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CA-800 Bias Problems After DOA Rebuild

Discussion in 'Yamaha' started by ekimezeerf, Jun 9, 2018.

  1. ekimezeerf

    ekimezeerf New Member

    Messages:
    9
    Location:
    Michigan
    Hey everyone, long time lurker, first time posting.

    A friend brought me a dead CA-800, would not power on and many signs of heat stress on main amp and power boards. Our thought was the PO has dumped a bottle of cologne on it due to it's aroma and the goop present everywhere inside.

    I've worked on the main cap board, power supply, main amp boards, and mode switch - deep cleaned, changed all electrolytic capacitors, signal + power transistors, fuse resistors, micro switches, trim pots, diodes...

    I've cleaned every switch, every pot, pre out- main switch, checked all grounds, re-soldered cracked joints on rear panel, re-worked all wires with signs of fatigue, checked all resistors I thought relevant...

    I've read almost every post on AK that even mentions a CA-800 or CA-1000 and learned so much more than I ever wanted to know about this amp. But, it came alive! So thank you to everyone who ever took the time to leave their experience and knowledge here. I'm definitely standing on your shoulders.

    Now, here's where I'm completely stuck. It's sounds terrible at any volume past ~ 10-15%. Distortion in the bass is most obvious at higher volumes but the treble is no better. When attempting to adjust the main amp boards (specifically the Class A/B Bias) in class B mode the lowest I can set the bias is 0.085V on both boards (but as high as 0.120V)

    In Class A mode, the highest I can set the bias is 0.130V (and as low as 0.080V)

    Differential voltage is right at 15V and the Mid-Point Potential is right at 0.0V. The +/- 50V supply is there and in Class B Mode I have +/- 45v and in Class A Mode i have +/- 20v (which is too high right?)

    The Bias problem is the same on both boards which made me think there was a problem with the power supply or maybe the mode switch. I've exhausted myself going over both, even took the mode switch completely apart and cleaned the rails and contacts inside.

    I have many more details and voltages to report if someone is interested in giving my a hand up. I'll include a few photos here of the before/after type. Maybe I've made a simple mistake, overlooked something, or maybe its a dying transformer...any help would be great!
     
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  2. avionic

    avionic " Black Knights " Subscriber

    Messages:
    43,015
    Location:
    Fort Dodge, Iowa
    Hmm..On the main amp boards there are 4 terminals on the top edge. A,B,C & S. Make sure they are all going to there proper destination on the mode switch board ( class A switching circuit). For right now don't even mess with class A bias adjustment trimmer until class AB is working properly. Set all 4 bias adjustment trimmers to there min. ie fully CCW I believe.
     
    Last edited: Jun 9, 2018
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  3. avionic

    avionic " Black Knights " Subscriber

    Messages:
    43,015
    Location:
    Fort Dodge, Iowa
    On the A,B,C and S terminals you can verify the proper switching of the mode switch by ohming out the wiring and switch in both Class A and Normal switch position.
     
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  4. avionic

    avionic " Black Knights " Subscriber

    Messages:
    43,015
    Location:
    Fort Dodge, Iowa
    Verify that you have the correct value trimmers installed.
     
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  5. ekimezeerf

    ekimezeerf New Member

    Messages:
    9
    Location:
    Michigan
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  6. ekimezeerf

    ekimezeerf New Member

    Messages:
    9
    Location:
    Michigan
    For testing the Ohms verifying the switch is functional, in NORMAL mode I should have something like this:
    Normal:
    B-C <1 ohm
    S-C ~470 ohm
    A-C >5k ohm

    Class A:
    A-C <1 ohm
    S-C <1 ohm
    B-C >5 kohm

    Or do I have that reversed?
     
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  7. avionic

    avionic " Black Knights " Subscriber

    Messages:
    43,015
    Location:
    Fort Dodge, Iowa
    Correct
     
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  8. avionic

    avionic " Black Knights " Subscriber

    Messages:
    43,015
    Location:
    Fort Dodge, Iowa
    Correct..
     
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  9. mbz

    mbz AK Subscriber Subscriber

    Messages:
    2,101
    Location:
    Melbourne, Victoria
    Thinking maybe the >5kohm are a little high, trimmers should be set to minimum, measuring path between pins A and B, expect <5k
    From another thread
    Mode------Pins--------L------------R-------
    NORMAL............BC.....0.4 ohms.....0.4 ohms
    ..................BS.....469 ohms....472 ohms
    ..................CS.....469 ohms....471 ohms
    ..................AB....1.52k ohms...2.38k ohms
    ..................AC....1.52k ohms...2.38k ohms
    ..................AS....1.99k ohms...2.85k ohms

    Class A...........BC.....1.52k ohms...2.38k ohms
    ..................BS.....1.52k ohms...2.38k ohms
    ..................CS.....0.3 ohms......0.2 ohms
    ..................AB....1.52k ohms....2.38k ohms
    ..................AC....0.3 ohms.......0.3k ohms
    ..................AS....0.3 ohms.......0.3 ohms

    TR604 been replaced?
     
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  10. ekimezeerf

    ekimezeerf New Member

    Messages:
    9
    Location:
    Michigan
    Here's the readings I got this weekend. I'll have to go back tonight and follow mbz's list of readings as they are more comprehensive:

    Mode------Pins------L-------R--------
    Normal---BC----0.9ohms--0.9ohms
    SC----480ohms--484ohms
    AC----8.06K ohms---8.1K ohms


    Class A
    AC----0.9ohms---0.9ohms
    SC----0.9ohms---1.3ohms
    BC----8.6K ohms---8.1k ohms


    Seems like the 8.6-8.1K ohms readings are pretty high. I'll have to double check that the trimmers were set to their minimum (which I swear they were) and follow the rest of the above listed measurements.

    As for TR604, Yes it was replaced with KSC2690 AYS
     
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  11. avionic

    avionic " Black Knights " Subscriber

    Messages:
    43,015
    Location:
    Fort Dodge, Iowa
    Lets see a close up of TR604 and TR605. Did you replace the TR605's?
     
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  12. avionic

    avionic " Black Knights " Subscriber

    Messages:
    43,015
    Location:
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    DSC04192.JPG You do know that those SONY TO-202 transistors ( EBC) are a different pin out than the KSC2690AYS (ECB)

    .
     
    Last edited: Jun 11, 2018
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  13. mbz

    mbz AK Subscriber Subscriber

    Messages:
    2,101
    Location:
    Melbourne, Victoria
    I don't see a problem with the A-C/B-C measurements. It just means the VRs were not at minimum.
    My concern was that one of the VRs had become intermittent/open, clearly new VRs installed.
    Need to double check any transistor replaced. TR605 on the heat sink catches some people out.
    You should use KSC1815 or KSC1845, pinout ecb, pin closest to the VRs (top pin) is base. Pix
    suggests correct orientation.
     
  14. ekimezeerf

    ekimezeerf New Member

    Messages:
    9
    Location:
    Michigan
    I did note the pin-out difference and installed them accordingly. And TR605 was replaced with a KSC1815. It was turned 180 degrees from original transistor orientation, so the pin-out should be correct. It may be difficult to see in my photo (because of the heat shrink I used) but TR604 and TR608 are installed just like the photo above.
    IMG_20180611_063720537.jpg
     
  15. ekimezeerf

    ekimezeerf New Member

    Messages:
    9
    Location:
    Michigan
    I also wanted to note that on the Right Amp board, when the unit is powered up in Class B mode, There is definite ringing (oscillation?) noise coming from the bi-polar 47uF capacitor that sits between TR604 and TR601.

    If I touch it with an insulated screw driver, the bias drops 1-5 mv and the ringing gets much quieter. :dunno:
     
  16. mbz

    mbz AK Subscriber Subscriber

    Messages:
    2,101
    Location:
    Melbourne, Victoria
    Maybe D607, R626, R627 out of spec? But both channels affected...
    D603,4 failed short?
    b/c/e voltages to GND for TR604,5,8,9,610,611 may be needed (or whatever
    you have already taken). Please don't probe between transistor legs.
    The 47uf cap...
     

     

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  17. mbz

    mbz AK Subscriber Subscriber

    Messages:
    2,101
    Location:
    Melbourne, Victoria
    Did the 2 fuses on the main amp board blow, they look new, would point to outputs/drivers/... maybe going intermittent
     
  18. ekimezeerf

    ekimezeerf New Member

    Messages:
    9
    Location:
    Michigan
    Yeah...because both boards are affected/behaving the same way I'm really lost.

    I'll go back and check D603,4 and start a list of the voltages for the transistors. (may take me a few days to gather and post).

    As for the fuses, F602 on the Left Amp Board was blown when I first got the unit. I don't recall if the same was true for the Right Amp Board

    TR610,11 were replaced with MJ21196 and MJ21195 (both sets). The original TR611 (2SA663) on one side was definitely shorted and failed.
     
  19. ekimezeerf

    ekimezeerf New Member

    Messages:
    9
    Location:
    Michigan
    Just checked some photos of the rebuilt amp boards...

    R626 & R627 are supposed to be 4.7 ohm according to my schematic. Can I get a sanity check on that?

    From my photos I can clearly see that I installed 4.7k ohm resistors in both location on both boards :no:

    Time to go back and check all values of all the things I've replaced...sigh
     
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  20. avionic

    avionic " Black Knights " Subscriber

    Messages:
    43,015
    Location:
    Fort Dodge, Iowa
    Explains why it doesn't bias. My board here has 4.7Ω 1 watt.
     

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