| Electronics > Beginners |
| NAD 218 THX amplifier Repair. |
| << < (4/12) > >> |
| Audioguru:
The Nad amplifier is high quality and probably uses matched transistors that you probably cannot buy. |
| exe:
--- Quote from: Audioguru on February 17, 2019, 08:25:52 pm ---The Nad amplifier is high quality and probably uses matched transistors that you probably cannot buy. --- End quote --- If these transistors impossible to buy, where does the manufacturer get them? ;) Anyway, transistors cannot be ideally matched in any case, simply because npn and pnp have different geometry, capacitance, etc for the same current. So, there is always a difference. If there is a mismatch, the amplifier still will work, at the expense of increased THD. But it's not granted the difference will be dramatic (or even audible). It's also not granted how well they were matched at the factory. Don't you think they buy a lot of transistors and throw away those that are not within 0.001% tolerance? And, finally, you can match yourself! May be, if you want both channels to be as equal as possible, you may want to match transistors also between channels, but I wouldn't go that far. |
| Richard Crowley:
--- Quote from: exe on February 17, 2019, 08:36:16 pm ---If these transistors impossible to buy, where does the manufacturer get them? ;) --- End quote --- Of course the complete question would be understood as "you can't buy matched PNP/NPN pairs of reliably genuine vintage BJT transistors in small quantities at retail". Clearly manufacturers buy those transistors by the hundreds or thousands. And it is easy enough to sort them by hFE and match transistors of similar gain. And even the $10 "POS tester" can do a competent job of simple hFE measurement for matching. Note also that those large white ceramic block resistors are designed to equalize the amount of current hogging so that you can operate many transistors in parallel without precise matching. A very common circuit design in power supplies and power amplifiers. The hFE matching for this amplifier would probably be to have all the PNP transistors that are in parallel to have similar gain to reduce the load on those resistors, and then to select the NPN transistors for similar gain. I would think that you would need to take all the power transistors out of the amp and test each one isolated out of circuit. It is possible that only some of the transistors are shorted. And also note that it is possible that the driver circuits could be causing the fault or indeed causing the failure of the output devices also. Many circuits like that are a "house of cards" where one component failure takes out everything around it. It is beneficial that the other channel still works so that you can take voltage measurements at various points in the circuit to see what should be "normal". |
| SantaClaw:
Right, The OEM transistors for this amp are impossible to get a hold of as they've been out of production for years. So the SC5200 + 2SA1943 are recommended replacement parts from the service manual https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=2ahUKEwjxwJSc18PgAhWQh7QKHQfVAbsQFjAAegQIChAC&url=https%3A%2F%2Fwww.vintageshifi.com%2Frepertoire-pdf%2Fpdf%2Ftelecharge.php%3Fpdf%3DNad-218-Service-Manual.pdf&usg=AOvVaw3GdCgBskytLATAhfokMBJM ^^ url from hell I know but it's the service manual, I also have a newer version of it that lists replacement parts. Sure, I can test if a transistor is broken, but I would have to de-solder all the transistors to do that. And I figure, if I have to de-solder all the transistors, I might as well replace them. How am I supposed to get matched pairs ? To describe the fault, I only have sound in the right channel. R345 blew I replaced it, I got sound back in the left channel. The R345 blew again after a couple of hours, I was told I had to replace the fets.. Also, I think the only adjustment I can do is the idle current as described in the service manual. I don't have a reliable signal generator for the alignment stuff. |
| Audioguru:
A manufacturer of high quality amplifiers buys millions of transistors and matches most of them, then sells the odd ones on ebay. My cheap Chinese LED flashlight has 24 white LEDs perfectly matched. They did not buy them matched, they matched them, and sold the odd ones on ebay. |
| Navigation |
| Message Index |
| Next page |
| Previous page |