Output transistors replacement - Need advice

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Hello guys,
This is my first post here, recently i've accidentally blown up two output transistors from my Marantz PM 7003 integrated amp. Unfortunately, there is no officially authorized Marantz service where i live so i went to a local repair service. I took back the amp several days ago and everything works fine, i cant see any difference except a bit of more heat while the amp is idle, but that's is
not what is bothering me. Yesterday i've decided to open the amp to see if the transistors are replaced with the original ones. I don't have any knowledge when it comes about transistors and repairs but if i'm not mistaken according to the service manual (https://bit.ly/2QNy5TD) the transistors should be two pairs of 2SC2837 NPN and 2SA1186 PNP. But when i opened the amp i realized that there is one pair of the original 2SC2837/2SA1186 transistors and one pair of 2SC3856/2SA1492 (images of the transistors).
My question is: Are these transistors worse than the original. Should i replace them with the originals?
 
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Don't worry. The replacement numbers you supplied are pretty close to the ratings that the originals were. Some ratings are slightly higher, but that won't make any difference.

The big question would be, are they real, or remarked (fake) transistors.

The higher temperature is a concern. It might indicate that the bias current is not set correctly, or if it is, they didn't check the other channel.

-Chris
 
Don't worry. The replacement numbers you supplied are pretty close to the ratings that the originals were. Some ratings are slightly higher, but that won't make any difference.

The big question would be, are they real, or remarked (fake) transistors.

The higher temperature is a concern. It might indicate that the bias current is not set correctly, or if it is, they didn't check the other channel.

-Chris

Hi Chris,
Thanks for your replay. "Some ratings are slightly higher, but that won't make any difference" - What does mean this? Are higher ratings always better/worse?

"The big question would be, are they real, or remarked (fake) transistors.". Is there any way i can check this?

The higher temperature is a concern. It might indicate that the bias current is not set correctly, or if it is, they didn't check the other channel.. I'm not 100% sure if the temperature is higher. The temperature on top on the grid of the amplifier chassis is around 35-40 Celsius while idle or listening at very low volume level. However, before the service i think the chassis was always cold at this level.

Should i visit the service once again?
 
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You could ask them after checking both sides for equal temperature. If the temperatures are different at idle, let them know.

The replacement transistors are "better" than the originals. This isn't true as better might refer to the quality they are made with. I am comfortable using them as a substitute.

Real or fake? Unless it is really obvious, this is not something you would be expected to know enough about. All you can do is trust that the shop knows enough about parts to be able to get the real thing.Re-marked transistors are a big problem. There area couple threads here that deal with this issue along with examples.
The last amplifier I worked on had fake transistors installed on both channels. The original repair was for a blown channel. They also caused a lot of circuit board damage. That technician was not qualified to work on this (Marantz) amplifier.

It's a real shame to see poor workmanship on a good product. It is also a lot of extra work for me in this case, more than I can charge for.

-Chris
 
The answer to the big question can only be answered two ways. Take them out and test them (compare to known good) or the smoke test. If it survives playing loud they are real. Neither is without risk.

The most likely scenario is the originals were unobtainium, and he got the subs from digikey or wherever. Service information may not be available to anyone other than factory authorized. Probably just set the bias to what is typically optimum, which will produce some heat and is harmless if the amp is well designed. The factory bias setting may very well have been below optimum (for distortion) which would make it idle cool. There may have been a reason for that - anything from reduced assembly cost to lazy to having problems with thermal runaway requiring a low bias. If the temp is stable over many hours, and returns to that safely after being temporarily played loud and run hot, there is probably nothing to worry about. If you know something about amps it’s easy to make these judgement calls, but the lay person not so much. Which is why you need a reputable shop.
 
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Hi wg_ski,
A straight repair should leave the bias current set to manufacturer recommended specs. Marantz didn't run the outputs too cold, and the optimum bias point is often less than what some folks would suspect. The closer the match is on the outputs, the lower bias current is needed. I have proved this on my own bench.

celestialsound,
The written worksheet should detail what was done to repair the amplifier. If outputs were blown and they didn't replace the drivers, it wasn't a proper repair. I was a Marantz authorized repair centre along with many other high end brands.

Understand that without expert knowledge, you have to trust teh shop. This could be good or bad. One thing is for sure, if the work isn't neat, the repair wasn't properly done.

-Chris
 
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There is a difference between the transistors types that should not have been ignored if the repairer knew anything about Sanken products, regardless of whether or not you can hear or measure any difference between them.

The original C2837/A1186 output pairs are LAPT type (Sanken's unique multiple discrete transistors on the die) which results in the best linearity from audio transistors that you can buy. The replacements are standard, triple diffused audio types like other manufacturers. Good and cheaper but not the same high quality device.

Because both types of transistor are in stock at Digi-key, there isn't a compelling reason to substitute, apart from needing to order the parts and the delay. Perhaps the repairer just figured that was more important than the correct type of parts?
 
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@anatech,
I've measured the temperatures on the both sides of the heat sink and they are almost equal (0.1 'C - 0.3 'C). I've turned the volume know above half of the maximum and waited for about half an hour. The temperature was 37.6 'C on the left side and 37.4 'C on the other side. Regarding the repairing work i didn't get any worksheet detail on what was done so i guess they didn't replace the drivers. I was told that only the output transistors were replaced and was charged 40 euros. If you ask yourself how did i blown the output, I've accidentally torn away the cable from the speaker (with my leg) while the amp was driving at very loud level and made short circuit between the plus and minus from the cable. IF you can judge by eye :) here is picture of the old and the replaced transistors https://imageshack.com/a/img923/2447/5GHkC1.jpg

Hi @wg_ski,
I've done several times "smoke test" listening at very loud levels but not for long time. As expected, the amp temperature would rise maybe twice and more at those levels but once i turn the volume down it begins to cool. I guess that's right.
 
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Hi celestialsound,
Well, it works. It seems to perform normally.

It doesn't look like the grease has been replaced and the installation is messy. That fits with the low repair price. I doubt they replaced the drivers.

Hi Ian,
True, but I doubt that will make any difference. Especially at audio frequencies.

-Chris
 
The replacements are standard, triple diffused audio types like other manufacturers. Good and cheaper but not the same high quality device.

Because both types of transistor are in stock at Digi-key, there isn't a compelling reason to substitute, apart from needing to order the parts and the delay. Perhaps the repairer just figured that was more important than the correct type of parts?

Perhaps just keeps one type in stock. If so I would opt for LAPT’’s even if they are a buck or two more. Chances are he didn’t know the difference and was just going by ratings. If you’re going to do that you may as well just buy Fairchild C5200N equivalents or ON 3281’s. If you’re going to pay for Sanken you might as well get the best ones they have. You can get 130 watt 15 or even 17 amp LAPT’s. At least these were all TO3Ps - and didn’t have to deal with the MT200 issue where you simply can’t get something that’s as good... period.

The amp will probably work fine with a normal load and triple diffused outputs. They are however less rugged and more likely to die instantly on a shorted speaker wire. The higher rating may be enough extra protection, but then again might not.
 
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I fail to understand why you would replace the outputs on this amplifier. There is more detailed work that should have been done, but the armchair quarterbacking on the output devices is just silly. Even to the point of second guessing the repair shop's motives.

Right now you have a working amplifier. Don't futz with it unless you are going to take it to a very well regarded shop where they will do the whole job correctly. I would have replaced the driver transistors, and also the diff pair unless they are JFet types. There is zero wrong with the output transistors that were used and they won't fail faster than the originals. Remember those? They failed when that output was shorted. I think that "blow easier" might be measured in msec. and not include parameter shift in the characteristics.

The biggest question is, are the output devices they used fakes or not. Well, you pounded the amplifier and it survived. About output device speed. 5 MHz is still much faster than 100 KHz (accepted audio bandwidth), so worrying about the fT of the outputs is really picking to find faults.

So, at this point, I would really highly recommend you just use it and enjoy the amp.
Messing around with outputs could easily result in a non-functioning amplifier. THe only other alternative would be to ship your amplifier to a really good shop and tell them what has happened so far and request that they rebuild it properly. I would only do this if the amplifier blows up again.

- Chris
 
You should be able to short an amp without blowing it up but I notice most DIYA projects do not include protection. I guess rebuilding amps is part of the fun?:rolleyes:
Looking at the Marantz service manual, it's s bit over complicated, and although it does have over-current sensing, the signal does not immediately limit the amp. Instead, it wanders from module to module and ends up at the micro controller, which I suppose disconnects the speaker relays??? If that is correct then I'm not surprised that by that time your output is dead. I'm sure it's fast enough to save your speakers. Maybe I belong to the "less is more" club. :D
 
Vi limiters have fallen out of favor in recent years. Or is it decades? Why? Beacause if they are actually set up to do their job you quickly find the need for more than one pair of output transistors. Use a speaker load that’s any more than one single driver whose impedance is higher than the nominal “8 ohms” over most of its range and it will be running into limit often enough to be noticeable and annoying. So you either have to double or triple up on transistors or relax (or dispense with) the limiting. You see which path manufacturers have chosen to go down. Modern transitions tend to be more rugged than they were in the 70’s but it’s still a gamble.
 
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Hi Steveu,
You should be able to short an amp without blowing it up
No, that is incorrect. You can overbuild an amplifier and it might still blow up if you short the outputs. Pro amplifiers often survive that abuse, but they are designed not to fail, not for sound quality. I have repaired many large well made professional amplifiers over the years. Speaker loads generally shorted by tripped cords, or driver burnout. Carver amplifiers stood a good chance of surviving those problems as one example. They are very complicated too.
Looking at the Marantz service manual, it's s bit over complicated
Huh? How so? Often those "over complicated" amplifiers have better sound quality as the complications linearize the active stages or shield them from supply noise.

Have you got Douglas Self's books on amplifier design? How about Bob Cordell's? They would provide a good explanation as to why you find some amplifier designs ... "complicated".

-Chris
 
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