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Old 29th January 2014, 03:51 PM   #1
ishiru is offline ishiru  Indonesia
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Default amp went protect after repair

hi there..
lately, my grandpa asked me to repair his car amp (power atlandis dc.350 which i guess it's korean/chinese amp)

i didnt test it first since i saw this amp was horribly repaired w/ someone else before (psu caps still in long leads w/o any heatshrink, fake transistor replacement, (maybe) fake smps mosfet, crappy caps everywhere which they supposed to be nichicons like what i owned before)


okay..so..i changed the 2n3055/2955 w/ njw0302/3281 original pairs, changinG the driver to mjl15032/33, replace the caps w/ panasonic FCs, and some muse in the input caps..

at the first, i exploded the negative rail caps.guess why?.the polarity on the PCB was reversed!goddamnit!

okay..change the exploded caps, they turned on flawlessly..

next, i measured the dc offset to make sure aaannnddd...another shock, dc offset was -19v both channel!


some checks leads me to the VAS driver was failed..so i ordered some BC546/556 pairs..
after the replacement, they went wrong again!.now, it's my fault to used the wrong tr on 1 side

pls note, i'm doing this only 1 channel..i just couldnt repair both at the same time

okay..check, check, turn on, and measureee...tada!..100mv offset.


now go to next channel, everything good...and check it for dc offset...


i gooootttt...-19v!.argh!


i tried to remove the input jumper from the tone controls to make sure if it's causing the dc offset but then, the amp went protect

tried to remove the diodes and smps mosfet but no luck..

my question :

why i got dc offset in both channel just after i done the another channel meanwhile i done only 1 channel, it works flawlessly?

at the 2nd channel, i got warmer finals than the 1st..is it caused by bias transistor?..any good recommendation for this?

and why the smps suddenly failed?..i didnt shorted any supply and i'm sure there's no shortage on my amp section.or maybe some of the diodes shorted and causing the tl494 broken?..

there's driver transistor for the irfz44n, i'll check it on friday..that amp was left on my workplace and i have something to do tomorrow..so i couldnt take any measurement tomorrow


last, i'm sorry if my post was waaaay tooo damn long

-add-

this amp has no bias and offset trimpots..
bias is easy to add, but how about the offset trimpot?..i have ('87?) krell ksa-50 schematic and there's simple offset adjustment circuit.is it applicable on my amp?..

thanks

Last edited by ishiru; 29th January 2014 at 04:19 PM.
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Old 1st February 2014, 05:23 AM   #2
timwebb is offline timwebb  United States
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pic of board?
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Old 1st February 2014, 04:19 PM   #3
ishiru is offline ishiru  Indonesia
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sorry for late reply!

i'll take it monday.it still on my workdesk

just in case it just the driver tr was broken, is it okay to use BC546 (or BC556?)?

big thanks!
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Old 3rd February 2014, 11:22 AM   #4
ishiru is offline ishiru  Indonesia
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here is the picture of the board
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