at 12.50 vdc, no matter what I did with the VR. I turned the unit off and gave it a DeOxit bath while working the VR, but that predictably didn't change things.

I have posted a pic of the backside with my test probe on what I believe is pin 15 for your confirmation. I also took a shot of the top side by the driver IC's to see if you can tell if anything is amiss.

Do you have any further troubleshooting advice?

Thanks!
 

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It looks like one of the 390 ohm resistors has been changed on the rearmost driver chip, could not see the front ones as the speaker out cables are hiding them. Check the 0.47 ohm emitter resistors just to the left of the driver chip, they are the square white ones, one may have changed value. The fact that the readings on the front driver chip pins 13 and 14 are so low means that something is amiss. Those pins are the output from the driver into the power amp chip and the readings on the rear chip are more like what you should see.

Rob
 
Rob, I've posted a better picture of the pcb around the drivers. Not sure if you can tell anymore from the pic. Does that red resistor (I long ago forgot how to read the value!)at the foot of the filter cap look burned to you?

I'm close to my technical expertise threshold on this one, which isn't saying much!
 

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That one with the reddish body is the one I figure was replaced, the code is Orange(3)-White(9)-Brown(x10) = 390. This is the driver that had to good readings, it was the one closer to the front that had almost no output on Pins 13 & 14. In the pictures there are the two 390 ohm resistors for the front driver and they are right at the corner of the Protection relay between the white three way wire and the chip itself. I don't see any sign of burning but you need to see if either of them is open.

Rob
 
I checked resistance on the four resistors by the driver boards as you suggested. Starting from the 390 working forward; 382,470,389,289. Obviously the second R value is wrong on one of the pairs. Could a resistor not at value cause this?
 
They are 5% resistors so anything 19.5 ohms either way is OK, but 470 and 289 are not acceptable. Whether they are the problem we will only know when they are replaced. So you need two 390 ohm 1/4 watt resistors, that reddish one is 1/2 watt so you might replace it while you are at it.

Rob
 
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