AVForums South Africa
Audio/Video Hardware => The Vintage Audio section => Topic started by: JohnnyIC on January 11, 2017, 09:08:41 AM
Does anyone have a service manual for a NAD 3125? I have to replace a couple of burned out resistors and a shorted transistor but I don't know their values and can't seem to find a service manual anywhere.
The only thing I could find was this crappy quality schematic, you have to pay to get the higher resolution image.
Okay I removed the faulty bits, now I know what type of transistor to get but I can't quite make out the colour bands of the resistors, both look the be the same value.
Where is the fault pre or power?
Can you post a larger pic of the resistors? I suspect from what I can see on the less charred ends see they very well may be 330 Ohm 5% (Some older NAD amp designs of this generation I repair always seem to have blown 33/330 Ohm near the pre drivers of the power sections).
On a side note its unusual for just that 2SC1815 transistor to fail all by itself without any other subsequent damage besides two resistors, usually (and depending on where it is), the short takes out several subsequent stages when they are turned full on after it shorts. If its in the power amp section, better to be safe and use a multimeter diode test to check the PN junction forward voltage drop on any related transistors that have been fed signal by the blown one.
Thanks for the reply Rick.
To be honest I'm not completely sure where the fault is, I only do this for a hobby and I'm still learning. It looks like the pre section to me though.
I think you may be right about it being a 330 Ohm resistor, I can't quite get my phones camera to focus clearly on a closer pic.
I have checked the other transistors in the vicinity of the shorted one and they all appear to be fine.
Here is a little trick which sometimes enables you to determine the value of a burnt out resistor, you need a fairly accurate ohmmeter. The chances are that the resistor has gone open circuit in just one particular spot. Put one probe as close to the middle on the burnt out resistor as possible and then check each end to see if there is any continuity,if so measure the resistance and the original resistance will be double the measured value. There is a chance the overheating may have changed the value of the conducting section slightly, it will still be close. Give it a try.
Just a further thought, looking at your photo of the PCB it looks like it ran hot before the tranny and resistors packed up, check all the capacitors around that circuit, a failed ie. Shorted cappy could have caused the failure in the first instance and taken out the other failed components.
Hope you manage to sort out the problem
Thanks for the tip. I actually read about that method and tried it the other day. After scraping away the coating I couldn't find any burned spots on either resistor, it seems as though they have been running hot for a while which made them appear burned out. One measures 3.3k ohms and the other one 3.9k so I guess they are 3.3k ohm?
3.9 k ohm is a standard value so replace it with the same value.
Now I can see, definitely a 3.3K (that second band will burn from white to grey but its still orange so it was never white before as orange burns to brown). I have probably changed too many burned resistors in my time....Bleh!
Knowing these old NAD amps quite well (but not really this particular one) I can guestimate as per picture above that top left is the phono section, top right the preamp, bottom right the preamp power supply, then bottom left the section of interest. From the pic, what can think is the "square" group of components that ran hot and burned looks something like two class A buffer amps between the pre and power section. Probably localised damage that didn't go any further then, replace both transistors and use 3.3K half watt resistors or even 1 watt metal film instead of the 1/4 watt that was in there. Measure all other resistors around the burned section don't trust they are ok even if not burned, sometimes they blow open circuit with no burn.
Many of these older NAD amps like to burn resistors, and very often they just shoved a 1/4w into where the service manual says a 1/2 w must go, see it all the time with them.
Hopefully you replace transistors, electrolytic caps and the resistors in that section and are good to go! (The green poly caps you can leave in, they last forever and I have never found a shorted one)
It may be a good idea to change the op-amp in the pre section (top right) just in case. With these things one needs to get all the possible blown components in a single shot otherwise the replacements keep blowing each other up and down the chain wasting time and confusing the issue.