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Marantz 7t an ac buz suddenly occurred on channel b. how do i find it with only a multimeter?

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  • pedersen Jan 13, 2008

    As suddenly as the buzz occured, it disappeared, but I'll take any advice so that when it reoccures I will be prepared. the volume control afftects the buzz level so the problem is before it. I have interchanged leads, etc. and it is definitely inside the preamp. I am thinking a loose coax grounding on one of the input cables, but have not opened it up yet. The multimeter pamphlet says it has recolution down to 200 mV. Is this enought to detect the buzz? I do have the schematic.

    While I do have you, the expert online, our model 15 Solid State amplifier has one bad channel. The local repair place says it is the output transistor, but is having a hard time finding a replacement. The Marantz schematic only gives the Marantz part # and they do not support the product anymore and do not have parts or information. Is there a cross reference guide somewhere of Marantz and manufacture parts #? Any other ideas would be helpful.

    Again, thank you.

  • drobertson17 May 13, 2009

    I am also trying to find a suitable replacement output transistor for the Marantz Model 15 amp. I have a service manual but it only lists the Marantz part numbers (DTM13 and DTM14). They are TO-3 type transistors. It also gives a voltage chart for base, emitter and collector.
    DTM13 = Base -42v Emitter -42v Collector -0.8v
    DTM14 = Base +42v Emitter +42v Collector +0.8v

    Does anyone know the cross reference for these devices?




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You could try to find the problem with a multimeter.... but i suggest perhaps some experience on vintage amps would be required also. ;) This old girl is a sought after collectable classic these days. I would first check that all connections to and from the preamp are ok by interchanging leads. If this does not point to the problem, is it independant of the vol control or not. I will need to ask a few more questions to help you from here

Posted on Jan 11, 2008

  • Graeme Ross
    Graeme Ross Jan 13, 2008

    It is likely that you have an earthing problem in the preamp ascociated with corrosion of eareth connections. Taking the lid off and carefully seeking this with a mechanical approoach(gentle tapping) may help you to find it. Original parts ar rare for this model (and vintage) but not unobtainable. Check out the links below for details of this guys approach to service and repair... taking note of the age related failures that can occur...particularly resistors and capacitors going out of spec. Rebuilding can often be the best approach to repairs for these old girls. I reccomend it if you can afford it! They carry value as collectables, and will always do so. I am in Australia, and am glad to help you with the work to be done if you are down under. The links I have given you are for a Chicago Tech that offers specialist service on your amps. Good luck. A "Fixya" rating would help my profile for my time spent to get you to this point:) Good luck mate!



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