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Old 29th May 2013, 01:31 PM   #1
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Unhappy Adcom GFA 555 II keeps blowing the main line in fuse any ideas?

Here is the situation. I was given this amp by a friend who replaced one of the fuses with one of incorrect voltage (the main, line in fuse). Here is a picture of the back of the amp for reference (it also shows the fuse ratings):
imgur: the simple image sharer
I also noticed one of the smaller capacitors seemed to explode:
imgur: the simple image sharer
Here is another picture of it:
imgur: the simple image sharer
This lead me to replace the 4 matching capacitors of this type. I bought high end Nichicon capacitors that matched the 47uF 160V of the busted capacitor. Here is a picture of the amp post swap:
imgur: the simple image sharer
I also replaced all fuses, with the correct type (AGC-6A/250v for the DC line, and 4 AGC-7A/250V for the left/right channels)
Now, whenever I turn it on, the power indicator flashes for a split second, and immediately after it turns on, the DC line fuse blows.
Any input as to why this is happening will be tremendously appreciated.
If you want to know anything else about the amp, let me know!
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Old 29th May 2013, 01:36 PM   #2
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Also, I found the circuitry diagram for this amp, if it helps:

http://www.drmarksays.com/wp-content...55mkii_sch.pdf
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Old 29th May 2013, 01:46 PM   #3
srinath is offline srinath  United States
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Voltage of the fuse isn't a big issue as long as it was higher. You can replace a 2 amp 125v fuse with a 2 A 250v - no problem. That voltage is the voltage @ which the plasma inside the fuse is likely to conduct. So a 125v fuse will be a straight wire @ 125V and up. For a 250, that is @ 250V, you got nothing to lose.

Your problem is likely in the output transistors. Them metal eyelet looking things. Get a transistor tester and test away. This amp is a bit like my flame linear 400 in its construction. It may be prone to some of them same failures.

Cool.
Srinath.
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Old 29th May 2013, 02:07 PM   #4
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I believe my current multimeter can test transistors, I will try this. Thanks!
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Old 29th May 2013, 03:43 PM   #5
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Disconnect all output transistors from amp board then check one by one, I say this because sometimes you read something else in the circuit and can confuse your readings, also check for shorted diodes. Good luck.
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Old 29th May 2013, 06:34 PM   #6
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Quote:
Originally Posted by lanchile View Post
Disconnect all output transistors from amp board then check one by one, I say this because sometimes you read something else in the circuit and can confuse your readings, also check for shorted diodes. Good luck.
I think you may be on to something. The diode that was attached to the visibly busted capacitor is not showing any voltage. the other 3 matching diodes have 0.1 V reading or less. Is 0.1v normal? Any advice on replacement diodes? I am not sure what kind are used. It looks like the markings on the diode say GP5110 but I am not certain...

On a related note, is there such a thing as audio grade diodes? I know capacitors are particularly important towards sound quality, but do diodes matter?

I have also gone through a number of the transistors already and they appear to be working properly (although I have merely inferred whether they are PNP or NPN based on voltage readings, and haven't looked up the actual transistors). I am getting about 0.6 volts registering on expected transistor circuits.


Fun stuff!
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Old 30th May 2013, 12:38 PM   #7
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Ok, so I think I found the busted diode in the following diagram:

http://akdatabase.org/AKview/albums/...%20Service.pdf

I need to find 4 EPG50D diodes (to replace all 4 matching ones)

Also, I used a more accurate fluke multimeter to test the diode again. It actually does register a tiny voltage, about 0.01v coming from BOTH directions. Definitely looks busted.

A cursory google search did not turn up anything. Does anyone have any advice on a replacement for this diode?
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Old 30th May 2013, 03:33 PM   #8
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Found this:

Radiobox - ???????/????????????? ?????????????? ????? EGP50D EGP50G

Yikes.... Anyone have any advice on how to proceed? Alternative places to buy the diode? Anyone know Russian?
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Old 30th May 2013, 04:11 PM   #9
llwhtt is offline llwhtt  United States
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Mouser Electronics has them.

Craig
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Old 30th May 2013, 04:22 PM   #10
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Quote:
Originally Posted by burdenofproof View Post
Found this:

Radiobox - ???????/????????????? ?????????????? ????? EGP50D EGP50G

Yikes.... Anyone have any advice on how to proceed? Alternative places to buy the diode? Anyone know Russian?
For diodes EGP50D you can find them at Digikey, they are cheap diodes ($ 2 USD) the problem is the shipping and handling...I guess you have to buy more parts
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