The sunn Forum
Sunn Musical Equipment => Q & A => Topic started by: mikejohn on May 03, 2007, 03:06:42 pm
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I have a beta lead head that has no output. Power light comes on and both channel LEDs come on when plugged in. I am receiving some hum when a speaker is plugged in. Output transistors have been replaced for one chennel (2N3055). What is the biasing for these - I'm uncertain that they are installed correctly. Also any other info would be greatly appreciated!!
Thanks
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I have a beta lead head that has no output. Power light comes on and both channel LEDs come on when plugged in. I am receiving some hum when a speaker is plugged in. Output transistors have been replaced for one chennel (2N3055). What is the biasing for these - I'm uncertain that they are installed correctly. Also any other info would be greatly appreciated!!
Thanks
Output transistors (2N3055s) are not specific to channel. And transistors are not biased the way tubes are.
What are you really trying to say?
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I just wasn't sure if it made a difference how the two posts on the transistors were inserted on the board. I did not do this work myself. This head was kind of handed off to me by a friend. I do have experience in electronics, but it's been awhile since I've worked audio. Just wondering if there might be a common fix or if somebody knows of a good starting point. 74C04N chips maybe????????
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There is a service manual for the Beta Lead and Bass located here:
http://sunn.ampage.org/site/svcmans/beta/ (http://sunn.ampage.org/site/svcmans/beta/)
I would check page 24, the "Troubleshooting" guide, really quick. That is, after you make sure the transistors are put in the correct way.
If you can't figure it out, I would take it to your local amp tech, who could probably tell you if they are in the right way. If there is no tech, I will take a look at the power board on mine and see if I can give you some hints as to how they should be placed.
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Thanks!! I already have the manual and schematics and I am working off of them just haven't dug in very deep. Cleaning the POTs right now seeing as how they've probably never been. Yeah, if you could tell me how your Transistors are installed that would be great. The channel A side has the original Motorola transistors, the channel B side has the replaced transistors - 2N3055. Maybe I can remove one from each channel and see if they read the same on a DVM.
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I repeat: there is no channel A or B side to the output transistors.
The pins on the bottom of the transistors are for the base and emitter connection. Depending on the manufacturer, there may be a "B" and "E" stamped into the case of the transistor indicating which pin is which connection.
The transistors should be installed with the base pin closest to the edge of the circuit board.
You need to troubleshoot to find a problem - not just swap out parts and hope you get it to work. Take it to a tech as Rick suggests.
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When I say channel A or B I'm reffering to the corrisponding channel inputs - A, B, or 1, 2, however the transistors are assigned. Two banks of transistors (3 each) - One bank for channel A or 1, the other for channel B or 2. I'm not Part swapping!! These parts were already installed just verifying thier placement. Nevermind - I looked up the T0 package on-line and the transistor mount compared to pin placement is oblonged, thus with them being mounted to the heat sink through threaded inserts they can only be mounted one way.
So now I'm only left with a no output problem and not component positioning.
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First thing I'd do is check the preamp out by running a line from it to another amp. From what you've said, I agree it's almost surely the power amp, but it's always best to make sure.
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Thanks Isaac I'll give that a shot and see what happens.
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Preamp appears to be fine. Reconnected the power amp board and noticed that Resistors R1, R3, R23, and R25 get HOT! Any ideas?? Maybe a shorted Bridge Rectifier: CR4 19-0010 6A 200V??
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Seriously, just take the amp to a qualified tech. You are just jumping around and guessing. It is quite obvious you do not know what you are doing.
The resistors you mention, in conjunction with a few other components such as Q1 and Q11 comprise a circuit known as a voltage regulator. The two formed by these components take a 40 volt supply as input and provide a 15 volt stable supply as output. What do you think happens to the "missing" 25 volts? Yep, it is dissipated as heat.
A systematic and logical approach is necessary for troubleshooting. Trying to troubleshoot an amp remotely is bad enough, but it is impossible to do when someone sticks to bad assumptions or ideas or concepts that have no factual basis. Your insistance on assigning the output transistors to a "channel" is a good example.
Also consider this: if you did check the preamp and it was providing a good output, what is making you think the bridge rectifier is shorted? Wouldn't a working rectifier be necessary to power the preamp? Is it logical to assume a bad rectifier? Have you even measured the power supply voltage? Guessing and hopping around is not very systematic.
It's your amp so please feel free to ignore what I am saying and continue your efforts to fix it. If you do, my crystal ball says one or both of two things are going to happen. You are going to trash the amp to where repairs are going to be more expensive... if even possible; or you are going to hurt yourself.