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Marantz 2015 Receiver Distortion

Discussion in 'Troubleshooting and Repair' started by KilgoreCemetery, Sep 4, 2018.

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  1. KilgoreCemetery

    KilgoreCemetery

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    Apr 12, 2017
    Everything on this receiver seems to work fine except when you hook up a load to the speaker connects. I just finished recapping and replacing a couple of bjt's in the FM section and finally hooked it up to some speakers so I could start enjoying it, but the music sounds horrible. It's not just FM either. Phono, AM, Aux.. doesn't matter. It's all garbled. I could have swore that I listened to it before doing all that work and it sounded fine, so I put it back up on the bench and plugged in a pair of headphones... and it sounds very clear.

    After doing some testing, I realized I can turn the distortion on and off in the headphones by pushing the button that enables the speakers. It doesn't matter if it's a real pair of speakers or a pair of dummy resistors, nor does it matter if it's "A Speakers" or "B Speakers". I finally circumvented the speaker buttons and jumpered some wires from a small speaker directly to the output on the main amp circuit board and it distorted the sound in that channel. As soon as I pulled the jumpers back off, the audio cleared up in the headphones.

    It doesn't go the other way around either; disconnecting the headphones has no affect on the quality of the sound coming from the speakers. It seems like the distortion is only present when there is a load. I'm using a pair of Pioneer CS-63DX speakers (8 ohms) or a pair of 8 ohm dummy load resistors depending on where the receiver is sitting. 8 ohm speakers are what this receiver is rated for.

    From there, I tried to adjust the trim pots according to the service manual, but step 1 under Audio Adjustments doesn't work. I'm supposed to be probing both sides of a resistor and adjusting a variable resistor until I get 7.5mV, but no matter how far I turn it in either direction, I get 0.1mV. This goes for both channels. They are 0.5 Ohm resistors and when I test them, they're spot on. I also sprayed the VR with some Deoxit and checked it for resistance. It goes up and down as I turn the center, just as I would have expected from a working unit.

    I really feel like I'm missing something silly, but I have no idea what. Does anyone have any ideas of things I could try?

    Audio Adjustment.jpg

    Amp Board Schematic.jpg
     
  2. Ylli

    Ylli

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    Jun 19, 2018
    Both channels? Kind of rules out a random component failure.

    When you randomly replaced the caps, are you sure you didn't get any of the polarized caps in backwards?
     
  3. duke37

    duke37

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    Jan 9, 2011
    Measure the voltage at the centre of the output emitter resistors.or the positive of the large output capacitor. Plug in a speaker and see if it changes.
    Does plugging in one speaker affect the other channel, using headphones as a monitor?
     
  4. razi

    razi

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    Sep 4, 2018
    Do you have a osszilliscope? Then connect it with your dummy-resistors and watch the signal. how does it looks like? Is it cropped? May be the power-supply doesn`t work correct ...
    Or messure the resistence of the power-transistor (Disconnect the power-supply before!) between emitter and collector, may be, they are out of work.
     
  5. WHONOES

    WHONOES

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    May 20, 2017
    Sounds like you (or rather your amp) are suffering from crossover distortion whereby the output transistors are not biased on. This could be caused by the thermal compensation diodes at H701 and H 702 having failed (or appear to have failed) short circuit. This would stop you being able to adjust the output stages quiescent current which is necessary to stop the disto'.
    Presumably when you were "servicing" your amp, you had the output stage(s) disconnected from the amplifier pcb. If so, I would suggest that you check the connections for any anomalies such a short or something similar in the region of J606, 607, 619 and 620.
    Also check the voltage at the junction of R639 and R633 then at the junction of R640 and R642. These should be close to 1/2 the supply rail.
     
  6. KilgoreCemetery

    KilgoreCemetery

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    Apr 12, 2017
    Thanks for all the replies! I'll try to go down the list in the proper order.

    Fairly certain. This isn't my first recap (if that means anything), but also, I took pictures prior to recapping and everything looks to be put in the right way.

    Measured from the positive side of the output capacitors, no change

    It only affects the channel with the speaker

    I do have a scope and a signal generator. The signal doesn't look perfect, but it's not cropped unless I turn the receiver volume way up. Somewhere above 3/4 on the volume knob.

    Interesting. I saw the diodes on the schematic, but kind of overlooked them, I suppose. Are they just regular diodes or is there a specific test I need to do for them?

    Yes. They were removed, cleaned and I applied fresh thermal paste to them. I will double check those connection points

    I'm not sure about those exact resistor numbers, but I'm showing J605 as the supply rail and it's measuring 49.9V. R640/R642 are showing about 26V and R639/R641 are showing about 25V
     
  7. WHONOES

    WHONOES

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    May 20, 2017
    Good news that you have got a scope. Connect you scope to J610 and ground or J606 and ground. With a suitable 8Ω resistor fitted to the speaker connector, apply a signal to the input, 1Khz I would suggest, and adjust the output to about 1/2 scale. Hopefully the signal will not be clipping. Now have a look at the waveform 1/2 way up. The transition should be smooth. If there is a kink, then you have crossover problems. You will probably need to adjust your scope time base so that you have just one cycle on the screen. If this is the problem then you have an issue with the output stage biasing.
    The diodes at H701 and H702 are probably straight forward diodes but I don't know that for sure without doing a lot of digging. There is no reason for them to be anything else. You mention that you removed them cleaned them up and applied new thermal grease. Have you inadvertently shorted them to the heatsink?
    The voltages that you measured are in the ball park and are of no concern for the moment.
     
  8. KilgoreCemetery

    KilgoreCemetery

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    Apr 12, 2017
    I'm getting some conflicting information about H701 and H702. They may not be diodes at all. According to the parts list in the service manual they're varistors. Part number SV-3A. In diode mode on my DMM, I get 0.L both directions on both parts. If I switch to resistance mode, I get 1.9M one way and 694k if I reverse the leads. On the other part, I get 3.6M and 934k reversing the leads.

    Haven't found a data sheet for them yet.
     
  9. KilgoreCemetery

    KilgoreCemetery

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    Apr 12, 2017
    There is, in fact, a small kink halfway up.

    I had previously wondered if I did something with the output transistors that could cause this, but as long as there is no load, everything sounds great through headphones. Very clear, no distortion.
     
  10. Ylli

    Ylli

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    Jun 19, 2018
    Two diodes in series - Your DMM may not have enough voltage to turn them on. Measure and post the voltage at J606 and J607 (and/or J620/J119 for the other channel).
     
  11. Ylli

    Ylli

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    Jun 19, 2018
    Last edited: Sep 5, 2018
  12. WHONOES

    WHONOES

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    May 20, 2017
    That small kink is the crossover distortion I was describing.
    The output would sound reasonable using headphones as the load will be quite small.
    Having had a bit of a trawl through some Marantz info in German, I have deduced that the diodes are indeed diodes. Probably something like 1N914's in series.
    You could take a brave pill and wire 2 1N4148's in series with a bit of insulation round them for each module and then very very very carefully observing correct polarity for the diodes, fit them in place of the ones already there. It would be a good idea if you do so to hang a small fuse rated at 100mA or so in the supply lead in series with a small switch (so that you can turn it off quickly if need be) plus a change of underwear. Connect your DMM on its current range in series with switch and fuse. I would expect the current at this point to be in the very low 10's of mA. Set R627 to minimum value and gingerly switch on. If the fuse does not blow (though the board capacitors may cause it to), make a note of the current draw and then slowly adjust R627 until in starts to increase. Stop adjusting and allow the amp to warm up a bit then re-check current draw. Set it so that it is 10mA or so above the original read.
    If that all works out, do the same to the other channel then re-check for crossover disto with your scope.
     
  13. KilgoreCemetery

    KilgoreCemetery

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    Apr 12, 2017
    J606 = 24.15
    J607 = 25.56
    J620 = 25.90
    J619 = 27.29

    This stays the same whether there is a load or not
     
  14. KilgoreCemetery

    KilgoreCemetery

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    Apr 12, 2017
    Hmm... I don't know if I have any 1N4148's on hand, but I do have some parts units that I can look into for something similar. I also have a current limiting setup that I built. Power has to run through a light bulb and a switch before it gets to the outlet. (Side note: the distortion is there whether I use this outlet or a normal one). Would the current limiter be enough, or would I still need the fuse?
     
  15. WHONOES

    WHONOES

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    May 20, 2017
    If you can get to the bases of H611 and H613, put you dmm on those points and measure the voltage. It should be close to four Vbe's or 2.4V. Maybe a little more or a little less but not substantially so.
     
  16. WHONOES

    WHONOES

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    May 20, 2017
    A current limiter would be a good idea as it would be more flexible than a fuse. I would stay with the switch though. Do you have any small diodes at all? The 1N 4148's and 1N914's are signal diodes. That is the sort of thing that you want.
     
  17. KilgoreCemetery

    KilgoreCemetery

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    Apr 12, 2017
    You lost me on this one, I think. Getting to the bases of those transistors is easy enough, but did you mean 24 volts from ground to base or 2.4V from one base to another?

    Measuring from ground to base, here's what I show:
    H611 = 24.74
    H613 = 24.4
    H614 = 26.0
    H612 = 26.35

    They don't appear to change with or without a load, but I took these measurements shortly after power it on and as the unit warms up, they're slowly decreasing.
     
  18. KilgoreCemetery

    KilgoreCemetery

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    Apr 12, 2017
    I have a small bag of mystery diodes. They appear to be zeners and have "j1 31 v" printed on them. Unfortunately, I have no idea what those codes mean.

    I do have a Marantz 2250B and a Marantz 2216 that I can pull parts from, but I would need to verify that they are good before using them
     
  19. KilgoreCemetery

    KilgoreCemetery

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    Apr 12, 2017
    Ah ha! The Marantz 2250 has the same SV-3A's
     
  20. WHONOES

    WHONOES

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    309
    May 20, 2017
    I meant between the bases.
     
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