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Old 27th November 2012, 02:08 PM   #21
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Default mosfets for bias circuit

Hi everybody!

Just a quick post really! Just wanted to know if anybody new where I could source some 2SJ43 mosfet transistors for this amplifier? I have bought some from ebay recently and sent them back twice because they were faulty! does anybody no of a decent source?

Many Thanks!

Ross
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Old 27th November 2012, 10:58 PM   #22
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http://www.littlediode.com/component...e982689739391b
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Old 29th November 2012, 08:58 PM   #23
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Default Bias problems!

Hi!

Thanks for your help Ian! Have done a couple of tests and found the mosfets were ok so won't need to replace them!

I have had a bias problem with this amplifier on the left channel for quite some time now! When it is switched on it starts to get quite warm after a short time! I have tryed adjusting the bias level but it gets to a certain point (about 10 mv) and won't go any lower (instead it starts to go up again) I have also carried voltage tests on the bias circuit itself and have found that the left side is definately different from right side but the results are very general and are not showing anything specific to be the problem! Just wondered if anyone had ever had this problem before and if so what might be causing it?

Many thanks!

Ross
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Old 5th December 2012, 11:22 PM   #24
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The schematic for this amplifier is large and necessarily fine and difficult to read the scan so it's not easy to suggest what to do. It is a credit to you that you have made it thus far with 2 working channels. I would have dropped the plan long ago without the experience I have gained over many years of building and repairing, often without the prohibitively expensive (for the layman) schematics.

The 10mV bias you quote doesn't mean anything unless you say where it is measured. Are you measuring the voltage drop across one pair of the large 0.47 ohm emitter resistors? Are test points marked on the PCB?

The emitter resistors on the output transistors are the usual reference point in an amplifier and it seems they are wired up as paralelled for 0.24 ohms. A voltage of 10 mV across this represents a bias current of 42 mA which is not unusual in an EF amplifier like this one. Yes, the heatsink will run warm but it should be well within ratings and acceptable so long as it stabilises after say, 15 mins. What is the same measurement in the other channel?

The bottom limit to your adjustment is set by the voltage reference of 2.8V provided by the diode string referred to earlier in the thread. Interestingly, the schematic shows two strings of four diodes in series and that means ~5.6V reference. Measure across points 6&4 or 5&3 on the PCB and so verify that the voltages you measure is similar for both channels. If it is higher on the channel you repaired, it may be reasonable to remove one diode from one string of 4 to get a more suitable range of control to match the other channel. Remember, the diode strings must be rigidly and thermally attached to the heatsink at the same or similar position as originally in order to get the reference voltage and hence bias current down to where it should be. They also must remain fully insulated electrically. Not simple eh?

I hope this is clear enough to follow and addresses your question
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Old 14th January 2013, 06:24 PM   #25
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Default bias problem!

Hi ian!

Thanks once again for your help and sorry for the late reply! sometimes life gets in the way! I managed to solve the bias problem! There is alot of solid core wire in this amp and it turns out that one of the wires connected to the biasing diodes had started to break causing the bias to go up to the level quoted on my last post! I prodded the wire by accident when trying to test something else and the lamp limiter that I had been using the amplifier with suddenly started to glow very brightly so I switched it off and soldered the connections and bingo bias problem resolved! I am now trying to resolve another problem that being humming on both channels sometimes its there sometimes its not! Im thinking that there maybe some AC voltage getting through the power supply filtering and rideing on the DC voltage (excess ripple). I just wondered how you would test for excess ripple and what you would look for to define whether it was a problem or not? how would you read the results?

Any answers greatly appreciated!

Many thanks again

Ross!
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Old 14th January 2013, 07:32 PM   #26
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Hi,
I have a simple check to check for ripples in the rails voltage. Some members do not agreed but any way it worked for me. Give it a try. Just read the voltage with the meter in AC and it should read zero volts. If you have voltage means that your capacitors need replacement.
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