Hi, I am working on a Nad 2200 and am having problems tracing the faults. I have found 90V on the outputs which should be 0V so suspect problems with the power transistors. No problem checking them but further checking shows the 62V rectifier output voltage on the -ve dc side is at -90v, should be -62V. This voltage sometimes drops back to -62V but then goes back to -90V for no apparent reason. What I don't know is if it is the rectifier that is shot or is this just a symptom and there is a more fundamental cause. The +ve side of the rectifier is stable at 62V.
Are you experienced in using it?
That's a more important question.
The output imbalance is due to something before the outputs, causing the problem.
Voltage checks comparing the service manual's specifications would likely narrow down the cause.
In that case, the "steps" required must be taken.
Just like walking up a stairway to get to the top.
The first step, at the bottom, is to inderstand the fundamentals of electricity.
1) This includes things like Ohms Law, electical current (AC and DC), and how it travels through conductors. (this has been documented in books available.)
It also includes Theory.
2) Experience though applying the above, starting with simple circuits.
3) Gaining helpful mentoring from accredited instuctors (not some lunatic online who "claims" to know things)
Owning an O-scope or other techical equipment is fine, but without the prior mentioned training is like trying to get to someplace in a car without filling up the gas tank. (skipping the steps)
It just isn't going to get you anywhere.
I've been tinkering for about 10 years and learn something new almost every day.
In it's most basic form, an oscilloscope is just a fast voltmeter, also, rather than
a numeric display it's graphical.
Post the make/model of your oscilloscope so we can give you guidance on
the controls etc...
The oscilloscope will have a Volts/division scale which is akin to the voltage
scale on your multimeter, ie, used for ranging. Normally beneath this will be
a selector for AC/GND/DC, select AC or DC depending on what you are
wanting to measure. GND for centering. There will also be a time scale
uS/mS/Sec per division.
You should have a probe with a ground wire attached, the GND should be connected to the GND/chassis of the amp being tested.
The oscilloscope may have a test point, typically 1kHz square wave.
Try powering up the oscilloscope, connect probe to test point and adjust
volts/div and mS/div.
It's reasonably easy learn to do the basics in maybe 30min (make notes)
Thanks MBZ, that's positive and encouraging advice. I am quite confident and have had a go with the scope after watching some of the many tutorials on YouTube. The scope I have is a Siglent SDS1052DL+ with two probes etc.
I'll have a go at the test point, there are a couple. At the moment I suspect a dodgy diode as I am getting -90V on the -62V circuit. Either that or the diode rectifier but not sure if a rectifier could put out _90V while at the same time putting out +62V on the other side of the rectifier.
I'll have a go at the test point, there are a couple. At the moment I suspect a dodgy diode as I am getting -90V on the -62V circuit. Either that or the diode rectifier but not sure if a rectifier could put out _90V while at the same time putting out +62V on the other side of the rectifier.
Hi Korpberget, yes the circuit is complete except for removing the wire connecting the output to the protection circuit. I have replaced all the capacitors as well as all the transistors and diodes except maybe for two larger diodes on the heatsink with the main transistors which tested as good previously. However I did initially have a good left channel but now both left and right are bad. I think I need to isolate each channel and do some more testing. I am also unsure whether the rectifier is good or not.
A dodgy diode would surely reduce the negative rail, not increase it? You have a voltage difference between the rails greater than the peak-to-peak voltage of the secondary apparently.
Have you checked the secondary ac voltage is correct?
Hi Korpberget, just saw this... Yes, the selector had gone bang in a previous life. I have it bridged out.
I think the AC is okay, I have 46V and 72V coming out of the rectifiers.
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