Yamaha CR-840 Diagnostic Help

SchweinHaus

SchweinHaus

Well-Known Member
Hey gang,

So I saw a Yamaha CR-840 for a very reasonable some online, and I went to take a look. At first there was no sound, and the former owner and I were toggling switches and fidgeting about trying to get it to work. Eventually, he cranked the volume all the way up, and the sound came on. But then it went off again after a few seconds. And then we fidgeted about some more, and when he cranked the volume up again, it worked for a few seconds. This happened once or twice more, and I chalked it up to corrosion in the volume pot, scooped it up, and took it home.

When I got it home, I immediately deoxed the snot out of the thing, especially the volume pot. I made sure to find the holes in the side and spray plenty of contact cleaner in there, working the pot again and again to clean it. When I fired it up again, no dice, but eventually I did get it to come on, but only for a few seconds. I cleaned all the pots again and worked all the switches. I don't see a tape loop switch on this one, as I know that's a usual cause for the sound dropping out like this. Same thing when I fired it up again. Did a third round of cleaning, and I'm still getting the same thing. Peeled off the face plate, made SURE I got into the volume pot, and I was still having the same problem.

So now I"m trying to move beyond the deoxit stage, which is a little bit past my bailywick. I own a multimeter, and I have some idea of how to use it, though I'm not a pro. I managed to do a pretty involved project on a Sherwood S-8900A with a fellow Audiokarma member, and I'm hoping I can get this guy up and running. I got a little googly eyed at the SNR on the phono section, and I really want to get this guy up and running without taking it in to a tech.

Any body willing to help out? My next thought was speaker relays. It's not fading in and out. It's coming on for a few under ten seconds, and then cutting back out again. One thing I did notice is that I swear I can hear music coming faintly out of what I believe to be the main filter caps. I don't know if that's actually a normal thing or not when there's no speaker output, but I could distinctly here the song I was listening to and then the cadence of the voice on the radio when I was using the tuner.

Any help is much appreciated. Thanks!
 
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I had read about the unobtanium STK in mits before hand and I’m really hoping it’s not that. I’ve read a bunch about units not “coming out of protection” and I’m wondering if it’s somehow related to that?
 
I just downloaded the CR-840 service manual from hifiengine having never seen this model before and I would suggest that you do the same.

Points:-

1. The amplifier has no electronic protection systems, only fuses.
2. The amplifier sections are conventional AB transistor circuits, not STK units..
3. The speaker outputs are selected by physical switches, not relays. These could be a problem area.
4. The headphone outputs do not mute the speakers so these won't be an issue for the reported fault. It may be worth plugging headphones in to see if the problem is there as well. If you get continuous sound through the headphones then the problem is with the speaker selector switches - doubtful in this case.

I would start by checking all the voltages coming out of the power supplies. It's quite evident on the circuits boards where these points are and the voltage values are given on the diagrams. You may have a voltage regulator circuit dropping out.

Please post if you want more information.
 
I'm not going to be able to stick with this, and to be honest my sense is that you'd be better off taking the unit to a tech, but I'll make a one-time set of suggestions:
  • Click the "Report" link at the bottom of your first post, and in the pop-up, ask for the thread to be moved to the Yamaha forum.
  • I don't think your problem is the STKs, but it might become that way if (as I suspect) you keep cranking up to volume to the point where you hear components "singing". My guess is that the singing is from the output packs being wildly overdriven.
  • If you are determined to DIY, get the service manual and perform the "Audio Section Adjustment". Note that there is at least one version of the manual online that has a completely incorrect schematic, but the rest of it is good. If you think a schematic would help you, get the CR-640 schematics from hifiengine. The power amp schematic is virtually the same, as is the protection circuit - just a few different component values/part numbers.
 
Powertech said:
downloaded the CR-840 service manual from hifiengine
Click to expand...

That is apparently the version with completely incorrect schematics, at least for the audio section. The CR-840 does definitely use STKs. The
CR-640 schematic can be used as a very good approximation.
 
ecluser said:
While the unit is powered, tap on the relay with something like the handle of a screwdriver.
Click to expand...

Tried this, no dice. I haven't tried popping open the speaker relay cap because it's a bugger to get to and I think I'd either have to remove the whole board or the heat sink. Is it worth trying to get it open, or if it were a relay issue would this do the trick?
 
chazix said:
I'm not going to be able to stick with this, and to be honest my sense is that you'd be better off taking the unit to a tech, but I'll make a one-time set of suggestions:
  • Click the "Report" link at the bottom of your first post, and in the pop-up, ask for the thread to be moved to the Yamaha forum.
  • I don't think your problem is the STKs, but it might become that way if (as I suspect) you keep cranking up to volume to the point where you hear components "singing". My guess is that the singing is from the output packs being wildly overdriven.
  • If you are determined to DIY, get the service manual and perform the "Audio Section Adjustment". Note that there is at least one version of the manual online that has a completely incorrect schematic, but the rest of it is good. If you think a schematic would help you, get the CR-640 schematics from hifiengine. The power amp schematic is virtually the same, as is the protection circuit - just a few different component values/part numbers.
Click to expand...

Okay, I've found the "audio section adjustment." and I see the TL and TR prongs, and I assume I put my read lead set to read low voltage DC on that, but what I don't see is where my black lead goes. Am I barking up the right tree? I also see where the A Left B Left etc are, but there are wires going into those spaces and I would need to go around the back of the board to get them. Do i just ground out the black terminal to whatever? Is this a case where I can measure the DC Bias out of the speaker terminals? From there, I see the trim pots and know how to adjust that back down as close to zero as possible.
I also do see a section on the board listed as "protect-er" so I'm wondering if there isn't a protection circuit that might be keeping things in check. It may be that there's just a resistor somewhere that's blown out that I can't find... I feel like I"m in the neighborhood but I'm not quite there.
 

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chazix said:
That is apparently the version with completely incorrect schematics, at least for the audio section. The CR-840 does definitely use STKs. The
CR-640 schematic can be used as a very good approximation.
Click to expand...

Maybe there were 2 versions of this receiver. The downloaded service manual definitely had CR-840 all over it. Annoying to say the least.
 
I have never worked with a CR-840 without STKs. The main fault on this receiver has been a defective STK-3042. This IC has been released in 3 versions (revisions) while the CR-840 was still on the market.
I still have a couple of these Yamaha "originals" unused in my possession.
 
Last edited:
marathonman said:
I have never worked with a CR-840 without STKs. The main fault on this receiver has been a defective STK-3042. This IC has been released in 3 versions (revisions) while the CR-840 was still on the market.
I still have a couple of these Yamaha "originals" unused in my possession.
Click to expand...

Welcome to AK. I have to say I think you are mis-remembering receiver models or STK types, though.

The STK3042 is a two-channel voltage amp. It was not used in any Yamaha CR-x40 series, though I think some version of it was used in subsequent models.

The CR-640 through CR-1040 used single-channel voltage amps that also included an over-current protection function. These were labeled with Yamaha-specific part numbers (IG029x0). They were undoubtedly produced for Yamaha by Sanyo, but I don't believe Sanyo ever offered them with an STK label.

My main point here is to emphasize that there is no STK part that is a drop-in replacement for a CR-x40 voltage amp.

Cheers
 
chazix said:
Welcome to AK. I have to say I think you are mis-remembering receiver models or STK types, though.

The STK3042 is a two-channel voltage amp. It was not used in any Yamaha CR-x40 series, though I think some version of it was used in subsequent models.

The CR-640 through CR-1040 used single-channel voltage amps that also included an over-current protection function. These were labeled with Yamaha-specific part numbers (IG029x0). They were undoubtedly produced for Yamaha by Sanyo, but I don't believe Sanyo ever offered them with an STK label.

My main point here is to emphasize that there is no STK part that is a drop-in replacement for a CR-x40 voltage amp.

Cheers
Click to expand...

You are so right! My bad!
STK-3042 is used in the next generation receivers, R-300/R-500/R-700.
I have become old and it is a long time since I left Yamaha as a service engineer.
I still have a lot of components for the old amps as transistors, diods, bulbs, IC and other stuff.
 
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