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Old 26th August 2011, 11:04 AM   #1
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Location: Krakow
Question help repairing JVC A-k11 integr amp

Hello!
I repaired many amps
but this is a mysterious for me.
After powering it burns fuse.
After unsoldering output transistors, it works “normal”: the rely switches on and its mode comes from standby to on.
I tried to power it in series with a 60W lamp and marked the EB voltage of Q719 and 720 is 7,9V when Q717/718 have 0,7V.
What is more strange, the voltage at the collectors of Q705/707/706/708 is +34V, should be much lower.
I checked all of the semiconductors and they seems to be OK,
Do you have the idea what might be failed?
Below is the link to service manual, the file has 2 rar parts, after saving the first file you have to save the second part.
JVC A-K11 Service Manual free download,schematics,datasheets,eeprom bins,pcb,repair info for test equipment and electronics
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Old 26th August 2011, 01:01 PM   #2
wahab is offline wahab  Algeria
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Quote:
Originally Posted by padamiecki View Post

What is more strange, the voltage at the collectors of Q705/707/706/708 is +34V, should be much lower.
If these transistors are the current mirror ones , the voltage is normal.

Does this amp have the same power part schematic that the RK11 receiver ?
As this latter one has voltages specified in the schematic.

JVC R-K11 | Owners Manual, Service Manual, Schematics, Free Download | HiFi Engine
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Old 26th August 2011, 01:12 PM   #3
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Quote:
Originally Posted by wahab View Post
If these transistors are the current mirror ones , the voltage is normal.
no they are VAS trans, on their collectors should be ~+2V.

Quote:
Originally Posted by wahab View Post
Does this amp have the same power part schematic that the RK11 receiver ?
I guess yes, on the rear panel it has the sockets to power outside tuner.
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Old 26th August 2011, 01:40 PM   #4
wahab is offline wahab  Algeria
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Can you post the amp s schematic ?..
My unzipper doesnt work correctly with the file you gave, and i tested two, with no sucess, as Acrobat says that one of the two compressed files is corrupted...
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Old 26th August 2011, 01:54 PM   #5
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Quote:
Originally Posted by wahab View Post
Can you post the amp s schematic ?..
My unzipper doesnt work correctly with the file you gave, and i tested two, with no sucess, as Acrobat says that one of the two compressed files is corrupted...
of course, please find attached, maybe you downloaded only 1 file?
Attached Images
File Type: jpg Picture2.jpg (99.3 KB, 61 views)
File Type: jpg Picture1.jpg (146.9 KB, 62 views)
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Old 26th August 2011, 06:17 PM   #6
wahab is offline wahab  Algeria
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Quote:
Originally Posted by padamiecki View Post
the EB voltage of Q719 and 720 is 7,9V
Are you sure of that 7.9V EB value ??

It cant be more than about 0.7V if the transistor is functionnal.
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Old 27th August 2011, 07:56 AM   #7
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Quote:
Originally Posted by wahab View Post
Are you sure of that 7.9V EB value ??

It cant be more than about 0.7V if the transistor is functionnal.
hm,
after this try I did not check if they are ok...
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Old 29th August 2011, 01:26 PM   #8
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Quote:
Originally Posted by padamiecki View Post
hm,
after this try I did not check if they are ok...
checked:
they are ok...
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Old 22nd September 2011, 02:11 PM   #9
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Hi
I am going mad,
all of the parts are ok...
Please look at simplified schematic with measured voltages (with output transistors unsoldered).
At VAS collector is far more voltage than needed.
Is it possible that the transistor when unsoldered measures ok and under certain voltage is broken?
What part should I check one more time?
Attached Images
File Type: jpg Obraz7.jpg (94.2 KB, 18 views)
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