Author Topic: NAD 218 THX amplifier Repair.  (Read 9605 times)

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SantaClaw

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NAD 218 THX amplifier Repair.
« on: February 17, 2019, 10:48:26 am »
I'm looking for bout 10 pairs of 2SC5200 + 2SA1943 transistors and I'm worried about getting fake trannies. I'm replacing some faulty ones in my Nad 218 THX amplifier..

I'm having problems locating a store that gives me confidence... lol

 
« Last Edit: February 22, 2019, 10:51:00 pm by SantaClaw »
 

SantaClaw

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Re: Where can I get a guaranteed genuine set of transistors?
« Reply #1 on: February 17, 2019, 10:54:41 am »
Right, I'm going to try this store: https://www.ebay.com/itm/263510261094

I mean they have over 10000 positive feedbacks so it can't be that bad?
 

Offline goldfinger

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Re: Where can I get a guaranteed genuine set of transistors?
« Reply #2 on: February 17, 2019, 10:57:13 am »
I'm looking for bout 10 pairs of 2SC5200 + 2SA1943 transistors and I'm worried about getting fake trannies. I'm replacing some faulty ones in my Nad 218 THX amplifier..

I'm having problems locating a store that gives me confidence... lol

I’ve got a lot of NOS jfet etc from bg-electronics-de an eBay seller from Germany can highly recommend...




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Offline exe

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Re: Where can I get a guaranteed genuine set of transistors?
« Reply #3 on: February 17, 2019, 10:58:49 am »
Just buy from any reputable distributor, there are plenty of them: tme, digikey, mouser, rs-online, farnell... Why bother with ebay? Most reviewers have no clue how to test if it's a genuine part or not.

BTW, there are different manufacturers of these transistors. I found 2sta1943 from ST to have better performance (read more gain and phase margin in common emitter mode) than Toshiba part in my stupid tests at current about 1A and Vce ~0.5V or so. Although I tested only one or two of them :)
 

SantaClaw

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Re: Where can I get a guaranteed genuine set of transistors?
« Reply #4 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..

I've sent a query to that German store, we'll see what they say..

I have no clue what to look for in a good transistor. I just want something that works... lol

I don't know if lets say: https://www.digikey.no/product-detail/en/2SC5200N(S1%2cE%2cS)/2SC5200N(S1ES)-ND/4815288/?itemSeq=284842507 and https://www.digikey.no/product-detail/en/2SA1943-O(Q)/2SA1943-OQ-ND/871383/?itemSeq=284842527 will work together...

The price is virtually identical to that Ebay store, I also seem to get free shipping from Digikey, I f I can use those parts I can give it a try...  I have no way of matching the transistors that I am aware of, I have an osiliscope multimeter thingymybob (Uni-T ut81c), and a basic component tester but that's about it.
« Last Edit: February 17, 2019, 11:16:18 am by SantaClaw »
 

Offline OwO

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Re: Where can I get a guaranteed genuine set of transistors?
« Reply #5 on: February 17, 2019, 11:18:58 am »
I would avoid ebay but aliexpress may be fine if you pick a seller with >98.5% positive feedback rate.
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Offline Yansi

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Re: Where can I get a guaranteed genuine set of transistors?
« Reply #6 on: February 17, 2019, 11:42:18 am »
Interesting to hear such an opinion from a Chinese guy direct. What do you base that opinion on, if I can ask?

(I also prefer Aliexpress for sourcing parts, due to numerous reasons, but not everything can be bought there, thats where the ebay comes in).

However, C5200/A1943 could be source from reputable distributors (with properly marked-up price of course). But I'd suggest to go with a MJL3281/1302 from Onsemi. Similar/Same spec, but more reliable and does not get so much attention from the counterfeiters, but slightly bigger price. (And change them ALL in the amp, you can't simply combine completely different types).
 
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SantaClaw

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Re: Where can I get a guaranteed genuine set of transistors?
« Reply #7 on: February 17, 2019, 11:53:08 am »
Aliexpress is out of the question.



I see the onsemi ones are out of stock.. , oh and 20 euro more than the toshiba ones
 

Offline Richard Crowley

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Re: Where can I get a guaranteed genuine set of transistors?
« Reply #8 on: February 17, 2019, 12:12:28 pm »
If you are looking for genuine, non-counterfeit transistors, Ebay or any of those mass-market Asian sellers is the LAST place I would look.  Stick with "Authorized Distributors" for the brand-name of the part.  Even there, the authorized distributors sometimes get suckered into sourcing and selling counterfeit parts.  Even "military-grade" parts sometimes turn out to be counterfeit. Which is a danger to many critical applications.  I believe there is even more than one space satellite that has failed due to counterfeit semiconductors.

I would also trust audio-savvy sources like Parts Express to source genuine audio power amp transistors.

https://www.parts-express.com/2sa1943-transistor--2sa1943
https://www.parts-express.com/2sc5200-transistor-npn-to-247l--2sc5200
 
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Offline Richard Crowley

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Re: Where can I get a guaranteed genuine set of transistors?
« Reply #9 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)
 

Online ebastler

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Re: Where can I get a guaranteed genuine set of transistors?
« Reply #10 on: February 17, 2019, 01:27:09 pm »
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.
 

Offline exe

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Re: Where can I get a guaranteed genuine set of transistors?
« Reply #11 on: February 17, 2019, 01:55:23 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.

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.
 

Offline Yansi

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Re: Where can I get a guaranteed genuine set of transistors?
« Reply #12 on: February 17, 2019, 04:14:59 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)

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.

 

Offline mcinque

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Re: Where can I get a guaranteed genuine set of transistors?
« Reply #13 on: February 17, 2019, 05:59:22 pm »
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.
« Last Edit: February 17, 2019, 06:02:05 pm by mcinque »
 

Offline james_s

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Re: Where can I get a guaranteed genuine set of transistors?
« Reply #14 on: February 17, 2019, 06:08:22 pm »
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..

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.
 

Offline Audioguru

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Re: Where can I get a guaranteed genuine set of transistors?
« Reply #15 on: February 17, 2019, 08:25:52 pm »
The Nad amplifier is high quality and probably uses matched transistors that you probably cannot buy.
 

Offline exe

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Re: Where can I get a guaranteed genuine set of transistors?
« Reply #16 on: February 17, 2019, 08:36:16 pm »
The Nad amplifier is high quality and probably uses matched transistors that you probably cannot buy.

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.
 

Offline Richard Crowley

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Re: Where can I get a guaranteed genuine set of transistors?
« Reply #17 on: February 17, 2019, 09:11:56 pm »
If these transistors impossible to buy, where does the manufacturer get them? ;)

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

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Re: Where can I get a guaranteed genuine set of transistors?
« Reply #18 on: February 17, 2019, 09:31:11 pm »
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.
« Last Edit: February 17, 2019, 11:22:04 pm by SantaClaw »
 

Offline Audioguru

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Re: Where can I get a guaranteed genuine set of transistors?
« Reply #19 on: February 17, 2019, 09:32:32 pm »
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.
 

Offline Yansi

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Re: Where can I get a guaranteed genuine set of transistors?
« Reply #20 on: February 17, 2019, 09:46:21 pm »
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.

You seem to be a bit paranoid, aren't ya?   You would be surprised what kind of awesome deals could be made with those chinese guys if you know them more.

But from your opinion, I see you have none experience on shopping in far east or negotiating deals with the sellers directly.

ESD damage of a single  component is extremely y unlikely, especially with something a BIPOLAR JUNCTION transistor, think why. (Having no oxide isolated MOS is only part of it).
 

SantaClaw

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Re: Where can I get a guaranteed genuine set of transistors?
« Reply #21 on: February 17, 2019, 10:52:09 pm »
Yeah, so I de soldered the transistors on the bad channel and put them in my cheapo component tester...

B1163 Test Results:  BJ T-PNP #1 B=84 Uf=557mV #2 B=88 Vf=553mV #3 B=86 Vf=547mV #4 B=96 Vf=538mV

D1718 Test Resilts: BJ T-NPN #5 B=117 Uf=554mV #6 B=79 Vf=561 #7 B=128 Vf=552 #8 B=128 Vf = 542mV

Am I wrong, or did all of them test ok ?
 

SantaClaw

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Re: Where can I get a guaranteed genuine set of transistors?
« Reply #22 on: February 17, 2019, 11:20:23 pm »
I only have some tiny transistors left on that channel to test, could it be any of them ?

One of the transistors #2, actually gave a reading as a 447 ûF cap at first, but I assume that's down to the crappy tester... btw I built it as kit so it's prolly faulty :D
 

Offline Richard Crowley

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Re: Where can I get a guaranteed genuine set of transistors?
« Reply #23 on: February 17, 2019, 11:22:09 pm »
Those results from the transistor tests seem OK to me.
What about the transistors that are driving them?  Take those out and test them.
How do the transistors on the good channel test by comparison?
Do any of the components on the bad channel get more hot/warm than on the good channel?
 

Offline Richard Crowley

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Re: Where can I get a guaranteed genuine set of transistors?
« Reply #24 on: February 17, 2019, 11:31:51 pm »
I only have some tiny transistors left on that channel to test, could it be any of them ?
Yes. The driver transistor stage could be driving the output stage into saturation.

Do you have a schematic diagram for the amp?  Can you measure the voltages at key nodes on the good channel to establish what "normal" should be?  Did you say that the amp operates for a short time before failing?  After establishing "normal" voltages on the good channel, can you then quickly probe a few key nodes on the bad channel?  That would be very helpful to drill down on the bad channel to see where it was going wrong.

Quote
One of the transistors #2, actually gave a reading as a 447 ûF cap at first, but I assume that's down to the crappy tester... btw I built it as kit so it's prolly faulty :D
It sounds more like the tester was touching only two legs of the transistor and didn't have secure contact with all three legs.
 


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