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Old 9th December 2007, 05:48 PM   #1
jol50 is offline jol50  United States
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Default substance on board, corrosion

I have an amp I been working on for a while that is becoming a sad story. Skip the next paragraph if you don't want the history.

It is an audiobahn 4ch and one channel was blown. I replaced the outputs and the resistors all looked good (I have new gate R to put in yet) and then found one of the power supply 220s' gate was soldered to ground (!) I know some of you see this but still amazing, and this amp was used for some time by the looks of it. The 4 220s are driven in pairs so the next one may have had its gate pulled down, anyway fixed and it checked good so I tried it on this 2.7 ohm resistor for power.

It idles at 10.4v off 12.6v supply and all is ok but have 60mv on the new channel on 20v rails, about 3mv on other channels or so. So I look upstream of the gates and in the center of the board under the fan I see something, some stuff on the board. Have not taken board back out yet to get fan off but it is under a couple of...bipolars? Never messed with them but the round black three leg with a flat on side...sorry... I wondered if they might have puked out stuff? It may have come in fan but fan/etc is very clean...maybe water? So think I will have to repopulate that area about size of small fingerprint.

I tested voltage up gate circuit and it is different than other channels up to this ugly area with a little dried residue on it. I can't get directly to it until the fan mount is removed. The gate resistors show same ohms (~4 ohms) unless they are bad under power and changing the whole circuit, still this area does not look good. Some of the resistors have corrosion on their legs, it is near center of board but op-amps are over with crossover filters to end of board.

Didn't try to play music, nothing got hot.
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Old 9th December 2007, 08:58 PM   #2
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I think there may be some confusion with BJTs/FETs here. Unless this Audiobahn amp is different from all of the others I've worked on, it uses BJTs for the output transistors and FETs in the power supply.

If that' the case here...
BJTs don't have gates. The 'input' terminal is the 'base'. The terminals are base, collector and emitter. If you are not sure what type of transistor you have, look up the datasheet and look at the lablels on the terminals.

The case style has no bearing on the type of transistor. The small TO-92 cased components can be BJTs, FETs, JFETS or a number of other types of devices like voltage regulators. Again, you need to look up the datasheet for the device.

For components that don't have the full part number, you may need to add a '2S' ot 'KT' prefix. Most of the older transistors use a 2S prefix. If the transistor has a large K on it, you should also try the KT prefix. For example, if the number on the transistor is B817, you would look up the datasheet for the 2SB817. To make sure that the 2S and KT devices are the same (they are not always the same), you would need to look up the KTB817 also.

If the 'stuff' is brown or green and looks powdery, it's probably corrosion (the board probably got wet). If there is corrosion, you need to clean the area thoroughly. Many times, when the board gets corroded, traces open which can cause all sorts of problems.

What model amp is this?
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Old 9th December 2007, 11:34 PM   #3
jol50 is offline jol50  United States
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I have no idea what they are and can't really see them under the fan, though I didn't try real hard either. It is a a4004t. I'm not familiar with some of the input components. Outputs I put in are BJT, I used NTE replacements for those #s.

It is green and powdery, I know a cap can leak but didn't know if anything else would. It could then be a few drops of water I guess, seems like soda or something would have been on the fan/case in order to get down there. With the PS towards you, it would be the lower right channel and problem is under lower right of fan. Ok, let me get a pic of that and the pa2 hold on.
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Old 10th December 2007, 02:18 AM   #4
jol50 is offline jol50  United States
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I replaced the two outputs off top of pic, SK C4466/A1693 with NTEs. Sorry, not a gate resistor for those.

What I mean is the devices in the area of gunk here, wondered if they made it or it is outside substance. (under fan, fan is in center of board)

Click the image to open in full size.

Here is the slight solder problem I found in the power supply grounding the #1 pin and maybe the next mosfet, pretty sure these are IFRZ something my PC is not near work area. You can see the traces for the gate resistors two on left and two on right though maybe it did not affect the one next to it, but must have shut this one off:
Click the image to open in full size.
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Old 10th December 2007, 02:45 AM   #5
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The green stuff is oxidized copper. It's rare to see it on an area of the board where there is no copper. It didn't come out of the transistors but it could be that the leads ozidized and somehow, the oxidation made it onto the board.

If the leads are the source of the oxidation, you should replace the transistors. If you clean the board well (would probably require removing the transistors), you may be able to do further troubleshooting with these transistors. For long term reliability, replace any components that show signs of severe oxidation on the leads.
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Old 10th December 2007, 02:57 AM   #6
ppia600 is offline ppia600  United States
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Looks like something dripped onto the board through the fan blades. Whatever it is probably was acidic and in combonation witht the voltage between the transistor leads corroded some of the copper from the surrounding components and some of the exposed through holes, to make the green stuff. Probably a couple of drops of soda. Cool that you found the grounded fet, haha it worked all this time one fet short. I wonder if quality control really sucked that bad or if someone has already repaired it before.
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Old 10th December 2007, 03:13 AM   #7
jol50 is offline jol50  United States
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It sure does look like corroded copper, it is harder to see in person. I'll get back to it soon, had to put a chair in my work area that is softer to sit on than the previous sub box...don't ask. Just use a contact cleaner on this? A swab?

I'd have to look but pretty sure it was still glued on the clamps. They got sloppy adding all that solder to the board is my guess, I don't know why anyone would reflow all that if they put PS transistors in it. They look like it was all done at once....with an iron much larger than the 30w I use because it will not melt it like that.

I also added solder to the transformer legs, I could see some of the wire sticking out though it likely had enough on it. Had to see in pic but on left side was bare spots below the shorted transistor.

What was I thinking...there is electricity in there lol, sure that would accelerate it.

If it does not zap me there must not be power in it...
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Old 10th December 2007, 03:21 AM   #8
ppia600 is offline ppia600  United States
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I would use a cotton swab with a little wd40 or liquid wrench on it. If its copper remnants those will wipe it right off.
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Old 10th December 2007, 03:26 AM   #9
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A brush (toothbrush or something with relatively stiff bristles) and contact cleaner would work.


I suspect that the amps don't get true QC. Sometimes, it seems that the QC labels are for the sheetmetal cover or heatsink and have nothing to do with the electronics.
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Old 10th December 2007, 03:35 AM   #10
ppia600 is offline ppia600  United States
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Quote:
Originally posted by Perry Babin
A brush (toothbrush or something with relatively stiff bristles) and contact cleaner would work.


I suspect that the amps don't get true QC. Sometimes, it seems that the QC labels are for the sheetmetal cover or heatsink and have nothing to do with the electronics.

Haha, yeah the chrome has to shine! Back when we sold audiobahn, I seriously think most of the problems they had were simple QC flaws. They actually had good performance for the price.
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