Electronics > Beginners
NAD 218 THX amplifier Repair.
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Richard Crowley:

--- Quote from: SantaClaw on February 18, 2019, 11:40:50 am ---Right, would anyone want to answer my question on weather I identified the transistors that drives the main fets correctly?
--- End quote ---
Not sure what you mean by "main fets"?  The output devices in your amplifier are ordinary BJT (bipolar junction transistors). They are not FET (Field-Effect Transistors)

On the positive (upper) half of the circuit, the driver for Q354 and Q350 is Q334.  The driver for Q338 and Q342 is Q330  And working "upstream", the driver for Q330 and Q334 is Q325. 

And you can apply the same logic to the negative (lower) half of the circuit where Q326 drives Q332 and Q336 which in turn drive Q340, Q344, Q348, and Q352.


--- Quote ---also I find the schematic confusing..... It says "Right channel" and "Left channel" but on the boards itself it says the opposite, even in the schematics... Where they drunk? I've dealt with the board that is marked Left channel on the PCB
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Don't sweat it. That is a common error in the published documentation.  Things often get re-named in the process of creating the service documentation and proof-reading is not a high priority.
Richard Crowley:

--- Quote from: exe on February 18, 2019, 12:35:02 pm ---Wait, that's not the matching I meant :). This matching looks to be easy to me, just buy enough bjts from a single batch. Usually this is the case when ordering from large distributors.

What I meant was matching between npn and pnp transistors. If this is not required, then task is easy (assuming no adjustment of bias current is required).
--- End quote ---

Right. Matching the PNP and NPN are more commonly found in smaller circuits where there is just one pair of output devices. 

But note that this circuit is DC-coupled throughout the power amplifier circuit. There is a special "sense tap" at the output which goes through R430, a 2M2 resistor for the "DC Servo" circuit which does the positive (PNP) against negative (NPN) balancing.


exe:

--- Quote from: Richard Crowley on February 18, 2019, 12:46:36 pm ---There is a special "sense tap" at the output which goes through R430, a 2M2 resistor for the "DC Servo" circuit which does the positive (PNP) against negative (NPN) balancing.

--- End quote ---

Oh, didn't know there is schematic:). Anyway, that's servo thing is beyond my understanding (I wasn't even able to find that dc circuit). But I also see balancing resistors (sorry if that was mentioned before). So, I think, just buying a bunch of transistors from a single supplier should be enough. May be it's worth buying a few extra just in case.

I'd also replace all transistors connected in parallel at once for better matching, even if just one failed. Unless measurements show new parts are a good match to existing.
SantaClaw:
I will be replacing all transistors on both channels. This amp has bridge mode, and I intend to use it.. I figure mixing parts would be a bad thing?
Richard Crowley:

--- Quote from: SantaClaw on February 18, 2019, 09:02:40 pm ---I will be replacing all transistors on both channels.
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Are you including the small transistors as well as the large output transistors?

With all the transistors (large and small) removed from the circuit, I would absolutely do a thorough comparison between the good channel and the broken channel.  I would print out one or two copies of the schematic diagram and go through all the resistors and measure each one (in-circuit, but without transistors).  With power removed, unplugged, and filter capacitors discharged, of course)
I would write on the schematic the actual measurement of each resistor. 

I would compare each reading on the good channel to the same resistor on the broken channel and write down the actual measurements on the schematic diagram page.  You could write the good channel values on one copy, and the broken channel values on the other copy.  Or use two different colors, etc.  There is a possibility that a resistor failed (open, shorted, or changed value) which caused the failure in the bad channel.

And with all the transistors removed, I would check each capacitor (in-circuit) with the "POS tester" to at least get a "sanity-check" good-channel vs. broken-channel comparison.  You may see things like high series or parallel resistance (or "leakage") by testing in-circuit.  But if you get the same reading on the broken channel as you measure on the good channel, that is enough for a cursory comparison.  Of course if you see a significantly different reading on the broken channel (compared to the good channel), then it is time to take that component out of circuit and test it isolated.

This is a high-power, direct-coupled circuit.  It is very easy for a small, very inexpensive component (like a small resistor or capacitor) to fail and cause the transistors around it (and all they way downstream) to fail along with it.  Or, conversely, it is possible that a transistor failed and damaged resistor(s) and capacitor(s) around it.


--- Quote ---This amp has bridge mode, and I intend to use it.. I figure mixing parts would be a bad thing?
--- End quote ---
I would feel much more comfortable with all new transistors, are at least with all the transistors tested out of circuit and shown to be good.  For that matter, I would test the new transistors as well before installing them.

When you remove the transistors (especially on the "good channel" be sure to label each one so you know where it came from.  The Q number will identify which resistor in the circuit, and "L" or "R" to indicate which channel it came out of.

Mr. Carlson has an interesting video on the subject of troubleshooting (BrEnglish: "fault-finding") audio circuits and using some of his inexpensive DIY test gear. 

https://youtu.be/jKr3rdX9oXQ
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