Yamaha CR840 tech info needed...

[GerryL]

Hi folks-
I found this site earlier in the week while searching for info on my problem. While waiting for membership to go through I've been reading the forums and I fear I've developed a vintage equipment addiction.
So anyway, I've had this CR 840 for about 3 years when I got it from the take it or leave it shack at our dump. It looks real nice, and worked fine until this week. I was listening to it out in my shop, and there was a sudden and short BZZZT, then all was quiet and dark. I took the cover off and looked around; I found a 5A fuse blown on a vertical board in the middle. I replaced it with another 5A, and tried again. Within seconds of being on, I see something letting it's smoke out, so I power it right down. I've figured out that it's a resistor getting very hot; I've shut it off quick enough that it hasn't discolored much, but it's definately what's letting the smoke out. I'll attach two photos with the resistor circled; can anyone give me a clue to what would cause this?
Thanks- Gerry

 

[SoCal Sam]

Welcome. Here is the owners manual and schematic. Hope this helps.

http://sportsbil.com/yamaha/cr-840-om.pdf

 

[GerryL]

Thanks for that, it should give me a good start! :thmbsp:

Gerry

 

[EPI-Center]

hey Gerry welcome aboard.....I have an 840 I need to look at as well and will be checking the DC offset shortly.......hopefully merrylander will chime in and get you on the right path.

 

[merrylander]

From the photo that looks to be R-718 a 2 watt 390 ohm resistor in the feedback path of the right channel. Under normal circumstances there is supposed to be no voltage at either end of that resistor, why a 2 watter I have no idea. I sounds like your power amp or pre-driver module is toast.

 

[GerryL]

If that's the case, are these parts still available?

 

[EPI-Center]

my understanding is that the power one is available..but the pre driver must come from another unit as a donor. I've read of a workaround but you'd have to do quite a bit of soldering.

http://www.audiokarma.org/forums/showthread.php?t=64571

 

[GerryL]

OK, I guess the next step is for me to get the service manual for it.
I'm not opposed to a lot of soldering, I've built quite a few kits over the years and enjoyed it. First I guess I need to determine exactly what is bad.
Does anyone have a suggestion on how to procede with the least risk of more damage? For example, I know that resistor is getting really hot, but I've been powering down before it toasts. Should I lift it to open the path? Or separate the drivers? Hmmm, maybe the service manual will give me more understanding and a plan of attack. I guess I need to know how to test the drivers/predrivers first.

Gerry

 

[merrylander]

If you remove that resistor it will separate the power amp and pre driver. The pre-driver is the flat pack on the circuit board, the power amp is fastened to the side panel. In the case of the pre-driver pin 5 is not soldered, that will orient you. Pin 15 will have about 45 volts DC on it, pins 13 and 14 should not have more than 5 volts on either one. Measure this between the pins and the chassis.

For the power amp you can measure at either of the two coils on the board - that is the output ahead of the relay. There should be little or no voltage there measured from coil to chassis.

 

[GerryL]

ML, that's very helpful info, thank you. I'm going to try to find time one night this week to look at that.
Gerry