Electronics > Beginners

NAD 218 THX amplifier Repair.

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ebastler:
Are "matched pairs" a thing for transistors in this application (audio amplifiers)? OP seems to believe that he needs matched pairs, and is hence reluctant to buy the two individual complementary transistors from a reputable distributor. When used in a push/pullstage in an open-loop configuration, matching the transistor parameters might actually matter?

@SantaClaw, please note that the ebay vendor you suggested does not seem to offer matched pairs either. They just offer "complementary transistors", i.e. one each NPN and PNP with nominally similar specs.

exe:

--- Quote from: ebastler on February 17, 2019, 01:27:09 pm ---OP seems to believe that he needs matched pairs, and is hence reluctant to buy the two individual complementary transistors from a reputable distributor.

--- End quote ---

I didn't people sell matched pairs. If they do, I'd ask curves from a curve tracer, that's the only real "matching". If they can't provide traces, then seller at last have to explain how matching was done and how close matched parts are.

I randomly checked a couple of ebay shops selling matching bjts, none of them provided any info on matching. I'd say they just sell complementary parts.

Yansi:

--- Quote from: Richard Crowley on February 17, 2019, 12:23:54 pm ---If you (or a friend or acquaintance) work in health-care, maybe you could get X-ray images of your original transistors and whatever you buy for replacement.

I would certainly have one of those $10 "ESR Meter Transistor Tester Diode Triode Capacitance SCR Inductance Tester" gadgets. You might find some particular parameter that would distinguish parts that might be more "genuine".  Or at least it would allow you to sort through your devices to match gain (hFE)

--- End quote ---

Highly unlikely you will measure anything interesting with the POS meter.

To distinguish between transistors, use a curve tracer. Not a POS meter, that measures the beta @0.5mA Ic of a 20A power transistor.

mcinque:
If you want to win high probability of fakes and inconsistent stocks, go on ebay or aliexpress.
Someone tells that ebay and aliexpress are fine and yes, you can be lucky and purchase a good stock or genuine transistors sometimes.
But if you place another same order after some days, even to the same seller, you can receive a mix of different faked products because the seller's sources are not the manufacturers but other resellers that purchase from other resellers who purchase from who knows.
And another important aspect is the packaging: it's completely worthless purchase genuine transistors shipped in a static plastic bag commonly used to keep anything else than electronic components.

If you need genuine products, consistent stocks and protective packaging, you MUST purchase from reputable resellers: for example RS, Farnell, Distrelec, DigiKey... there is no other source other than the manufacturers.

At least... if you do it to manufacture and sell a product. If you do it for hobby and don't care for reliability or esd damages... go for the cheapest seller. But you've been warned.

james_s:

--- Quote from: SantaClaw on February 17, 2019, 11:08:48 am ---I'm not an expert at all, I just know one channel is dead, and a protection resistor keeps blowing, so I figure replace them all..

--- End quote ---

There are good reasons not to use the shotgun approach and replace them all if you don't even know what's bad. Transistors are very easy to test, you don't need to be an expert to accomplish that. If you are unable to test a transistor then I would recommend taking the amplifier to someone who is experienced working on them, otherwise you are likely to make it worse. In many amplifiers you have to use reasonably matched pairs and then adjust the bias after replacing parts in order for it to be reliable and sound right.

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