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Yamaha G100 B212 dc offset

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  • Yamaha G100 B212 dc offset

    Hi folks,
    I have a series 1 G100 in beautiful physical condition which I’m hoping to get working
    and using. Previous owner sold it as not working complaining of distorted sound on clean setting. Its 2200uF filter and output capacitors had been removed although it came with the originals and some newer but used spares. The capacitors seemed to test ok i.e. resistance steadily rising on a multimeter.

    Amp fires up fine with a low glow on the lamp
    limiter. Both input channel preamps seem fine, sending a clean signal to an audio probe with all eq and reverb controls working fine. Output transistors (TO3s so easy to remove) test fine.

    But there’s a stack of DC at the speaker terminals - 50V!!! This is present with both the old and new capacitors fitted. This doesn’t seem right!!! - what should I expect with a cap coupled output with no input and no load? Are all the caps shot? I’d be grateful for your thoughts. Thank you!

    Schematic here:

  • #2
    Cap coupled speaker outputs will float until a load is hooked up. It's sort of a phantom voltage. Hook up your speaker and try it. The only way the speaker could be damaged is if the cap i shorted or leaky. The cap's job is to prevent DC from reaching the speaker.
    "Yeah, well, you know, that's just, like, your opinion, man."


    • #3
      Thank you, The Dude, for your reply. I thought it might be something to do with the cap coupling - but I wonder how this squares with the advice given in this discussion: ?

      Relevant bits:

      "So disconnect the speaker and pray that it was not damaged. Also, at this point do not connect the speaker up until you have repaired the amp and confirm that the amp is not putting DC voltage on the speaker output."

      "With the speaker disconnected you can measure across the speaker outputs to see what the DC offset on the amp measures. The DC offset on the output speaker should be low around 250mv or less."

      Note that this is referring to the G100 series 2 - but this still has the cap coupled speaker output afaik.

      Thank you!


      • #4
        That advice is for a directly coupled output stage. 50V is the voltage on the +ve terminal of the output coupling capacitor. If there's no load any capacitor leakage will cause a reading of 50V on the speaker output as The Dude said. Check the voltage on TR24 collector. It should be about 54V. TR18 collector is at 56.6 volts and the amp output is 3 emitter follower Vbe (3 x 0.7V) down from there.
        Last edited by Dave H; 07-10-2020, 02:20 PM.


        • #5
          Temporarily shorting the REC OUT (right side of 560R/3W) resistor to ground would allow the output coupling cap to charge and should bring output DC to zero.
          - Own Opinions Only -


          • #6
            Any DC across the speaker terminal with a speaker load means the coupling capacitor is leaky, (2200uF 80v). or there is no load and it has not charged up yet.
            Use the 560R headphone output resistor to load the amplifier, then take another reading.
            The manual explains the settings. Page 3.
            Last edited by Jon Snell; 07-10-2020, 07:01 PM.
            Support for Fender, Marshall, Mesa, VOX and many more.


            • #7
              Many thanks for the responses.

              Grounding the 560R does indeed bring the voltage down to a few mV, so I'm guessing I don't need to be concerned about connecting up the speakers! Good news.

              Collector of TR24 = 50.8V
              Collector of TR18 = 33.5V so lower than Dave suggests. A bit tricky taking this reading as I understand that the pins of this 2SA818 are EBC rather than ECB. The 33.5V reading is from the pin nearest the DC board i.e. uppermost in the amp and nearest the bottom on the schematic. For reference the other pins read: E=88.8V and B=87.8V.

              The +CO figure referred to on page 3 of the service manual reads about 50.3. The service manual suggest that this should be at 31V +/- 1V. I'm not clear whether this figure should be taken loaded or unloaded. In an unloaded/no input condition adjusting VR2 only brings this down to ~37V minimum. Readings above were all taken with this figure at 50.3V.

              Idling current reading is -0.05mV (should be +5mV +/-1mV).

              Some advice how to take these latter readings (loaded/unloaded) and what the other values from the transistors tell us would be much appreciated.

              Many thanks!
              Last edited by Rhod; 07-11-2020, 05:51 PM.


              • #8
                FWIW, I have a G50 (almost identical circuit) and was chasing biasing issues for a couple of weeks, and it actually came down to leaky caps on the gates of TR14 and 15. Here is the thread about it.


                • #9
                  That's helpful to know, thank you. I had a read through - I agree with you about the awkward nature of the power amp board in the amp, with access pretty tricky. I like the way Yamaha build things generally, but that's not my favourite design choice of theirs!
                  What would be the best way of testing the theory about the caps near TR14 and 15?

                  The audible symptoms on this amp are pretty similar, i.e. horrid distorted noise but unlike yours mine doesn't play nice at all, not even for a short while. The +CO voltage (amp side of output cap) seems pretty stable if higher than spec.


                  • #10
                    Just had a look in my parts drawer. Looks like I've got a pack of 1uF 450V caps - is it worth me swapping those ones out just to see what happens?


                    • #11
                      Originally posted by Rhod View Post
                      Just had a look in my parts drawer. Looks like I've got a pack of 1uF 450V caps - is it worth me swapping those ones out just to see what happens?
                      Nothing wrong with replacing 35+ year old electrolytic caps in my book.


                      • #12
                        Before any intervention on the Yamaha G100 B212 check the amp on the other speaker box if there is a distorted sound.

                        Clean possible oxide on fuse holders and check for cold solder joint at points 1 - 6 on power PCB (NA02919) and DC PCB (NA02918)

                        Before any power stage adjustment, replace the speaker capacitor 2200uF / 80V between point 5 (YE wire on power PCB NA02919) and point 1 (WH wire on speaker connector)

                        If the DC voltage measured between point 5 and point 2 (power PCB NA02919) is half the supply voltage measured between point 6 and point 2 (power PCB NA02919) there is a high probability that the power stage is correct, it is only necessary to bring it to the correct operating mode. (see main amp adjustmens page 3)
                        Who does not know and knows that he does not know - teach him Confucius)
                        Who knows and does not know that he knows - wake him Confucius)


                        • #13
                          Thank you, both.
                          Glebert - fair point. Have replaced those caps. Time invested although with no immediate benefit!

                          vintagekiki - No alternative speaker but I have checked the original drivers with another amp and all is well with the speakers. Fuse holders nice and clean. I did check those points 1-6 on both boards and all looked and tested OK, but now reflowed to make sure.

                          DC pt 5 - pt 2 = 50.2V
                          DC pt 6 - pt 2 = 92.8V

                          So the voltage between pts 5 & 2 voltage is 3.8V more than half the supply voltage. Is that close enough?


                          • #14
                            Oh, and the 2200uF/80V coupling cap has been replaced. The amp came with the original and the filter capacitor removed but with the originals and a pair of replacements (both 2200uF/100V) provided loose. The replacements are newer but used so I guess we don’t know for sure if they’re good, but the symptoms are the same with either cap fitted.


                            • #15
                              Hi folks,
                              I've reviewed the posts so far on this and summarised the outstanding questions below. Maybe the subject line needs changing or a new thread needs starting as I started with a specific question i.e. DC offset but now am seeking support for the repair. Anyway:

                              1. What's good:
                              a) Amp powers up fine, lamp limiter on low glow.
                              b) Speakers have been tested with another amp and are known to be good
                              c) Both Preamp sections seem to be fine, putting out clean audio with all volume, tone, distortion, reverb controls affecting the sound.

                              2. What's not good:
                              a) Amp puts out a horrid low volume distorted sound, all inputs.
                              b) The CO+ terminal voltage referred to on page 3 of the service manual reads 50.3V. Should be at 31V +/- 1V. In an unloaded/no input condition adjusting trimpot VR2 brings this down to ~37V minimum.
                              c) Idling current reading is -0.05mV (should be +5mV +/-1mV).

                              3. What have I done?
                              a) I have tested the TO3 power transistors out of circuit - they test good.
                              b) I have replaced the filter and output coupling capacitors with newer than original but second hand spares that were supplied loose with the amp. Reinstalling the originals makes no difference to the sound.
                              c) I have also replaced a couple of the electro caps in the power amp (near TR14 and 15) as this was an issue identified in another thread - no improvement.
                              d) I have reflowed solder on the male and female connectors between the DC board and the Main Amp board, and cleaned up the fuse holders.

                              4. What else do I know?

                              Some readings taken on the advice of some of the folks here:

                              a) Voltage readings:
                              DC pt 5 - pt 2 = 50.2V
                              DC pt 6 - pt 2 = 92.8V

                              b) Reading from Transistors on power amp:
                              Collector of TR24 = 50.8V
                              Collector of TR18 = 33.5V

                              5. Questions!

                              a) Should the terminal voltage (at CO+) be taken loaded or unloaded?
                              b) vintagekiki suggested that if the lower value at 4 a) is half of the higher one then the power stage should be OK and we just need to adjust. Are the values of 92.8 and 50.2 close enough to this ratio?
                              c) Dave H suggested that the voltage at the collector of TR18 (reads 33.5V) should be slightly higher than than on TR24 (50.8V). What does this lower voltage on TR18 tell us?
                              d) If this was your amp and you wanted to try and get it working, what would you do next?!

                              Please let me know if it would be worth me starting a new thread as we've moved away from the original query in the subject line! Thank you!


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