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Soundcraftsman MA5002 Bias
Soundcraftsman MA5002 Bias
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Old 16th January 2005, 06:02 PM   #11
sam9 is offline sam9  United States
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Why not just specify a currentn or voltage?

Another speculation. There is enough random individual parameter variation between in-spec active devices that the optimal value would be a little different for each amplifier. Hence, while such a procedure as you suggest may give an easily implemented "Good enough" setting, it would not be the the very best except for a few random units.
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Old 6th February 2019, 03:48 PM   #12
Depaj is offline Depaj  France
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Hi Guys,

I repaired an MA5002 a while ago on a different thread here but when I asked about biasing I didn't get any answers, I'm hoping some of you might help me out here.

I put my meter across R31 (0.27ohm 5W emitter resistor, I tried the other 0.27ohm resistors as well but same reading) and read 0.6mV on one channel and 0.2mV on the other channel no matter the position of the variable resistor. the pot itself is good (read clean from 500 to 5ohms) the bias transistors (Q6) test fine. When turning the pot I can tell the bias increases (VBE on Q6 increases from 2.99v to about 3.4V with the pot half way and the buzzing of the transformer dims down and voltage across R28 10ohm, increases).
Am I doing something wrong here ? Could you tell me where to measure and at how much to set it ?

Thank you
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Old 9th February 2019, 08:44 AM   #13
Depaj is offline Depaj  France
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Any ideas ?
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Old 10th February 2019, 04:01 PM   #14
epicyclic is offline epicyclic  United Kingdom
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Quote:
Originally Posted by Depaj View Post
When turning the pot I can tell the bias increases (VBE on Q6 increases from 2.99v to about 3.4V
Do you mean VCE on Q6 .......VBE should stay approx. 0.63V What voltage range do you get across R28?
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Old 11th February 2019, 08:25 AM   #15
Depaj is offline Depaj  France
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Yes sorry VCE ranges from 2.99 to 3.4(half way and 3.7V at 3/4 of the bias pot)
across R28 (10ohm) I'm getting from 0.3mv with pot all the way down to 120mV half way to about 220mV at 3/4 of the way and I don't want to go any further.
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Old 11th February 2019, 09:20 AM   #16
epicyclic is offline epicyclic  United Kingdom
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Just to clarify the amp is now working , you cured the large DC offset on the output ?
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Old 11th February 2019, 09:31 AM   #17
Depaj is offline Depaj  France
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Yes everything is working (except for one of the vu meters to seriously lag behind but I do have equal ouptut so it has nothing to do with the amp itself).
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Old 11th February 2019, 09:48 AM   #18
epicyclic is offline epicyclic  United Kingdom
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Ok ...........however the voltage drop across R28 needs to approach 600mV for the outputs to go into class AB. Set the bias pot to 3/4 (3.7V) and measure the VBEs of Q7,8,9and 10 and the voltage drops across R20 and R25.
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Old 11th February 2019, 10:32 AM   #19
Depaj is offline Depaj  France
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Ok so I got up to 620mV across R28 and then measured across R31 and hello 15mV .
When I first got the amp the bias pot was a tad before half way so I was afraid to move it much further than 3/4. Now I guess changing the outputs might have influenced the vbe multiplier a bit.
Now the question is what voltage to aim for ?
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Old 11th February 2019, 10:54 AM   #20
epicyclic is offline epicyclic  United Kingdom
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Good news ......( Now the question is what voltage to aim for ? ) that is where the distortion analyser comes in ............15mV across 0.27ohms (55mA) seems a reasonable value to me .
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