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Old 27th February 2012, 03:37 AM   #1
diyAudio Member
Join Date: Feb 2012
Default Adjusting Bias on solid state poweramp.

On my GK 800RB bass head, I'm getting a weird hum and slight distortion. I called my tech (who's overseas for two months) and he told me to check the bias (I'm not electronics wiz, but I know my way around a voltmeter).

I checked the service manual and it told me to do the following:

Set bias:
A) Adjust R38 TP to get 5mV across R48, R49, & R50 (the most pos. of the 3), and R54, R55, &R56 (the most
neg. of the 3)

On the PCB, R38 is a pot and R48, R49 and R50, R54, R55 and R56 are large white 5W resistors. How do I check the bias, I'm a little confused how this is worded. Do I want to test each individual resistor?

Here's a copy of the service manual I'm looking at.
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Old 27th February 2012, 07:17 AM   #2
diyAudio Member
Join Date: May 2010
Ok, so it's asking you to first measure the voltage across the resistors 48,49 and 50 and adjust the r38 pot so that the most positive of those voltages = 5mV. This should also cause the most negative of R54, 55 and 56 to be at roughly -5mV. The polarity displayed on your voltmeter will depend on which way around you put the probes.
Be very careful while doing this, for one thing in the guts of the amp are potentially dangerous voltages and if you make the bias too high you can cause your output devices to overheat.
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Old 27th February 2012, 09:46 AM   #3
DF96 is offline DF96  England
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Join Date: May 2007
Bias would probably have to be seriously wrong to create hum. Does the amp appear to be getting much hotter than usual? The solution would be repair, not adjustment.

I would measure the bias, and then talk to your tech before trying any adjustment.
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Old 27th February 2012, 10:38 AM   #4
diyAudio Member
Join Date: Feb 2009
Location: Bangalore
If it is to do with hum and slight distortion then check for grounding, most probably where you insert/pull out- Must be a minor issue

Do not touch the bias, it may end up with serious damages, if at all there is a faulty condition or a component waiting to oscillate, bias adjustment can jump start it.
Check for offset voltage at the outputs
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