Yamaha R-700 help!

savv

savv

Super Member
Got this on Ebay with 3 missing knobs. (They were $16 only)
Checked for DC offset and I am getting these readings:
R channel - 194!
L channel - 14

Haven't opened it up yet. I have adjusted the offset on the CR 600 and CR 640. Is the DC offset adjustment for the R-700 similar?

I haven't hooked up speakers yet nor have I done any Deoxit cleaning. Should I clean it up before attempting DC offset adjustment?

Thanks in advance for your responses.
 
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I don't think you can adjust the DC on this reciever. I've got one that reads similar. I'm under the impression that some transistors have to be replaced.
So of course I'd like to hear differently.
Do a search, the schematic is posted at least once here in Vintage SS.
 
There is no adjustment on these receivers, you either get sound or nada. If the protection relay operates and the sound is clean then you are fine. If the sound is distorted on one or both channels you probably need new STK1050IIs or the STK3056.

If the sound is clean then a potential source of DC on the outputs could be the 2SA999 and 2SC2320 used in a feedback circuit having gone bad.

Rob
 
merrylander said:
There is no adjustment on these receivers, you either get sound or nada. If the protection relay operates and the sound is clean then you are fine. If the sound is distorted on one or both channels you probably need new STK1050IIs or the STK3056.

If the sound is clean then a potential source of DC on the outputs could be the 2SA999 and 2SC2320 used in a feedback circuit having gone bad.

Rob
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Priceless information. Absolutely priceless.
I had put this r-700 up in the closet early on in my fixer-upper phase when I discovered the triple digit dc, but when very little of this made sense. Pulled it out yesterday and popped the hood and so much more was clear. Zeroed right in on the STK modules and breathed a sigh of relief. I'm guessing the 2SA and 2SC are the transistors on the little 1" heatsinks. I'll have a look tomorrow?
But I'm curious:
By "if the sound is clean you are fine" do you mean it's fine to play and it won't cook your speakers? Or "fine" as in you don't need to find STKs?
 
Well 194 is high and will definitely offset your voice coil so that it won't be linear, but if you get clean sound (which I would doubt) then there is nothing you can do. The only adjustment on the R-700 (and I imagine the R-900 since they use the same chassis) is the 25 volt supply, and that is a small variable resistor in the power supply section.

Rob
 
Well my DC isn't that high - I think it's ~90 in each direction (-L +R) - but still not right.
I'll have a look for replacements on the transistors and see if that makes a difference.
If not, with the tuner issues I created, it'll probably become a parts unit.
Thanks for the info Rob.
 
Merrylander and all,
Are you familiar with the Yamaha bulletin that changes amplifier coupling from DC to AC??? If not, I'll post instructions. This will influence DC offset, but more importantly, keeps it from blowing up.
 
I have seen some Yammies with a switch on the back that does this but have not seen the service bulletin, It would be interesting to have.

Rob
 
merrylander said:
I have seen some Yammies with a switch on the back that does this but have not seen the service bulletin, It would be interesting to have.
Rob
Click to expand...
My Yammy M-4 has such a switch and it's for input coupling not output. If you look at the specs you can see that when it's in the AC coupled position there's a minor impact on the bottom end. With a good preamp it should to safe to leave it in the DC coupled positon which is where mine has always been.
 
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