Marantz 8B broken bias meter

Discussion in 'Marantz Audio' started by kwingylee, Aug 19, 2017.

  1. kwingylee

    kwingylee Active Member

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    261
    Location:
    Chicago, IL
    Guys:
    I have a Marantz 8B amplifier with a inoperable bias meter. Is this something that is repairable? Or is praying for one to appear eBay my only option?

    Otherwise the amp has been rebuilt and sounds great.

    Appreciate anyone's input.
     
  2. kcin

    kcin Well-Known Member

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    Location:
    Canada
    The odds of you personally repairing is slim. Depending on what is wrong you may or may not have luck with it. You should be able to hook it up with the appropriate load ( I don't know the FS delflection) and determine whether it is really the meter or the switch.

    Try contacting VAC amps in Florida. They built the reissue and had identical meters made and may have spares or guide you. Otherwise you can- and I have thought of this a bit - surgically remove the movement from a chinese meter on ebay into the casing of the marantz and make it work.

    Or EchoWars on this site posted a successful meter repair here:

    http://audiokarma.org/forums/index.php?threads/analog-meter-repair.730325/

    Let us know how you make out. I have a lazy meter on a Model 2 but I carefully cleaned it and it works well enough now.
     
  3. Pio1980

    Pio1980 AK Member Subscriber

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    I think these meters were made by Readrite, and are probably simple iron vane type with a stationary bobbin coil.
     
  4. kwingylee

    kwingylee Active Member

    Messages:
    261
    Location:
    Chicago, IL
    Thank you for the pointers! I will give it a shot.
     
  5. kwingylee

    kwingylee Active Member

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    261
    Location:
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    I took some pictures of the meter. Compare this to a repro VAC unit, the original has 2 bolts/studs in the back for mounting
     

    Attached Files:

  6. kwingylee

    kwingylee Active Member

    Messages:
    261
    Location:
    Chicago, IL
    Does anyone know the full scale sensitivity and the current at the bias point of the meter is? I contacted the instrument meter specialist company alluded to by echo wars and they suggested a rip and replace using a donor Simpson 125 meter. They requested for this information....
     
  7. Pio1980

    Pio1980 AK Member Subscriber

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    I checked the service manual, and while all test info was ref'ed to the meter, there was nothing specific on the meter itself, or what the readings corresponded to current-wise.
     
  8. kwingylee

    kwingylee Active Member

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    Location:
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    Thanks for trying ......
     
  9. Pio1980

    Pio1980 AK Member Subscriber

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    I might recommend careful disassembly and inspection by a specialist, it should be a fairly simple design.
     
  10. kwingylee

    kwingylee Active Member

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    261
    Location:
    Chicago, IL
    Trying, I am looking at some use Amico meters on ebay to possibly do a swap myself. Anyone know the best way to take these types of meters apart without destroying it?
     
  11. Pio1980

    Pio1980 AK Member Subscriber

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    22,368
    Location:
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    Looks to be be crimped at the bezel rim, so would have to be carefully uncrimped, cut, or ground off first. Then I think the innards are unbolted at the rear.
    Surely someone here has been inside one of these.
    Let's all try a rigorous image web search for some help, different searches by different folks can get differing productive results.
     
    Last edited: Sep 12, 2017
  12. Pio1980

    Pio1980 AK Member Subscriber

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    Location:
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    Last edited: Sep 12, 2017
  13. kwingylee

    kwingylee Active Member

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    261
    Location:
    Chicago, IL
    There is a vac meter on eBay but I don't know how to fit that on the 8b as its missing 2 bolts for the mounting bracket. I would buy that if I know it will work
     
  14. Pio1980

    Pio1980 AK Member Subscriber

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    22,368
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    Can't find it on e-Bay.
    I had a pic of the underside of the VAC amp that apparently got vaporized in a bulk file transfer, I think the meter in the reissue is secured with a 'U' bracket with holes and insulated grommets on the terminals. You'd need the bracket and grommets to mount it, or something like.
    Here's a top pic of the VAC amp. $_3 (7).JPG
     
  15. kwingylee

    kwingylee Active Member

    Messages:
    261
    Location:
    Chicago, IL
  16. Pio1980

    Pio1980 AK Member Subscriber

    Messages:
    22,368
    Location:
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    The reissue uses the terminals with a bracket with insulating grommets to secure it to the chassis. This meter doesn't have them with it.
    The bias point is the line, the dot is reference level for the balancing procedure in the service manual, and I don't know the recommended idle current for the output tubes for the 8B. Probably same as the 8.
     
  17. kcin

    kcin Well-Known Member

    Messages:
    544
    Location:
    Canada
    Most meters are designed to be FS deflection of 50uA or 1mA with few exceptions and there are no shunts on this.

    I've got to look at the Marantz schematic I believe the reading is calibrated in DC volts across a resistor for bias.

    Hook up a digital multimeter set to its lowest current range in series with the meter and a power supply. Measure the current through the voltmeter. If the DMM over-ranges, change its setting.

    Adjust the supply so the needle reads on some identifiable l calibration point- say 1/2 way you don't need full-scale. Divide the current reading on the DMM by the deflection factor (50%. For example, if you get 10 microamps at 50% deflection, then FS = 10 / 0.5 = 20 microamps.

    Thats the way to do it.


    Parts connection has small meters that deflect to 100uA for about $20 that mechanisim could be transplanted after testing

    The 8B has a 6.8 ohm cathode resistor and .68v @ the cathode so 100mA/ pair or 50mA ea.
     
    Last edited: Sep 19, 2017

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