Electronics > Beginners

NAD 218 THX amplifier Repair.

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james_s:
Ebay is a crapshoot for components, I've gotten some that seem fine but quite a few others are questionable if not obviously counterfeit. Sometimes a transistor that may be close enough to work in some situations is relabeled as some other considerably harder to find/more expensive transistor. The thing is, if you lack the expertise the evaluate the performance of the part, you never really know what you're getting and it's not worth the frustration. If you're just hacking around building hobby circuits it may not matter if you get a fake component but other times it does. I saw some tests somewhere on a potentially fake linear regulator, turned out it was indeed a regulator, but it did not come close to performing as well as the one it claimed to be.

exe:

--- Quote from: Richard Crowley on February 17, 2019, 09:11:56 pm ---And even the $10 "POS tester" can do a competent job of simple hFE measurement for matching.

--- End quote ---

Shouldn't matching done at least at quiescent current of the amplifier? Power transistors optimized for higher current, measuring hFE at a few milliamps is not representative, imho. I'd say take 100mA (ideally several bias points, if curve tracer is not available).

Another thing is, I noticed that transistors from the same batch don't have much spread. So, buying "millions" may not help with binning :). (but, of course, I never bought myself such quantities to draw conclusions)

And how much is a good match? :). Is it even THAT needed? Do these expensive amplifier have a negative feedback to compensate for mismatch?

Richard Crowley:

--- Quote from: exe on February 18, 2019, 07:34:23 am ---Shouldn't matching done at least at quiescent current of the amplifier? Power transistors optimized for higher current, measuring hFE at a few milliamps is not representative, imho. I'd say take 100mA (ideally several bias points, if curve tracer is not available).

Another thing is, I noticed that transistors from the same batch don't have much spread. So, buying "millions" may not help with binning :). (but, of course, I never bought myself such quantities to draw conclusions)

And how much is a good match? :). Is it even THAT needed? Do these expensive amplifier have a negative feedback to compensate for mismatch?
--- End quote ---
We can see for ourselves how closely the  transistors are matched by the measured values. The only "outlier" in my view appears to be that #6.

The purpose of matching is to equalize the "effort" by the transistors in parallel. So that one transistor isn't passing a much different current (more current or less current) than the other three transistors.  @SantaClaw hasn't yet measured the transistors on the good channel to see what a "normal" spread should be.

SantaClaw:
Right, would anyone want to answer my question on weather I identified the transistors that drives the main fets correctly?


The top and bottom plate of this amp is held in place by no less than 36 m3 screws... lol

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

exe:

--- Quote from: Richard Crowley on February 18, 2019, 11:07:10 am ---The purpose of matching is to equalize the "effort" by the transistors in parallel. So that one transistor isn't passing a much different current (more current or less current) than the other three transistors.

--- End quote ---

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).

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