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Zx bias
Posted: Tue Feb 17, 2015 7:06 am
by Blad09
Can I re-adjust this myself?? I changed caps out of a zx250 and 350. The 250 reads fine but I'm up around 90mV?? On left channel of the 350. Any guidance for me lol?
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Re: Zx bias
Posted: Tue Feb 17, 2015 8:27 am
by trickyricky
The dc offset may not be caused by the bias needing adjustment. The predrivers may be leaking, try some air in a can and spray each predriver while monitoring the offset.
Re: Zx bias
Posted: Tue Feb 17, 2015 9:31 am
by Blad09
Lol no idea what predrivers are, post a pic? I'm what u could call a rookie with amp repair and such
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Re: Zx bias
Posted: Wed Feb 18, 2015 9:00 am
by Blad09
Should I just leave it alone?
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Re: Zx bias
Posted: Wed Feb 18, 2015 9:53 am
by trickyricky
The predrivers on the ZX250 are on the daughter boards.
Looking at the pic below, they are at the very top end of the daughter boards you can't miss them.
Re: Zx bias
Posted: Wed Feb 18, 2015 10:18 am
by Blad09
Thanks man!! I'll try air can when I get some time hopefully soon
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Re: Zx bias
Posted: Wed Feb 18, 2015 11:23 am
by trickyricky
Just make sure your extra careful with loose wire strands and use a 5amp fuse. Let the amplifier warm up then check the bias if its still at 90mv then spray some air on each transistor ONE at a time, as each one will cause it to drift in either way (negative or positive). I would not recommend anyone adjust the bias, its only needed when replacing the outputs and such.
Zx bias
Posted: Wed Feb 18, 2015 1:53 pm
by Blad09
Thank you for your time Tricky! I haven't been into the amp for months now but I believe I did have to replace caps, gate resistors, ps fets and a couple diodes. There was also some board damage from a blown cap. I should have specified in initial post but I forgot what I actually did to this amp. Back when I wasnt workin and had time to tinker I powered it up,with 10 amp fuse, after all the repairs and all seemed to be good (green light) until I measured dc offset and got the 90mV. I dont even know when I'll have time to play with again but I will try and get everything out this wknd. I've never repaired amps before this, I just read alot of info on this phorum and gave it a shot. I'll update when I have one
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Re: Zx bias
Posted: Wed Feb 18, 2015 6:28 pm
by vwdude
DC offset and bias are different things. The bias is able to be adjusted on these, but I highly advise against it. Adjusting the DC offset is not possible on gees, hence why Ricky recommended cooling those transistors off while measuring.
Re: Zx bias
Posted: Thu Feb 19, 2015 4:09 am
by Blad09
Ok thanks! Still alot to learn obviously so again, thanks guys
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