Adjusting Offset/Bias and other basic? things on Cr 2040 (help?)

Discussion in 'Yamaha' started by 0r0b0r0s, Nov 14, 2017.

  1. 0r0b0r0s

    0r0b0r0s New Member

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    Hi all,

    The good people in the general board suggested I open one up here, in the dedicated Yamaha forum. This was my initial post, suggesting I test the offset and how to do it:

    http://audiokarma.org/forums/index....estroy-a-yamaha-cr-2040.796319/#post-11023662

    I got 203MV and 107MV DC using his technique.

    I am a true amateur and basic multimeter use is about all I can do. I MIGHT be up for adjusting the offset/bias if you think it's a good idea, otherwise I just want to install new bulbs and not break anything.

    As I said in the main thread, I don't care about the FM tuner, I'm just using it as an amp for digital music coming off my computer - it will live under my desk, unseen but not unloved.

    Any hints? Suggestions?

    --Oroboros
     
  2. 0r0b0r0s

    0r0b0r0s New Member

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    Oh also a quick question: they suggested I use compressed air and blow out the crud inside; can I just use a vacuum with a soft attachment? As long as I don't whack anything too hard will this be OK?
     
  3. 0r0b0r0s

    0r0b0r0s New Member

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  4. macyjrm

    macyjrm AK Subscriber Subscriber

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    Disconnect speakers. Make sure you are measuring the same speaker outputs as you have selected on the front panel.
     
  5. clinic-audio

    clinic-audio all on YAMAHA untill 1990

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    Hello

    I got 203MV and 107MV DC using his technique

    where are you taking measurement ?? What voltmeter do you use ?
    203mV and 107mV is not possible if your relay is working after switch the power on .
    Bias and offset are measured without signal , level at zero and with load connected on A speaker (with speaker selector on A position) on CR2040
     
  6. 0r0b0r0s

    0r0b0r0s New Member

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    Oh, hmmm I had some cheap speakers hooked up to A, and I was told to check B after setting output to A+B. Is this incorrect; I should set the speaker output to A but check B (A has speakers wired in?)?
     
  7. mbz

    mbz AK Subscriber Subscriber

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    First up, RELAX. If you've made a "mistake" in the past then so what.

    Download the service manual and follow the procedure for checking the dc offset.
    203mV is certainly too high but probably not high enough to trip protection so you can continue to measure at the speaker terminals.
     
  8. macyjrm

    macyjrm AK Subscriber Subscriber

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    Disconnect all speakers. Volume knob to zero. Select A speakers on front panel. Measure DCV at A speaker terminals on rear panel.
     
  9. avionic

    avionic " Black Knights " Subscriber

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    Or use the designated test points in the service manual.
     
    Hajidub likes this.
  10. 0r0b0r0s

    0r0b0r0s New Member

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    Woo, I freaking rule!

    I was going to replace the bulbs because they didn't work, and because it seemed easy. I ordered some replacements, and they came, and I went to town. I now know they were wired in serial because I took the first three off the panel display and they were all good, I was like oh no. Something else is wrong. Then I got to the last bulb and sure enough, shot to hell. Black as a witch's heart at a wedding, etc. I put all the new bulbs in, struggled mightily to get the rubber gasket seal thing/holders/whatever into the stamped holes in the chassis, but finally got 'em in. I replace all the easy screw out/in ones on top too. This is what I have now, and best yet -- NO MORE SPEAKER POPPING when I turn it off. Woooo.

    Then the scary part. I had been told to adjust the DC offset so by God I was going to adjust the DC offset. My readings were pretty bad from the speaker output terminals; like 130/230 MV for L/R. So I downloaded the service manual, read various threads on here, and ordered some little grabbers for the multimeter. Because I had been warned about everything blowing up everything while doing this, I also slid some tape between the pins in the process -- just to make sure. Because if I can screw it up, I will.

    The pots are super sensitive. They are crazy. Like 1/100 of a MM makes a 10 MV swing on the zeroeing out one. Anyway I messed with it and got them close enough; the whole process scared me; I wanted the cover back on pronto. I did what I could, turned it off, and checked the offset stuff again from the speaker terminals.

    WOOOO. I freaking' rule. 2.6 and somethign like 5.2 for left/right. Compared to 120/250 that's gud enuff for me, way too good in fact. I quickly put in the screws and battened down the hatches - er, as it were. Point is, no more fiddling, I'm sure that's close enough.
     

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  11. avionic

    avionic " Black Knights " Subscriber

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    So you didn't check the bias (idle current) ?
     
  12. 0r0b0r0s

    0r0b0r0s New Member

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    I checked it. The manual said 60+- 2v, and mine read 60.X where X was somewhere between 1-7 (don't remember). I was like, eh, eff it, close enough, don't wanna break it. I did adjust the something or other on each R/L channel (red to E left to TP or something) to get them to 10MV or 10V (don't remember). They were out by a bit; one was like 14 and the other was 7 or something. The pots were a lot more gentle; it was easy to get them near 10. I did that. And I felt really proud. SO yeah. Awesome. It's pretty close to factory...? I guess. I mean it's way better than before, and it worked fine for my ears before...
     
  13. OMGCat!

    OMGCat! AK Subscriber Subscriber

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    If the bias was .something close to spec I wouldn't worry about it. Those old pots are a pain in the butt to adjust.
     
  14. avionic

    avionic " Black Knights " Subscriber

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    10v.......:yikes:
     
    Last edited: Nov 23, 2017
  15. 0r0b0r0s

    0r0b0r0s New Member

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    Ok I put it under my desk, and it's driving 4 speakers + subwoofer from digital music output from "the old computer." It replaced a Pioneer Sx 7something that always made me feel unsettled about the balance adjustment for some reason. Just always seemed off. Anyway, lugged the beast up (nice of them to put 6 feet on the base!, it almost did not fit on the wood sitting on top of the subwoofer which is it's dedicated home). Def. seems a lot more calm. I don't have to fiddle with anything to get happy output. Seems like I'm done for the foreseeable future! I can't imagine something better turning up on Craigslist anytime soon.
     
  16. 0r0b0r0s

    0r0b0r0s New Member

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    BTW is it really possible to blow out speakers? This thing is loud as eff. If I have the input on near max, I can't get above "2" without cringing (I am standing in front of the speakers working at my computer). I have some Andrew Jones bookshelf speakers and Klipsch 3.5's. If I wanted to see how loud it could get at, like "5" (forget "9"), do you think it would be safe?
     
  17. OMGCat!

    OMGCat! AK Subscriber Subscriber

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    Volume attenuator position doesn't really matter as far as amp output. The issue a lot of people run into is the inputs are more sensitive on older units than newer ones so you'll have a CD player or something driving an amp to ridiculous levels at a low setting on the knob. Luckily with your computer you can turn down the output to the amp which should give you a more useful volume knob range.
    BTW, it's very possible to blow out speakers. I've actually seen little drifts of smoke come from voice coils of speakers I 'experimented' with way back in the day.
    If it's getting cringey loud then it's not safe for your ears rather than your speakers. That amp has enough power to do damage to your speakers though I'd bet. If you keep cranking them you'll likely bottom the woofers and that's no fun.
     

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