Tweet
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
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
Comment