B2 amp - can't set bias

Discussion in 'Yamaha' started by endeeinn, Mar 11, 2018.

  1. endeeinn

    endeeinn New Member

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    One channel of my B2 works and adjusts correctly while I cannot adjust the bias current or DC balance on the other channel. The bias just reads 0.000 and won't adjust. The power supply provides proper + / - 85v and I've removed and checked every transistor on the drive board with an Atlas DCA55 checker and all are good. I've also checked all the diodes as well. A visual of all the resistors don't show anything burned out and have checked them in-circuit for open and all seem OK. I also removed the VFET's and checked them per the method described by Ecowars. Have not checked all the caps on the drive board as of yet as I thought I might get some opinions before I went any further.
     

     

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  2. rottalpha

    rottalpha Yamaholic Subscriber

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    Replace the trimmers
     
  3. endeeinn

    endeeinn New Member

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    I've removed the trimmers (10 turn units) and they measure correctly and measured across the wiper and resistance changes accordingly. I've ordered new trimmers and will try them but I don't think that's it.
     
  4. rottalpha

    rottalpha Yamaholic Subscriber

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    nevermind, I thought you still had the original trimmers.
    so you have a problem in the circuit. Start checking resistors.
     
  5. Oilmaster

    Oilmaster Drillers do it deeper Subscriber

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    Both DC and bias adjustment showing dead makes me wonder if any electron juice is present in the first place.... even with one more faulty parts some tension will be present across the DC/bias terminals.

    So.... did you confirm that there is incoming power at the +/-B and +/-B2 rails ? The latter coming in via de bridging PCB between VFET PCB and amp PCB (is that print well inserted, and do all terminals and pins have no solder fractures?) You can simply measure the B2 rails by putting the unit upside down, remove the bottom plate and put a DVM on the solder isles of the pin connectors.
     
  6. endeeinn

    endeeinn New Member

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    I'm out of town for a few days and will check when I get back and report results.
     

     

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  7. endeeinn

    endeeinn New Member

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    The incoming power is to drive board is +85.4 and -85. Checked the PCB that connects the drive board to the VFET board and it provides good continuity, all solder connections are good as well as the pins. Checked voltages and get the following B+ = 58, B- = -58, next pin is unmarked 0 v, NG = -17.5, NS=0, NF=0, PG = +12, PS=0. B+ & - should be +/- 56v so not too bad but NG should be -13.5v and PG should be +13.5v so I guess that means that there is still something causing it on the drive board?
     
  8. Oilmaster

    Oilmaster Drillers do it deeper Subscriber

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    Hmm, really strange case, more measurements needed.

    The fact that NS-NF-PS are all zero means that:
    - either the VFETs remain switched off (no bias current flowing through source resistors R701/702),
    - or the NS-PS rails are ground connected (due to some faulty component)

    Can you measure the voltage directly across R701 and R702 please ?

    The fact that NG is at -17.5 is also not ok; something is going on in the associated driver circuit.
    Can you measure the red-coloured voltage levels around all driver & bias transistors (TR117~TR124) as indicated in the schematic and the voltages across D110 and D111?
     
  9. endeeinn

    endeeinn New Member

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    I measured across R701 & R702 and they are both 0 v DC.

    As far as the rest of the voltages, I'm not sure how to get them with the drive PCB in place. Can I disconnect the drive board from the bottom PCB strap and remove it from amp and still get meaningful readings? Once out of the amp I could place the drive board in a PCB holder and run the +-85v to it and take measurements that way. I assume that at least the input ground has to be connected to the drive board as well?

    Thanks for help and guidance.!
     
  10. Oilmaster

    Oilmaster Drillers do it deeper Subscriber

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    Eh no... don't take out the driver boards without the bottom PCB to the VFET board. The B power rails come in via the VFET board, amongst others.
    See if you can take/lift the entire "block" partially out and power it up without making a shortcut anywhere between the block and chassis.

    Since this can be tricky, don't follow my suggestion if you are uncomfortable with it, and send the amp to recognised AKer instead.
    I don't want to be blamed for leading you to a potential catastrophic event.
     
  11. endeeinn

    endeeinn New Member

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    Don't worry I can't blame you for anything that I do. That said it is going to be difficult to get voltages but I didn't think of pulling the whole block. I've pulled the block without the drive board attached to check the VFETS but so I don't see why it isn't feasible to do the same with the PCB attached. I'll try it with the B2 unplugged to see what has to be protected from short circuits and go from there.

    You didn't comment on the 0v DC measurement across R701 & R702. I hope that the 0 voltage is a result of a bad drive board and not fried VFET's that I didn't pick up with Ecowars' VFET check method.

    Again thanks for your help!
     

     

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  12. clinic-audio

    clinic-audio all on YAMAHA untill 1990

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    Did you check value of R701 & R702 ?
     
  13. endeeinn

    endeeinn New Member

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    Yes, both R701 & R702 are .22ohm on the nose with the caveat that I checked both without removing them from the circuit.
     
  14. clinic-audio

    clinic-audio all on YAMAHA untill 1990

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    One question :

    did you compare DC V on top and bottom contacts on the heatsinks ?

    DSC08185-RR.JPG DSC08186-RR.JPG
     
  15. endeeinn

    endeeinn New Member

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    I haven't had much time to work on this between business trips and other things. One thing I just noticed after staring at this thing for a while is that in the process of changing VR102 the single solder pad that leads nowhere is gone. When you look at the drive PCB the pad does appear to not be connected to anything, however, when you look at the schematic one end of VR102 is attached to R153 and the VR102 wiper is also attached to R153. The other end of VR102 connects to C121. I can trace this connection on the PCB except for the where the wiper is attached to the end of VR102 that goes to R153. Can this be it? What am I missing?

    VR102 PCB.jpg VR102 SCHEMATIC.jpg
    [​IMG] [​IMG]
     
  16. Oilmaster

    Oilmaster Drillers do it deeper Subscriber

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    That is one of the many mistakes (differences) in the service manual vs PCBs in reality.
    Nothing to worry about, in reality one side of the pot's end is indeed "floating"
     

     

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  17. endeeinn

    endeeinn New Member

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    Thanks Oilmaster! I thought that might be the case but wasn't sure. I've obtained replacement transistors and diodes with subs as found elsewhere in AK and have begun replacing them and also have replaced VR101 and VR102 with burns multi-turn. Next I'll go through all the passives.
     
  18. endeeinn

    endeeinn New Member

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    Well after dealing with other issues I finally got back to working on the B2 and finished changing all the transistors and diodes on the drive board including new multi-turn pots. After all that work I decided I'd better double check everything else I did and pulled the VFET's to recheck them and all was good until the last 2SJ26 when I figure I must have made a rookie mistake when I checked them before. The source to the drain is supposed to be about 1 ohm or so and it first looked like it was right on until I noticed that my auto-ranging fluke meter said 1.025 k. When I checked them previously i must has missed the k and figured it was around 1 ohm. So now, I'm sorry to say, is that all I can do is start looking for a partially dead/derelict B2 to salvage a VFET from if that's even possible but the good news is that I have a good mono amp and 3 good VFETS. Thanks to all who have contributed help and ideas. John
     
  19. rottalpha

    rottalpha Yamaholic Subscriber

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    tears
     
  20. clinic-audio

    clinic-audio all on YAMAHA untill 1990

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    but the good news is that I have a good mono amp and 3 good VFETS

    Do you mean that you have :
    - one working chanel ?
    - 3 working VFET ?
    -one VFET missing to get the other chanel working ?
     

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