bias Roksan Kandy MK3 (mosfets:2SJ162/2SK1058)

[eightdigitword]

Hi,

opened up a kandy Mk3 for repairs, repairs now complete, but during testing i noticed the left heat sink barely got hot while the right got warm.

DC measure at the speaker posts was minimal, listening it sounds balanced.

should i look to the biasing? if so how do i go about this? I have read the data sheets on the mosfets and read every post i could find on the kandy and these mosfets and still dont know where to take the measurements from.

any advice would be most appreciated.

2SJ162 data sheet: https://www.renesas.com/en-us/doc/products/transistor/003/rej03g0847_2sj160ds.pdf

2SK1058 data sheet: https://www.renesas.com/ko-kr/doc/products/transistor/003/rej03g0906_2sk1056ds.pdf

this post has a pic of the amp board: http://www.diyaudio.com/forums/solid-state/243606-roksan-kandy-mk3-upgrades.html

http://www.diyaudio.com/forums/solid-state/96954-roksan-kandy-mkiii-tweaks.html

 

[Mooly]

100ma bias current per pair is typical for lateral FET's. If there is a low value resistor in the source or drain of either FET then simply measure the volt drop across that resistor having first calculated the desired target value (no signal and no speakers connected).

Those large white resistor look typical. Look at their value and measure the volt dropped across one of them.

 

[eightdigitword]

Hi Mooly, Thanks for the response and the instructions. I successfully set the bias on both channels. then went back to it the next day to check after some use and was unlucky enough to have the clip slip and touch a HV pin killing the right channel. lesson learned on caution. new mosfets now fitted but new issues have arose. I will be posting a new thread on this topic!

thanks again!

 

[Mooly]

Oh dear although its probably something we have all done in some form or another.

So now you have the challenge of fixing it

 

[eightdigitword]

true, its too easy to get complacent when everything goes well for years. its been a good reminder and lesson on areas i didn't know about and had to look into. thankfully its working now and playing music as it should.

The short killed a 7812 and (i believe) effected the logic chip/eprom that controls the HV relay and motorized volume. without any real knowledge on eproms or where to start reprogramming it, ive had to by pass all of the steps stopping the amp from powering up and disconnect the power to the motorised volume (constantly turning up when connected).

so, remote will not work on the volume and standby mode is all power to everything. so amp off when not in use. main thing is it works!

I have contacted roksan about a replacement eprom but i have little hope, none of the numbers on their website work!

 

[Mooly]

I thought you had just zapped the power amp. Zapping logic stuff isn't good, not so much because its hard to fix but because the parts are unique as you have found out. Typically any microprocessor will use a dedicated ROM mask identified by the last digits on the part, those digits being the firmware version. The EPROM may or may not be pre-programmed as it may simply store the last used configuration when you power off.