Transistor Leads That Turned Black

MarkMeyer

MarkMeyer

Active Member
On a couple of boards in my under-repair (plus total recap) AU-9500, there are several transistors where the leads have turned black over time.
Specifically, three boards that are like that are the F-2006 equalizer board, and the two F-2029 driver boards.

Black leads on components are generally an indication of overheating, but neither the transistor bodies themselves nor the entire remainder of the board or it's components show any sign at all of toasting.
Other than the black transistor leads, the boards could pass as brand new condition (visually).

As a general question; if these transistors test good, am I better off leaving them on the board?... or should I plan on replacing them, having to go through all the usual time consuming investigations trying to find a proper substititute?

TIA
 
If you have a schematic, and figure out what the voltages should be between the leads, and you measure those approximate voltages, you should be good-to-go.
 
I have seen a few transistors with what looked more like oxidation the heat related color changes on Sansui's.
The TO 220's were pretty dark on the tab. But seemed to have no effect on there operation.



Barney
 
jetblack said:
If you have a schematic, and figure out what the voltages should be between the leads, and you measure those approximate voltages, you should be good-to-go.
Click to expand...

Well, that's not really a concern at this point, because I'll do what I have to do to check the various voltages when the time comes, but what I'm wondering right now is if I should plan on replacing those darkened-lead transistors just for the heck of it even if all the voltages fall into place.

What I'm saying is: If the black leads are an indication of past overheating, then should I assume that those transistors may be in marginal condition ("ready" to fail) even if they test good and the voltages in their circuits are good?
Or, are transistors robust enough that even if they saw some heat in the past, that they should be OK for the long term as long as they're in spec?
 
Black leads are a sign of the corrosion of the tinned part of the leads. This can happen at any temperature, but happens more easily at high temperatures.
 
In that case, the black leads aren't necessarily an indication of the transistor itself being heat stressed, so if they pass a diode check and the voltages in the circuits are good, I'm prone to leave them alone.

Thanks guys!
 
transistors

MarkMeyer said:
In that case, the black leads aren't necessarily an indication of the transistor itself being heat stressed, so if they pass a diode check and the voltages in the circuits are good, I'm prone to leave them alone.

Thanks guys!
Click to expand...

I would agree, try something maybe...put a small amount of tarnex or any silver polish on a qtip and swab a blackened lead...see if it doesn't clean right off. Would Sansui use silver solder on high temp joints like a transistor may be?
Casey:scratch2:
 
Tarnex has sulfuric acid in it, and I would not let it near a PC board. Brasso is 100x safer.

No silver solder on vintage gear. Or 99% or new gear either, for that matter.
 
I've found in the past with experience that transistors with black legs generally lead to becoming noisy or give trouble eventually, also depends what part of the circuit they are in. Preamp/phono stages are prone to this.

I have read that the black corrosion eats it way into the junction. I guess that cleaning the legs with brasso is definetly a good idea to slow the eventual breakdown.

As for me I just change 'em, it saves having to open the thing up again at a later stage when one channel starts hissing or crackling!::smoke:

Just my two cents worth :)
Cheers
Glen.
 
Glenz75 said:
I've found in the past with experience that transistors with black legs generally lead to becoming noisy or give trouble eventually, also depends what part of the circuit they are in. Preamp/phono stages are prone to this.

I have read that the black corrosion eats it way into the junction. I guess that cleaning the legs with brasso is definetly a good idea to slow the eventual breakdown.

As for me I just change 'em, it saves having to open the thing up again at a later stage when one channel starts hissing or crackling!::smoke:

Just my two cents worth :)
Cheers
Glen.
Click to expand...

Those were all my original thoughts, so I'm back to square one at changing them out.
Of course, changing them out is child's play. It's the agony of going through all the specs and finding the right ones that turns out to be most of the work.

I continually fear that a wrong choice will affect the sonic performance of the amp, so if I can leave alone what Sansui did, I like to do so.
On the other hand, if tarnished leads will eventually lead to signal or power degradation over time, then it seems I really do need to change them out even if the transistor itself isn't suffering any heat damage.

But yea, I'm still having fun. :sigh:
 
IHMO, black leads aren't a sign of anything but black leads. I've seen no correlation between these mildly corroded leads and transistor noise & failure (other than many failed transistors are old, and a lot of old transistors have black leads). The tin plating (essentially solder) that covers the leads is not present in the internals of the device (devices are bonded internally, not soldered), so the mechanism that allowed the corrosion to take place ends at the transistor body.

If it really bugs you, get some Brasso and a pile 'O Q-Tips and get busy (and I'll admit I've done this on some boards to improve the appearance of the components and the board in general).

If you're simply looking for justification to replace the parts with new ones to make you feel better, that's fine. Go for it.
 
EchoWars said:
IHMO, black leads aren't a sign of anything but black leads. I've seen no correlation between these mildly corroded leads and transistor noise & failure (other than many failed transistors are old, and a lot of old transistors have black leads). The tin plating (essentially solder) that covers the leads is not present in the internals of the device (devices are bonded internally, not soldered), so the mechanism that allowed the corrosion to take place ends at the transistor body.

If it really bugs you, get some Brasso and a pile 'O Q-Tips and get busy (and I'll admit I've done this on some boards to improve the appearance of the components and the board in general).

If you're simply looking for justification to replace the parts with new ones to make you feel better, that's fine. Go for it.
Click to expand...

Hi, you have raised some good points about this issue, and very true that the transistors we are looking at and questioning are now pushing 35 years plus so there are bound to be issues with some of the transistors just being 'old' and not because of the black leg syndrome.
I've come across several transistors in gear that I've serviced that are noisy and no sign of the corrosion anywhere on the leads so I guess its just luck of the draw really, especially with older hifi, there are bound to be problems with aging components.... This discussion could go on for quite a while..hehe... Gee I hope I haven't opened up another can of worms!:D

Cheers
Glen
 
I remember years ago being in a shop and seeing a tech pulling transistors from the preamp of an AU20000. He was dipping them in muratic acid to clean the black off the legs. He said it would cause noise in the circuit. Dont know for sure if he knew what he was talking about, but I thought I would throw that in...
 
Silver solder

EchoWars said:
Tarnex has sulfuric acid in it, and I would not let it near a PC board. Brasso is 100x safer.

No silver solder on vintage gear. Or 99% or new gear either, for that matter.
Click to expand...

That makes sense...I didn't know it had sulfuric acid as an ingredient.
Casey
 
BrandXRC said:
I remember years ago being in a shop and seeing a tech pulling transistors from the preamp of an AU20000. He was dipping them in muratic acid to clean the black off the legs. He said it would cause noise in the circuit. Dont know for sure if he knew what he was talking about, but I thought I would throw that in...
Click to expand...

Interesting way of cleaning the legs, I wonder how his fingertips took it?:thmbsp: I'm sure he used a small pair of pliers or tweezers!

Cheers
Glen
 
I've read read several things about the blackened legs, got a bit worried, was dubbing on replacing them because.
So I tested them, hFE was well within spec, no leakage current, cracked them open (C1124 and c1364), too see if the oxidation travels into the substrate :bs:

Imo no indication on the state of the transistor other than the material used to plate them oxidates due to X factors
Look at the pics.
 

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EchoWars said:
No surprise here...oxidation ends where the encapsulation starts.
Click to expand...

Where is the evidence for your statement?

Ever see rust eat it's way through metal under paint? One would think the bond between the transistor case and metal leads would be compromised with incurring oxidation thus compromising the hermetic seal even further. Theoretically anyway, the corrosion could work it's way into the body of the semiconductor and compromise the silicon doping. In addition, the tiny internal wires that connect the component leads of a transistor to the silicon "chip" in the device could degrade as a result. I don't think it's outrageous at all to believe that and certainly, high-volume shops have already come to that conclusion. Is there a correlation? I bet there is.
It also begs the question why some do it and other's not? Quality of tinning?
 
Could this be akin to the unsolved mystery of tin whiskers? Eventually, everything will revert to its elemental state, but the question is whether it will take tens, hundreds, thousands, or millions of years.
 
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