170dc !!

Discussion in 'Marantz Audio' started by The Fuxtor, May 17, 2017.

  1. The Fuxtor

    The Fuxtor AK Subscriber Subscriber

    Calgary, Alberta
    Hey guys, new to the Marantz forum.
    I am picking myself up a 170dc amp today and yesterday I noticed that in the SM there is no biasing procedure?? Did I just miss it ???
    Also it will need new lamps, wondering what new lamp options tend to look the best?? LEDs are most likely what I will get but what color/shade works best??


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  2. catrafter

    catrafter Marantz Specialist Subscriber

    The bias on the 170DC is not adjustable.

  3. KutzlerTrans

    KutzlerTrans Jennie & I with Buddy and Barney. Subscriber

    I like the blue led fuse style units. Make sure to use a good defuser paper.

    DC offset your only adjustment. I believe you can check your bias across 753/4/5/6 emitter resistors. your looking for 40 - 50 ma across the resistors. Dividing the voltage by the resistance should give you the current.
    If i am not mistaken, if everything is correct in the amp, there is a way to set r719 to +/- 0.6vdc
    which should set your driver (q713/714)base voltage @ -/+0.6vdc
    R732/733 are iirc diodes with resistors in parallel.
    As Tom said, the circuit sets the bias, but it would be nice to check it before you experience thermo runaway.
    According to Dr*Audio, and i quote
    ".R732 and R733 are used to adjust the bias. They must both be changed at the same time, using the same value. Raise the value to lower the bias current. Small changes in value make a large difference in the bias current. You will probably need to use 1% resistors to get it close. Order some values around 8.2K, above and below, to have them on hand. Remember you will need 4 of each value."
    This info is an accumulation of wisdom i had gathered from postings by Tom, (cat), Dr*Audio and Ron(jblmar) back when i worked on my 300dc.

    The Driver heat sinks on these units get hot as hell. That appears to be normal operation. I am thinking 130 deg or so.
    Last edited: May 17, 2017
  4. The Fuxtor

    The Fuxtor AK Subscriber Subscriber

    Calgary, Alberta
    40-50 ma? Just making sure! I've had mix ups with millaamps and mV..
    Thanks for the info! I plan on recapping eventually as well. Anyone have advice on possibly upping the voltages say at the power supply and maybe upping the capacitance in the coupler??? No known issues in this guy???

    As far as bias goes, has a anybody put in 10k trim pots in place of R732-33? Would make life easier down the road???
    Last edited: May 18, 2017

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