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Old 31st January 2010, 01:33 AM   #1
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Default transistor bias

Can anyone what tell me what is involved in setting the bias on a transistor amp? What type of test equipment is neccessary? I have a Phase Linear 200 series II that needs new power supply caps. It's in a local shop and they want $240 to do it. I can replace the caps easy enough. But they say they have to set the bias and I'm not sure I can do that.
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Old 31st January 2010, 01:45 AM   #2
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Im going to hazard a reply here.

Ideally you should have:
A service manual (probably downloadable for free or cheaply purchasable online)
A digital or analogue multimeter

Thats it!

Read the service manual to find out where to stick your probes, turn the amp on and read off the current current / voltage readings, shouldnt be miles off the service manual specification... Tweak as needed.

Might also be a good idea to let it run for a few hours and recheck the bias hasnt drifted up too high.

Save yourself the cash I say
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Old 31st January 2010, 02:01 AM   #3
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If it's only the power supply caps that need replacing I doubt that it would need
the bias re set.
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Old 31st January 2010, 03:50 AM   #4
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Thanks guys. I think I'll do it myself.
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Old 31st January 2010, 04:31 AM   #5
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Bias voltage is usually between emittors of complimentary pair of output transistors ( Also It can be collectors of them in some amps)- you can trace from speaker output. Connect your volt meter between junctions of resistors and emittors(collectors-my Marantz has this). Bias are normally set at few milivolts- some mosfets may have much higher bias. You should not adjust bias when amp is hot or cold. I use 5 minute warm up time from cold before adjusments.
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Old 31st January 2010, 06:43 AM   #6
Mooly is offline Mooly  United Kingdom
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The caps won't alter the bias one iota.

"Bias" is setting the small value of current that flows all the time in the output transistors to minimise crossover distortion.

Without knowing the circuit details (bjt, MOSFET, EF CFP etc) it's impossible to guide you on that. Read the official manual... but as mentioned, changing caps won't alter it at all.
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Old 31st January 2010, 06:46 AM   #7
djk is offline djk
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If the PL 200II is quasi-comp (like most of the other models), then the bias voltage on the base of the top outputs is set to exactly 340mV (class AB+B).

If the PL 200II is full-comp (like some of the new ones), then the bases of the top and bottom side outputs will be around 600mV. On these type set the voltage across the emitter resistors to about 10mV (class AB).
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Old 31st January 2010, 12:34 PM   #8
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for 240 usd in greece you will probably have all capacitors replaced not only main caps plus all Ceramics remooved If any existing which will probably include fine tuning of everything beyond parts upgrade

replacing capacitors in the main psu will be probably one hour of work to do and lees than 100 usd in parts ( please correct me if i am wrong )

i wonder why some companies like to charge so much ....

regards sakis
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Old 31st January 2010, 04:20 PM   #9
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Or you'll get even another complete PL 200II for that $240
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Old 31st January 2010, 04:47 PM   #10
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Quote:
Originally Posted by mr2racer View Post
Can anyone what tell me what is involved in setting the bias on a transistor amp? What type of test equipment is neccessary? I have a Phase Linear 200 series II that needs new power supply caps. It's in a local shop and they want $240 to do it. I can replace the caps easy enough. But they say they have to set the bias and I'm not sure I can do that.
It depends on the manufacturer.

Some just need a multimeter to measure the voltage acroos the output resistors.

Personally I set mine up using a scope and increase bias until crossover distortion just goes.
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