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Old 18th December 2012, 10:54 PM   #11
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UPDATE!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!


I just installed a 3w 1ohm resistor in series with resistor 309 making it 6ohm total. I am now at 300mv dc at speaker, MUCH BETTER!. So resistor 311 is 5.3 and total resistance for 309 is 6ohm, is this because the transistors are not matched that they need different value gain resistors?

Last edited by famousmockingbird; 18th December 2012 at 11:07 PM.
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Old 19th December 2012, 02:51 AM   #12
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I suspect theese transistors either is very warm, or defect.
The values of the 1W-resistors seemed very high. A 3,9 Ohm resistor should not reach 4,5 Ohms. And with 6 Ohms on R309 the voltage would increase quite a bit ther, over 1,5 Volts or something like that now, is it?
It should be reduced to 3,9 Ohms to get the voltage down, and the same for R311
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Old 19th December 2012, 03:10 AM   #13
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When resistor 309 was increased to 6 ohms the voltage went down from 1.7 to 1.
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Old 19th December 2012, 06:10 AM   #14
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Match all the components. Start with resistors.

Gajanan
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Old 19th December 2012, 06:48 PM   #15
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I swapped Q303 and Q305 with Q304 and Q306 and the problem did not follow the transistor.

So I am assuming that the transistors are fine but there is something wrong with the components around Q303 and Q305.

I will order some resistors and keep my fingers crossed.
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Old 20th December 2012, 12:47 AM   #16
JMFahey is online now JMFahey  Argentina
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I do not suspect the transistor for now.
There is no DC balancing circuit there; the centerpoint/speaker_out is centered brute force by the string of resistors R308/309/310/311 , just like that.
To be more precise:
1) both rails must be symmetrical respective to ground.
2) you must have the same voltage drop across R308 and R310
3) you must have the same voltage drop across R309 and R311.
If not,replace them with proper value.
Your matching the higher value is dangerous, you will overbias and overheat them; stick to design values.
4) anyway, I'm quite sure that your 2V offset will disappear or drastically reduce when you connect a speaker as a load.
Without load yes, you may have somewhat higher offset, but that causes no problems.
EDIT: which of the big VOX amps is it?
Do you have the original cabinet?

Last edited by JMFahey; 20th December 2012 at 12:49 AM.
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Old 20th December 2012, 06:11 AM   #17
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That's my plan is to match resistors on both sides.

Edit; it's a super Beatle, original cabinet and all. I plan on installing a fuse in series with speaker out, I think 8A should be fine.
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