NAD 218 THX amplifier Repair.

[SantaClaw]

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

 

[SantaClaw]

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?

[goldfinger]

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...




Sent from my iPhone using Tapatalk

[exe]

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]

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.

[OwO]

I would avoid ebay but aliexpress may be fine if you pick a seller with >98.5% positive feedback rate.

[Yansi]

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).

[SantaClaw]

Aliexpress is out of the question.



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

[Richard Crowley]

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

[Richard Crowley]

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)

[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]

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.

[Yansi]

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.

[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]

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.

[Audioguru]

The Nad amplifier is high quality and probably uses matched transistors that you probably cannot buy.

[exe]

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.

[Richard Crowley]

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]

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.

[Yansi]

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]

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]

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

[Richard Crowley]

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?

[Richard Crowley]

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.

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

It sounds more like the tester was touching only two legs of the transistor and didn't have secure contact with all three legs.

[SantaClaw]

1 - Those results from the transistor tests seem OK to me.
2 - What about the transistors that are driving them?  Take those out and test them.
3 - How do the transistors on the good channel test by comparison?
4 - Do any of the components on the bad channel get more hot/warm than on the good channel?

1. Well, that's what I thought too.

2. This will be the small transistors type C4793, (Component Q333, Q329, Q321) type A1837 (Q331 and Q335) type A1837 ?
I haven't de-soldered them yet, it's past midnight.

3. I haven't de-soldered them either, though I plan on changing them too if I have to replace any of the others.

4. Yes, the resistor that blew... strangely enough. I never got the chance to see if the transistors got any warmer, as it failed before I could do that.

I am pretty clueless. The closest to an electronics education I have was when I did a year's school to become a car mechanic..

 What I can say that it was powering a set of Dali concept 10 speakers at the time it failed (6 ohm's) this amp is rated 4 ohms min. It produced low/muted/distorted sound on the left channel when it failed, so there was some sound, but not very good sound, no bass/punch and distorted high tones. Could it be the speaker relay somehow ?

There is a diagram posted further up in the thread, I posted a link to the service manual as a pdf

[Jwillis]

B(Beta) HFE or Gain are generally the same thing when describing BJTs so the 2SD1718 transistor might have a current gain anywhere between 60 and 200. The gain of the 2SD1718-Q will be in the range from 60 to 120, 2SD1718-S ranges from 80 to 160, 2SD1718-P ranges from 100 to 200.
The 2SB1163 transistor might have a current gain anywhere between 60 and 200. The gain of the 2SB1163-Q will be in the range from 60 to 120, 2SB1163-S ranges from 80 to 160, 2SB1163-P ranges from 100 to 200.
Your test is fine .If your  tester shows the pins 1-2-3 as resistors or open then the transistor is toast.
Download the data sheets for each transistor your putting to test and compare your results.

From experience ,Ebay is not bad for electronic components from China.But here's a tip.If the seller is only selling electronic components or items that are in that general area including modules ,test equipment ,ect.Your probably Ok .
If the vendor is also selling Lingerie and other weird stuff then you probably want to avoid them.
I have purchased probably 90% of my components from Ebay have have found no problems with them.
Not to say that i havn't received fake stuff ,but believe it or not it came from western Europe.I was indeed shocked.
Distributors like  Digikey is good but finding older components or obsolete products is tough. 

[Fire Doger]

The replacement resistor was 0.5 watts as schematic says?

[SantaClaw]

The replacement resistor was 0.5 watts as schematic says?

No, it was a 5w one, i didn't have any 0.5w 82K-ohm resistors on hand... Stupid as I am, I asked a radio amateur friend, and he said it would be fine.

I did order some, and I replaced it now when I de-soldered the transistors with a 0.5 watt one.

[Fire Doger]

The replacement resistor was 0.5 watts as schematic says?

No, it was a 5w one, i didn't have any 0.5w 82K-ohm resistors on hand... Stupid as I am, I asked a radio amateur friend, and he said it would be fine.

I did order some, and I replaced it now when I de-soldered the transistors with a 0.5 watt one.

5W is more than enough, if everything was ok it would be fine as your friend said.
I thought that you maybe used a 0.25W...

*For feature reference, replacing parts with higher maximum ratings is not always "the bigger the better", for example if you replace a 100mA fuse with a 10A fuse, a working circuit will work fine but in case of fail fuse won't blow up.
Accordingly a 5W resistor on a faulty circuit will put more stress before blowing up than a 0.5W one and may damage additional components or pcb tracks etc...

[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]

And even the $10 "POS tester" can do a competent job of simple hFE measurement for matching.

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]

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?

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]

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.

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).

[Richard Crowley]

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

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.

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

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]

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).

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]

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.

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]

I will be replacing all transistors on both channels.

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.

This amp has bridge mode, and I intend to use it.. I figure mixing parts would be a bad thing?

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

[SantaClaw]

I already labeled all the transistors I removed and the spot it came from

I'll have the chance to do more work on this tomorrow.

The uni-t 81c is actually a decent meter for normal measuring, by that I mean when it comes to functions, the oscilloscope part sucks though..  I also have an aneng an8009.

My component tester: https://www.banggood.com/DIY-M12864-Graphics-Version-Transistor-Tester-Kit-LCR-ESR-PWM-p-986954.html
I have two, I built a kit for this case as well https://www.banggood.com/Supporting-Shell-For-DIY-M12864-Graphics-Version-Transistor-Tester-Kit-p-989949.html
As with my meter, I can double check any value with the other one.

Though with regards to the component testers, they are calibrated using a capacitor... and since both where calibrated using the same capacitor, I would think the results would be damn near identical/with the same potential faulty readings.

The amp has been without power for 6 months or more, so I assume all caps are dead, though I'll measure the main filter caps just to be sure. I am a fan of mr carlson, though most of it goes way above my head

[Jwillis]

Arbitrarily replacing transistors is a bit extreme and expensive.

On an earlier post you stated that a "Protection Resistor" keeps blowing .Which one on the schematic is that .What is the reference number?

Bad regulation can cause components to burn up . Bad rectification can cause components to burn up. Shorts can cause components to burn up.
Did you confirm that the output voltages from Main power supply are correct? The schematic shows what voltage should be present at certain points.
I find that printing out the whole schematic helps reduce confusion.

Simply checking the resistance between transistor pins can point you in the right direction .If you get zero resistance between two pins on a transistor, reverse the probes and check again .If it is still  zero ohms that transistor is Shorted . If you get several Kohms between pins and Open or several ohms  when probes are reversed that transistor is probably OK.Move on to next transistor do the same .This can be done with components on board. You do not need to remove the components for this test..

Why do we do it this way? Removing and replacing components over and over Damages the epoxy gluing the traces to the board. So you want to avoid removing components unless you absolutely need to .

We can worry about Pairing transistors or where they come from after we find out which, if any, are at fault.

[SantaClaw]

Arbitrarily replacing transistors is a bit extreme and expensive.

On an earlier post you stated that a "Protection Resistor" keeps blowing .Which one on the schematic is that .What is the reference number?

Bad regulation can cause components to burn up . Bad rectification can cause components to burn up. Shorts can cause components to burn up.
Did you confirm that the output voltages from Main power supply are correct? The schematic shows what voltage should be present at certain points.
I find that printing out the whole schematic helps reduce confusion.

Simply checking the resistance between transistor pins can point you in the right direction .If you get zero resistance between two pins on a transistor, reverse the probes and check again .If it is still  zero ohms that transistor is Shorted . If you get several Kohms between pins and Open or several ohms  when probes are reversed that transistor is probably OK.Move on to next transistor do the same .This can be done with components on board. You do not need to remove the components for this test..

Why do we do it this way? Removing and replacing components over and over Damages the epoxy gluing the traces to the board. So you want to avoid removing components unless you absolutely need to .

We can worry about Pairing transistors or where they come from after we find out which, if any, are at fault.

I was told when this resistor blew, it was proof the transistors where shorted/broken, R345 it's a 82kohm resistor

[Jwillis]

Ok . Work your way from the obvious blown resistor outward .
That resistor ties the bases of Q317 and Q319 .That's a lot of current running out of  the Base of a transistor to fry a 1/2 watt resistor . Check Q317 and Q319 for short between emitters and base and collectors and base.

Looking at the transistors 2SA1478 / 2SC3788 label reading left to right Pin 1 is Emitter Pin 2 (center pin) is Collector and right most Pin 3 is Base .
What are your readings.

Check series Diodes D301,D303,D305,D307 and series diodes D308,D311,D313,D315 with Diode Setting of DMM. Forward voltage of 1N4148 should typically between 550mV and 650mV.

[SantaClaw]

Right, all set up, I'll print out the schematics and start work tomorrow.

[SantaClaw]

Right, time to get to work.

Cleaned it a bit

[SantaClaw]

No connectivity between any pins on Q317 and Q319 other than a short blip. I made sure I had good connections

D301 .636v  - Right Channel: D302 .637v
D303 .640v  - Right Channel: D304 .637v
D305 .637v  - Right Channel: D306 .638v
D307 .640v  - Right Channel: D308 .639v

D309 .631v
D311 .634v
D312 .537v
D313 .638v
D317 .658v
D319 .661v
D321 .594v
D323 .535v
D325 .669v

So the diodes I could find, looks good ?

The small transistors:
C4793:
1 Q333 B=143 Vf = 614mV
2 Q329 B=143 Vf = 614mV
3 Q321 B=173 Vf = 605mV

A1837:
1 Q331 B=160 Vf = 603mV
2 Q335 B=188 Vf = 571mV

So far no fault,  I suppose starting to check resistors is next ?

[SantaClaw]

Any suggestions ?

[Richard Crowley]

No connectivity between any pins on Q317 and Q319 other than a short blip. I made sure I had good connections

Not sure what this means?  What does "no connectivity" mean? 
Tested how?  Out of circuit?
With what? POS tester? with a DMM? with a curve-tracer?
What does "short blip" mean?

So far no fault,  I suppose starting to check resistors is next ?

I would say that "no connectivity" and "short blip" seem to indicate that Q317 and Q319 are blown.
Unless we are misunderstanding your testing methods and results?

What do you find when the corresponding transistors (Q316, Q318) in the other (good) channel measure by the same method?
Out of circuit in all cases.

[SantaClaw]

I put my DMM meter in diode check mode, and it briefly beeped when I measured between the legs.

So I need to de-solder q317 and q319 ?

[james_s]

What DMM? I have an older Fluke that has an audible diode test function where a short beep indicates a good junction and a steady tone is shorted. Every meter works differently though, it's important to know how to properly use your test gear in order to interpret what it's telling you.

What happens when you measure a known good diode?

[SantaClaw]

I used my Anegh 8008

https://www.aliexpress.com/item/ANENG-AN8008-True-RMS-Digital-Multimeter-9999-counts-Square-Wave-Backlight-AC-DC-Voltage-Ammeter-Current/32810125781.html

From what I understand it's a relatively good DMM for it's price and type.

It's 2 in the morn, so I'm going to sleep now.

I do appreciate the help

[james_s]

It may be a very good DMM, I don't know, either way the first step is to learn how the diode test feature works and then you'll know how to interpret the tests you do. Find a good diode and test it with the meter to see how it responds, usually it will display the forward drop which is somewhere around 0.6V for a normal silicon diode.

[Richard Crowley]

I put my DMM meter in diode check mode, and it briefly beeped when I measured between the legs.

That is an extremely crude way of "testing" a transistor.  It is not good enough for the kind of analysis and repairs you are attempting.

In reply # 21 you said "my cheapo component tester."  Perhaps it is time to look properly at your test gear and how you are using it. 

As I mentioned before, this is a high-power, direct-coupled circuit with an expensive set of transistors. It would be easy to make a minor mistake and blow up $100s worth of transistors and other components instantly. You could end up spending much more on replacement transistors than the amplifier is worth.

So I need to de-solder q317 and q319 ?

It is the only way of properly comparing the transistors in the good channel with the equivalent devices in the failed channel.

Repairing a high-power amplifier puts you well above the level of sophistication where you can use a cheap DMM in diode test mode to evaluate transistors.  At best, you might be able to use a diode-test DMM to make a very crude go/no-go test.  But unless you can get something that really tests a transistor using all three leads, you can't really repair this amplifier properly. At least IMHO.  Even one of those $10 component testers (which some people here refer to as "POS") is infinitely better than trying to test a transistor two leads at a time with a "diode test".

[SantaClaw]

Well, I have been using a cheap transistor tester with all the transistors I have de-soldered.

I figured using the DMM was ok for testing for shorts

[Richard Crowley]

Well, I have been using a cheap transistor tester with all the transistors I have de-soldered.

WHAT "cheap transistor tester". Specifically.

I figured using the DMM was ok for testing for shorts

We are past that stage. We need better detail at this stage.
Maybe those transistors are not dead.  But how close are they to their twins over on the good side?

[SantaClaw]

I have two identical kits, one in a nice plastic case, the other just as is.

https://www.banggood.com/Supporting-Shell-For-DIY-M12864-Graphics-Version-Transistor-Tester-Kit-p-989949.html

https://www.banggood.com/DIY-M12864-Graphics-Version-Transistor-Tester-Kit-LCR-ESR-PWM-p-986954.html

Basically, a kit. It seems to be accurate from what I can tell, with repeatable values that measure the same on both kits.

[Richard Crowley]

OK. going forward, we are going to use the component testers to get more detailed intelligence of what is going on inside the transistor package.
1) To confirm that they are behaving properly as a bipolar NPN or PNP transistor (as appropriate for each device)
If they have developed some weird behavior, even those cheap transistor testers will produce results that appear bizarre. That is an indication that the device is no longer behaving the way we want it to do
2) To compare the details (particularly Beta = hFE = gain) for parallel array matching. Specifically those places where there are four power transistors working in parallel. But also to compare how alike the transistors are to their twins over on the good channel for troubleshooting purposes.  That is what we are trying to do now. Compare the failed channel vs. the good channel. Component by component, and starting with the transistors as they are the prime suspects in this mystery.

[SantaClaw]

OK. going forward, we are going to use the component testers to get more detailed intelligence of what is going on inside the transistor package.
1) To confirm that they are behaving properly as a bipolar NPN or PNP transistor (as appropriate for each device)
If they have developed some weird behavior, even those cheap transistor testers will produce results that appear bizarre. That is an indication that the device is no longer behaving the way we want it to do
2) To compare the details (particularly Beta = hFE = gain) for parallel array matching. Specifically those places where there are four power transistors working in parallel. But also to compare how alike the transistors are to their twins over on the good channel for troubleshooting purposes.  That is what we are trying to do now. Compare the failed channel vs. the good channel. Component by component, and starting with the transistors as they are the prime suspects in this mystery.

Right, I'll start de-sodlering, labling, and posting the comparison numbers here.
Thanx for the help, I really appreciate it. As I said I'm a newbie at this.

[Doctorandus_P]

https://octopart.com/search?q=2SC5200