Marantz PM350

Discussion in 'Marantz Audio' started by Poskok2010, Sep 11, 2017.

  1. Poskok2010

    Poskok2010 New Member

    Messages:
    22
    My friend give me this one for try to repair it. There is low and very distorted sound on both channels.

    I started to check voltages on the board, in middle of checking i've noticed that schematic what can be found on the net doesnt fit in this part: resistors R723 and 724 (2,2K) are not in this amplifier, they are changed with something that look like thermal switch or something like that, on both chanels there is this green part with sign 17A on it, which are under the heatsink, on picture i pulled the left one out.

    Does anybody know what is that?

    I try to google it but google doesnt know what is it.The resistance is 3,5MOhm.
     

    Attached Files:

  2. mbz

    mbz Well-Known Member

    Messages:
    796
    Location:
    Melbourne, Victoria
    Probably a diode or several diodes mounted on the heatsink so it's voltage drop tacks that of the bias transistor Q709.
    Suggest that you put your multimeter on the diode function then measure the voltage drop, expect about 0.7V. If it
    reads open circuit then change the polarity of the MM probes. If the volt drop is 1.3V then it's a 2 diode package. Have
    seen upto 4 diode in such packages.

    As with all diodes, needs to read OK in one direction.

    With the symptoms you've described suggest you give all pots/switches a thorough clean.
    Since both channels affect the problem may be in the power supply. Looks like the preamp
    uses an OPAMP (QE01, QE02). Probably the power supply to these OPAMPS is faulty so
    both channels would be affected. Check power +/-17V supply.
     
  3. Poskok2010

    Poskok2010 New Member

    Messages:
    22
    yea ok, but why the schematic show the resistor on that place, and also all the inside pictures what i can find on the net of this amp doesnt show this diode pack it shows resistor? So was this amp somebodys playground or this is factory thing?

    checked voltage of psu and everything is ok,i even have 11mV bias on q731 and q732, cleaned pots and all switches through.

    the voltage drop is 1,11 v
     
    Last edited: Sep 11, 2017
  4. mbz

    mbz Well-Known Member

    Messages:
    796
    Location:
    Melbourne, Victoria
    "Improvements" during production are quite common, too expensive to update the service manual, which probably includes the phrase
    "Subject to change...". In any case the use of the diode over the resistor is seen as an improvement. Service manuals have errors as do
    the screen prints. Components (caps, resistor, tr) often differ, get used to it, work off what is infront of you if it looks original.

    Good sign that the amp bias up ok and psu voltages look ok.

    I assume you don't have an oscilloscope, simplifies tracking down noise source.

    More likely to be one fault than two, since both channels affected need to look at what's common, ie, psu and opamps in preamp.
    Important that you measure opamp voltages pins 4&8, measure at adjoining resistors R415, R416, +/-16Vdc

    You could try breaking the audio path between pre and power stages by lifting one leg of R701 (left) and check for noise. Would
    indicate problem in power stage. Repeat for R702 (right channel)
     
  5. Poskok2010

    Poskok2010 New Member

    Messages:
    22
    thank you very much on your help. i will do that.


    i bought osciloscope month ago but still did not find the time to learn how to use it.:)
     
  6. mbz

    mbz Well-Known Member

    Messages:
    796
    Location:
    Melbourne, Victoria
    Suggest you use the oscilloscope rather than previos suggest. It will be much quicker.
    What make/model number is it?
     
  7. Poskok2010

    Poskok2010 New Member

    Messages:
    22
  8. mbz

    mbz Well-Known Member

    Messages:
    796
    Location:
    Melbourne, Victoria
    Oscilloscopes are esentially very fast volt meters, with a few bells and
    whistles. It's very easy to get started and just do the basics.
    - Probe with earth (aligator?) clip goes in X.
    Connect earth clip to chassis/GND of item under test
    - Probe without earth clip goes into Y

    Key areas that you need to become familiar
    Bottom of screen CH1=... CH2=... M... etc...
    These are the current settings, the switches don't have settings they just
    cause step up/down.

    The controls in the highlighted yellow box is what you will mostly use
    - Intensity, keep low/minimum
    - Vertical, channel 1 and/or 2, same as voltage scale on your MM
    - [1] [2] to select channel 1 or 2
    - Up/down adjustment below [1] and [2]
    - Horizontal, this is the time per division (each small 1cm? square)
    - Position below Horizontal for left/right adjustment

    Try the following,
    - connect probes, also earth clip
    - power ON, adjust intensity
    - select channel 1 by [1]
    - Adjust CH1 vertical so it reads 2 or 5 or 10 volts, displayed bottom left of screen
    - Adjust horizontal so it reads 500uS as in the pic
    - connect X probe to metal tab with 1KHz square wave symbol (bottom right corner)

    You should get a square wave like what's displayed in the pic. You may need to
    adjust the CH1 vertical (voltage scale)

    Same basic method applies for hunting down the noise/distortion. Probably use CH1=100mV
    and the horizontal set to 500uS. Test points should be the base of transistors and preamp
    out stage. Prefer to measure at adjoining risistor rather that actual transistors, 1 slip...

    cro.JPG
     
  9. Poskok2010

    Poskok2010 New Member

    Messages:
    22
    Thank you very much on your help i will try my best this weekend.
     

Share This Page