I have a NAD 320 integrated Amp in need of repair. Does anyone have service manuals for this? The PSU seems OK - +/- 44V DC. If I feed signal into an input I get a much attenuated version out from the Pre-Out terminals. Volume control varies the level and source switching seems to work. I think the Pre-Out level should be much higher. If I input a signal into the Main-In terminals I get no output on the speaker terminals. Any suggestions or data welcome.
Thanks
Graham
GrahamH: You didn't happen to leave on a tape monitor switch or a mute switch ?
The beautiful thing about repairing STEREO equipment is that you have redundancy in the fact that there are 2 identical channels and corresponding circuitry. It would be quite unlikely that there are identical failures in both the left and right channels.... a good suggestion is that you should look for things that will affect BOTH channels, like switches, jumpers, special modes of operation, and the power supply, etc. Based on your last comment regarding the Main-In terminals the problem is most likely in the Main Power Amp (not the Pre-Amp) since it does not output any signal to the speakers when you applied a good line level signal to the Main-Amp inputs. If both channels are affected the place to start looking is the power output stages.... if just one of the channels is bad, the speaker relay will not enegize thus both channels will appear dead. By the way, as you said... you checked the main B+ voltages of +/- 44 VDC, but what about the B+ that supplies the lower regulated voltages to the preamp and drivers? A lot more investigation can be done without a schematic just yet. You might want to go to the website for this newsgroup and look over the weath of troubleshooting and repair tips...
No, I checked that ;)
Sound suggestions. I have focused on the PSU. All looks OK, +/-44V for power stages, +/-36V presumably for pre-amp, 5V for control logic. If I feed a signal in to an active input I get the signal out of the TapeOut connections. This works L & R for Tape1 & Tape2. Still nothing worthwhile from Pre-Out. In fact if I feed a square wave in the Pre-Out signal is narrow spikes corresponding to the edges of the square pulses. Peak amplitude of these spikes is half the square wave amplitude at maximum volume setting with no external load on Pre-Out. The links from Pre-Out to Main in are external and removable so the power stage should not be affecting what I see on Pre-Out at this stage. The volume and balance controls vary the amplitude of Pre-Out.
Thanks for your suggestions. I think that to move forward I need a schematic so that I can trace the signal. Can anyone help?
Ahh, progress! I can get a signal on the internal power stage output busbars. It is low level though. My current guess is that the soft-start circuit is faulty so that pre amp is muted and the speaker out relay is off. Now, where is the S-S circuit and how do I fix it, or bypass it!?
Graham
I would first test the AF power output transistors -- unless you are dealing with an integrated module (can't remember what NAD used in your model)
gb
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.