Re: NAD320 amp
- From: "GrahamH" <pknvdw@xxxxxxxxxxxxx>
- Date: Thu, 01 Sep 2005 17:18:50 GMT
<electricitym@xxxxxxxxx> wrote in message
news:1125372761.618036.293960@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> GrahamH:
> You didn't happen to leave on a tape monitor switch or a mute switch ?
>
No, I checked that ;)
> 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.
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.
> 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...
> http://www.repairfaq.org/
> plan to spend some search time there and dust off your test meter and
> oscilloscope and perform some more troubleshooting.
> electricitym
> .
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
.
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