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Help with bias adjustment.
Help with bias adjustment.
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Old 9th December 2012, 09:59 PM   #1
victo is offline victo
diyAudio Member
Join Date: Dec 2012
Default Help with bias adjustment.

I recently bought a JVC A-X9 amp. There is no problem with the sound and no difference between the channels. But one of the channel's heat sink gets much hotter than the other. I checked the DC offset and bias voltages as described in the service manual. I am able to adjust the dc offset for both channels. When it comes to bias, I can adjust the bias of the channel which is getting hotter, but the cooler channel's bias pot is not adjusting, the mV reading from the test point is always constant.

All the capacitors are original and I cant see any leaking ones.

What to look for? What can be the problem?
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Old 9th December 2012, 10:12 PM   #2
whizgeek is offline whizgeek  Ireland
diyAudio Member
Join Date: Feb 2010
Start with mechanics, first the pot. then wiring to the bias transistor (not sure if there is one separate)
then measure and compare volts at the bias pots.
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Old 2nd September 2014, 09:54 AM   #3
padamiecki is offline padamiecki  Poland
diyAudio Member
Join Date: Mar 2004
Location: Krakow
Thumbs up mine too, recap/recommendations?

I bought last week JVC A-X9 too
but as chassis and parts source.
the pcbs were broken, I had to glue them, then resolder the tracks, etc.
I was lucky, opt stage was not destroyed, all of the transistors were OK.
After checking a few times iall of the the connections I decieded to start the amp with bulb limiter...
...the amp started!
I am so glad
I set zero offset at the outputs and right bias, and now I am enjoying the sound
Phono section in this amp is extraordinary well designed and built. It is powered from +/- 59V DC power supply and has its own power stabilisers.
Lots of headroom.
Unfortunately there is no upper and cover, I have to "find" something.

The only thing which worries me a little is very soft hum when headphones are plugged in.
I guess that ~35 old capacitors are some kind dried (or there is still broken track on pcb).

What do you think, Elna Silmic will be OK for replacement with old ones caps or should I keep old caps and find and replace only failure cap?

thank you in advance

PS I have right heatsink warmer than left too
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File Type: png jvca-9xphono.png (117.3 KB, 48 views)
regards, Pawel
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