Adcom GFA 555 II keeps blowing the main line in fuse any ideas?

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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!
 
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.
 
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!
 
Ok, so I think I found the busted diode in the following diagram:

http://akdatabase.org/AKview/albums/userpics/10004/Adcom GFA-555II Service.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?
 
Just replaced the EGP50D diodes, but unfortunately the fuse is still blowing. Also, after removing the diodes and testing them again they magically started working (lesson learned, don't test components in circuit, as lanchile mentioned). Now onto the output transistors. I ordered 5x 2SB554 as well as 5x 2SD424 Toshiba transistors from china (these are the same ones in the system now). When I remove them to test them, it will be nice to have replacements on stand by. I will report back in a week or two once I am finished this step. Thanks to everyone who has helped thus far!
 
Just replaced the EGP50D diodes, but unfortunately the fuse is still blowing. Also, after removing the diodes and testing them again they magically started working (lesson learned, don't test components in circuit, as lanchile mentioned). Now onto the output transistors. I ordered 5x 2SB554 as well as 5x 2SD424 Toshiba transistors from china (these are the same ones in the system now). When I remove them to test them, it will be nice to have replacements on stand by. I will report back in a week or two once I am finished this step. Thanks to everyone who has helped thus far!

Live and learn right? do not worry that happens all the time, That is why I mentioned! I have seen some people check for bad caps with E.S.R meters and when they read the caps on the boards they look good, but as soon as they take them out and read them....they read bad!. That is when you should use a multimeter to see if the caps are measuring some kind of short...because you can read other parts like a transformer and you are reading the transformer impedance in your E.S.R meter thinking that cap is fine. But you can always unsolder one leg to measure when you are not sure(caps, resistors, diodes etc) Now you should take those output transistors and check them ...one by one. That is not all the worries...Now you have to check if those output transistors that you ordered from China are ORIGINAL.:confused: Check for the S.O.A.R. I hope they are original transistors other way they will blow in no time or they will work, but not as well as the originals (Performance). Good luck!
 
I don't know how much you paid for the Chinese copies but you don't need that many of them so it's only a couple of bucks more if ordered from Mouser. Any of the 3 pairs I mentioned will work, the MJ21193/4s were developed from Toshiba's technology so your right back with Toshiba, sorta. If the Chinese xsistors are installed and the amp still has problems, is it them or something else? All of the Adcoms are popular for one reason or another so there are MANY threads about them here and other forums. So take some time and do some searching to see what others are doing or suggesting.

Craig
 
I don't know how much you paid for the Chinese copies but you don't need that many of them so it's only a couple of bucks more if ordered from Mouser. Any of the 3 pairs I mentioned will work, the MJ21193/4s were developed from Toshiba's technology so your right back with Toshiba, sorta. If the Chinese xsistors are installed and the amp still has problems, is it them or something else? All of the Adcoms are popular for one reason or another so there are MANY threads about them here and other forums. So take some time and do some searching to see what others are doing or suggesting.

Craig

Thanks for the advice, I cancelled the order and ordered a couple of the recommended output transistors from mouser....

I will report back once I have had a chance to test them all.
 
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