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 Post subject: amp repair
PostPosted: 03 Feb 2012, 16:08 

Joined: 30 Dec 2011, 13:27
Posts: 2
Location: Novi Sad, Serbia
Hello everyone !
I hope you can help me with diagnosis and repair of my hitachi ha-4800 amp.
So far I managed to find that both STK 8250 and STK 3062 burned, and also the output transistors on both chanels (Q701L/R, Q702L/R, Q901L/R). I managed to find replacement parts. All parts are the same except STK3062. I found STK 3062 III.
I replaced all the parts except STK8250 on both chanels.
When I turned on amp, protection circuit turned on relay. Next thing was to measure voltage on STK3062III pins, and readings looked like this:
4= 42.5
5= -42.5
7= -43
9= 43
11= -42.8
12= 42.9
13= 42.1
14= -0.9
15= -0.20

I thought that readings are too much unequal, so my next assoumption was that some capacitor is broken. Since i really don't know how to find broken capacitor, I decided to change them all in a power amp block of both chanels. I thought that the idea of ​​changing all the capacitors is good, since they have worked in amp for over 30 years :eek: ... so, I changed all 28 capacitors :eek: ....

During that capacitor changing, I discovered some irregularities on schematics and printboard. On schematics, pin 4, on STK 3062, is attached to pin 6 over cap C713R. In reality, pin 4 is attached to pin 5 over cap C713R. Left chanel is just like on schematics. In datasheet of STK3062, aplication circuit confirmed that this must be mystake by manufacturer, so I changed that on printboard. I reely don't know did I made a mystake by changing that. Few schematics pointed that this was right thing to do, and now, L and R chanels are same...

After changing of capacitors , i took measurment again, and reading on STK3062III was like this:

1= 0
2= 0.03
3= 42.6
4= 43.5
5= -43.5
11= -43.5
12= 42.9
13= 42.2
14= 0.03
15= 0

How i had equal measurments on both sides of STK, so I decided that I can install STK 8250 on right chanell and give it a try...
I turned power on, and after few seconds I heard relays "click" :thumbsup: !!! So, I turned it off, and attached a speaker and sourse. I turned it on again, relay clicked again, and after few seconds fuse blowed !!! :mad: :hot: ..... It was clear to me that STK8250 burned again... so I desolder him, and discovered that it is shorten between pins 2 - 3, and 8 - 9 ....

Now I'm on dead end, and reely don't know what to do! :bawling: I reely have no idea what did I do wrong ?!

Can someone help me with this pleeeease ! :(

Thanks ! :)

P.S. I hope that my problem description is't too much confusing ... :)

P.P.S. On this link you can find amp's schematics: ... -4800.html

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