Marantz PM350

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

  1. Poskok2010

    Poskok2010 New Member

    Messages:
    27
    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.
     

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  2. mbz

    mbz AK Subscriber Subscriber

    Messages:
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    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:
    27
    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 AK Subscriber Subscriber

    Messages:
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    Location:
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    "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:
    27
    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 AK Subscriber Subscriber

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    Location:
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    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

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  8. mbz

    mbz AK Subscriber Subscriber

    Messages:
    1,103
    Location:
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    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:
    27
    Thank you very much on your help i will try my best this weekend.
     
  10. Poskok2010

    Poskok2010 New Member

    Messages:
    27
    Hi mbz,

    i did what you told me and as i understand i check the bases of every transistor, and on a base of QN05 voltage is higher than in manual (4,5 V normal and 6,6 measured) and the signal is not a flat line like on everything else it is like on picture attached. Is he a problem?
     

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  11. mbz

    mbz AK Subscriber Subscriber

    Messages:
    1,103
    Location:
    Melbourne, Victoria
    QN05 is in the power supply, the CRO waveform looks like rectified ac ripple, looking at the schematic, no regulation so waveform is considered normal.

    Since both channels are affected by noise, the "rule book" says to look at what's common, ie, the power supply. However since you have a CRO, I'd be
    tempted to test the theory. If you have a signal generator (or audacity pc software), you can feed the amp a low level 1KHz sine wave then using the cro
    test the pre and power amp stages for noise. Start with the speaker posts (low volume) and then work back. Some other test points being,
    - R701, 702
    - Q725,7base
     
  12. Poskok2010

    Poskok2010 New Member

    Messages:
    27
    MBZ you where great help to me , with your inputs i finally understand some of OCR possibilities, and i finally manage to find based on you inputs the noise source. It was just switch, i opened and clened every switch and potentiometer in this thing but i missed this one, because he is completly sealed and i did not think that he can be a problem, as you can see on pictures the contacts are pretty messy.

    Anyway the problem is solved, the amp sound normal, but i f..k up something, the trebele works fine but mid and bass potentiometer doesnt react, there is not change in sound when i move them?I removed them from the board and they are all right. What did i burned ( I changed both opamps 4558 with new OPA2604 )


    switch.jpg Switch1.jpg
     
  13. mbz

    mbz AK Subscriber Subscriber

    Messages:
    1,103
    Location:
    Melbourne, Victoria
    If the only problem is with the MID and BASE controls, amp plays OK otherwise, then maybe these sliders are dirty.
    You can kind of check the MID by, power off/unplug amp. Measure resistance RE19 (either side) and RE13 (either side).
    Move the MID slider, the resistance should move up and down, will need 200Kohms scale. It should vary from maybe
    20K (maybe 0) to 100-120K. You can do the same between RE19 and RE11, either side. Looking at the artwork, you can
    access solder blobs from behind rather than disassembly.

    pm350b.JPG
     

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