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NAD 7250PE Integrated Amplifier - Troubleshooting the power amp

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  • NAD 7250PE Integrated Amplifier - Troubleshooting the power amp

    Hi Folks,

    I need some help thinking through next steps for the repair of the power amp section of a NAD 7250PE stereo. It's a bit more of an advance design than the solid state guitar/bass amps I have worked on in the past. Here is a link to the service manual with schematic on HiFi Engine (account required). The power amp schematic is page 17 of the PDF. Note that it is the LEFT channel that is fully illustrated. Components in the right channel have 50 added to their component numbering.

    What I have found out / have done so far:

    1. Amp is not blowing fuses.
    2. The preamp section is passing signal and seems to be fine.
    3. Power amp is dead. Nothing looks obviously burned that I can see.
    3. The speaker protection relay (RY801) is is not getting activated.
    4. The output protection IC (IC801, a uPC1237H) is showing -2V on pin 2 (the output offset detection pin), so it seems to be doing its job.
    5. The output of both R & L channels upstream of the relay is showing -66V!
    6. This amp has dual power rails (I assume this is associated with the "power envelope" feature) -- a +/- 72 VDC rail (at least that's what I measure) associated with FWB D856, and a +/- 35VDC rail associated with FWB D855. Note that these rails service both the L&R channel, which is presumably why both the L&R channels have the same crazy output offset -- if these rails are fully or partially shorted for left, they will be also for right.
    7. The +/- 72V rail seems to be OK, producing 72V and -72V at the D856 diode outputs and elsewhere.
    8. The +/- 35V rail reads wrong. I get the expected +35V at the + end of the D855 diode package, however, the - end is reading -72V (or was it -66V?). On the scope there doesn't seem to be any significant AC component - filter capacitor C863 may be hanging in there despite being well over its 50V rating?
    9. The diodes feeding each "pull" section's -35V rail (i.e., both right and left channels; D712 in the L, D762 on the R) have -72V on the anodes, and -66V on the cathodes/tab. So I'm definitely getting something close to the "outer" rail's voltage on the negative side of the "inner" rail.
    10. I can't seem to get a reliable in-circuit measurement of the capacitance of C710 (left) / C760 (right) 100nF caps that form a path directly between the high and low voltage rails. I can read what seems to be about 2.4K of DCR across these, but I don't have any confidence it's a "real" reading. So one or both of these could be suspect. Power transistors Q707 and Q757 (I'm guessing these provide the "envelope power") are also effectively in parallel with these (although the collectors are on the other side of the diodes D712 and D762 mentioned earlier), and so I suppose could be culprits (not too sure I can get at these without some more disassembly).

    These are all measurements with the amp live and only partially disassembled. I'm not sure how much more I can do a this point without disassembling further, but at that point it's going to be much harder to get live circuit measurements. So I have a few directed questions:

    1. What other measurements would it be desirable to try and get before pulling it apart?
    2. Based on what I have described so far, what should I be focusing on as likely proximal causes of the failure?
    3. Based on where the big negative voltages are being found where they shouldn't be found (i.e., the outputs, as well as what should have been the -35V rail, which is instead is closer to -70V), what other components here are likely to have fallen prey to a cascading failure?
    4. What are my general next steps?


    All advice much appreciated. As usual I anticipate it will be a great learning experience.

    Cheers,

    Paul





    Last edited by Wombaticus; 07-31-2022, 09:32 PM.

  • #2
    OK, so amp is stuck in protect due to outputs slammed over to Vneg.

    I won't be signing up for an account at that web site, perhaps if you have downloaded the schematic, you could post it here yourself?

    A cap can be electrically leaky and allow DC to pass, but a wrong value of a cap is not going to let DC pass.
    Education is what you're left with after you have forgotten what you have learned.

    Comment


    • #3
      The svce manual is too large to attach. Here is a screenshot of the power amp area.
      Check Q707/708 and equivalent in other channel.

      Click image for larger version

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      • #4
        Hi Enzo,

        Delighted that you've got eyes on this. I figured out how to extract just the power amp section of the PDF, please see the file attached here, it shows a bit more than G1 uploaded, although I'm not sure it's required. For sure I will be checking Q707 / Q757, and C710 / C760, these are obvious suspects. I'm especially interested in what else is likely to be dead as a cascading failure given the output got slammed to -66V, or other stuff I should try and get a measurement from BEFORE I tear the thing fully apart and will have a much harder time getting live measurements..

        Many thanks,

        Paul Fawcett / Wombaticus



        Originally posted by Enzo View Post
        A cap can be electrically leaky and allow DC to pass, but a wrong value of a cap is not going to let DC pass.
        Yes, naturally. While I don't do a whole lot of solid state repair, I have a reasonably strong general knowledge.
        Attached Files

        Comment


        • #5
          Hi All,

          An update here but not the progress I hoped for. With the "envelope power" components Q707/Q757, Q708/Q758, C709/C759, C710/C760 (lower number of each pair is left channel, higher is right) taken out of the circuit, each component tested good. With everything listed there removed, and measuring between where C710/C760 would have been, I am now seeing that one end is at -72VDC corresponding to the expected value for the outer rail, while the other node is at about -37VDC, which is approximately what I would expect for the inner rail at -35VDC. HOWEVER.... BOTH the right AND left outputs are still getting pulled negative. This time they are getting pulled to the inner rail voltage of -37V rather than ~-70VDC as before. So my original idea that both right and left were affected because of a failure in the envelope power section might have been a red herring. I need help figuring out what is going on here... do I have two separate failures in each of right and left channels that are resulting in both outputs getting pulled negative (seems unlikely), or is there still a single failure that is pulling both negative (what?!).

          Cheers,

          Paul

          Comment


          • #6
            All four 68R rail supply resistors measure good (R627/R677 and R632/R682). The rail side of each is as expected, ~72V and -72V. The right channel has 0.7V across the +ve and -ve rails (10.2 mA for each), whereas the left channel has 0.97V dropped across the positive 68R (R627; 14mA), and a bit less (0.93V gained; 13.6 mA) across the -ve (R632).

            Confirming also that the +/- 20V rails from D857 measure as expected.

            These measurements are with the PE components still pulled. Both outputs still pulled to inner rail Vneg.

            Comment


            • #7
              Have you pulled the loop jumpers and verified there is no DC being sent to the power amps?

              Comment


              • #8
                Never mind that, I see the inputs are cap coupled, I was reading the schematic wrong.
                What do you have for DC at pins 4 and 8 of IC501 ?

                Comment


                • #9
                  G1, great idea, I will check. The IC851 powered +/- 20VDC rails that in turn feed the lower voltage opamp supply are OK, but I didn't yet check downstream of R857/R858. I guess it's possible that maybe one of the Zeners D853/854 that clamps the voltage here is kaput? I'll report back. BTW, the jumpers from the preamp are pulled, and indeed, the preamp is behaving as expected - but as you note, it's cap coupled...

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