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Marantz PM94 phono question

  1. Jul 10, 2016 #1

    I repaired this amp and have everything working except one channel of the phono input. The left channel works fine.

    Any ideas of how to troubleshoot this issue?


    I am still have trouble trying to post a part of a schematic from a PDF here.


    I found the issue with the PDF. It is password protected and can not be copied. So...the only way to look at the schematic is to download it from the web site.

    The part of the schematic in question is 6.2 Phono Amp Input Sclection...I know..PITB..lol
    Last edited: Jul 10, 2016
  2. jcsd
  3. Jul 11, 2016 #2


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  4. Jul 11, 2016 #3


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    Figure 6.12 on pg.13 is Phono pre-amp schematic. Test points A and C, at right edge of schematic near J410, are outputs of the pre-amps; a good place to start signal trace or signal inject. The pre-amp inputs are J405 at the left edge of the schematic, another good place to inject a signal. Good luck tracing thru all those switches and relays!
  5. Jul 11, 2016 #4
    Hi Tom,

    I don't think I have ever tried to work on a phono issue. I assume I can safely inject a signal of only less than 2 mV. I will be lucky if the fault is at or before the op-amp.

    There are Alps rotary switches that connect via a plastic flat cable with a metal strip encased in the center of the cable that actuate a slider switch. I don't assume at the moment that they are the issue. There are no numbers on the switches, just Alps/Japan. I have no idea if they can be bought or how to identify them. Also the signal inters a small transformer which has no part numbers (well..it has a Marantz part number on the schematic)...lets hope the issue is not there!!

    I don't typically try to repair vintage solid state stereo and what little experience I have has been pretty frustrating. I need to replace certain capacitors in this unit for example which are no longer made. How to re-design something using parts that are currently available is more or less pushing the limits of the skills I currently have.

    I see the test points you refer to...thanks for pointing them out. I will start there.



    EDIT: I found part of the issue at least. The Alps switch (S401) is not working correctly. No signal from the switch to the right side transformer. Digikey and Mouser do not sell them. Arrow.com sells Alps but nothing like what I need.
    This seems to be the place I always wind up with vintage stereo amps....found the fault....can't find the part.

    Is this typical for anyone repairing vintage stereo gear??


    Last edited: Jul 11, 2016
  6. Jul 11, 2016 #5


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    This is typical for anyone repairing vintage stereo gear?? anything. I once tried to rebuild a motorcycle engine, from a pre-WWII, German bike. Fat Chance!!
  7. Jul 11, 2016 #6
    Hi Tom,

    I guess I could make most anything out of metal. I guess casting and machining a piston would be pretty hard. Not sure I would want to try to do that or a crankshaft for example.

    The switch issue with this amp is a bit the same. These issues of no replacement parts are in my experience pretty much related to solid state electronics. I have rebuilt many radios built in the 1930's and 1940's without issue. Of course the radios were less complex and generally have more space to work in. There is even a company in the Czech Republic that will make a custom vacuum tube for a price.

    EDIT: I decided to bite the bullet and crank up the milling machine and make a new switch. That is crazy I know...lol I designed the part in Solid Works software and now if I can get the CAM software to work correctly I can set back and watch the computer cut the part. At least I have a friend who has Solid Works software and I did not have to pay the thirty grand the over priced software cost!!! A good 3D printer would be handy about now.

    My grandfather use to tell me " boy, you can take three hundred dollars worth of tools and make a three dollar ironing board". Well...that has gone to thirty grand to make a three dollar switch...lol

    All this is messing up my fishing time!!!...lol


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