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Old 27th February 2012, 03:41 AM   #1
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Default Solid State poweramp Bias.

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.

ftp://downloads.polaraudio.co.uk/Tec...B/SERVMAN1.PDF
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Old 27th February 2012, 04:25 AM   #2
davada is online now davada  Canada
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Set you voltmeter to the mV range. Measure across the 5W resistors and find the resistor with the highest reading. Use this resistor to set the bias for 5mV across the resistor. Proceed with this only if all the resistors read in the mV range. If one or more resistors read extremely high compared to the others, you may need to get the amp services. If this is the case then don't do anything.

David.
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Last edited by davada; 27th February 2012 at 04:31 AM.
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Old 27th February 2012, 04:41 AM   #3
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Sounds like power supply electrolytics. A 'scope becomes invaluable. How old is the head?
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Old 27th February 2012, 04:46 AM   #4
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Quote:
Originally Posted by cyclecamper View Post
Sounds like power supply electrolytics. A 'scope becomes invaluable. How old is the head?
It's about 7 years old, but this head is a classic on the talkbass forum, many people have had this head 20+ years without it being in the shop once.

I checked the bias and it was off, about 23mv. I pushed it down to 5ish, but I live in an apartment building so I can't test it this late at night.
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