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Old 7th April 2013, 03:43 AM   #1
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Default SC480 Amp Power Supply and Voltages

Hey Everyone

I've had a sc480 amp on the shelf for about a year and i have decided to complete it.

I am having trouble with the power supply i have connected my Toroidal i got a 25v - 0 - 25v 160VA MT-2114

to the power supply which is KC5347 which is suppose to give me +-40v dv rails, now because i have a 25 volt transformer and not a 28 volt one i know it will be less, when i put a mulit-meter set to dc on to measure the rails coming out of the power supply i put my negative in the middle which is the ground (0v) and i put my red on either side.

Now this is were i get confused my +40v side gives a reading of -34.4/-35v and when i put it on the -40v side i get around 34/35v is that correct? because to me that doesn't make sense, i thought i would be cleaver and swap the wires around but that just resulted in smoke from the amp so i changed it back.

now on to the amp, vr1 which is used to set the quiescent current level, does the screw move up or down because it doesn't look like anything is happening all i get is a clicking if i screw it all the way clockwise, now my voltage readings from the resistors in testing are all over the place..

it was like this before i had smoke from it, also at first power on the output transistors never got warm or hot...

Is there a guide to reading the voltage diagram that silicon chip have because i am finding it really hard to work out were i should be getting the voltage readings from?

Hoping someone can help me figure this out.
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Old 7th April 2013, 12:19 PM   #2
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Are you sure you have your meter leads plugged into the correct sockets on the meter? Do you have a DMM or a moving coil multimeter? I guess the former as you are leaving the same lead on the CT and noting the change in sign.

If you have the rectifier diodes wired in back to front the electrolytic capacitors would normally be very distressed and likely explode. I am surprised that no fuses have blown? The same remark would apply to the amp electro's under reversed polarity.

Try doing something simple like measuring a torch battery to sort out the polarity issue and report back on amp etc damage. Good luck.

Keith
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Old 7th April 2013, 12:43 PM   #3
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Well it was the leads, that's embarrassing, should have guessed as the power supply didn't blow up.

there is still constant volts change from the testing resistors, not even sure what to start replacing, i know i would start with the parts that were smoking from the reversed polarity, i might replace the output transistors and the vr1 trimpot as well.

Anything that would effect random voltages from the test resistors?
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Old 7th April 2013, 03:24 PM   #4
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Provided you look at the winding information on the transformer and pay attention to the order in which the ends must be connected so that windings add rather than cancel their outputs, it's very hard to go wrong wiring a rectifier because the polarity is clearly marked on the diode bridge and this is the only thing determining DC polarity. It seems you got that right if you measured +/-35V from the rectifier anywhere, regardless of the meter lead mix-up.

I guess you are asking about the resistors fitted across the fuseholders for set-up adjustments. It is anybody's guess what is happening now because the resistors only limit current to the amplifier so that you can safely test and adjust it before use. If it is damaged, and it should not have been with the resistors fitted, any number of circuits could be operating among the shorted or open circuit parts and this could be giving intermittent effects like charging and discharging caps which could appear as random voltages.

The other possibility is damage to your meter which you can check on any other circuit, power supply etc. There is some good news if the resistors were fitted when you first powered up, in that there should not have been enough current available to kill the output transistors but the front end small transistors are more likely to be the ones that failed, i.e. those in the VAS and input stages. They are easy enough to remove and test if you have a transistor test feature or alternatively, you may be able to measure Vbe, the 0.65V diode voltage between base and emitter, whilst the circuit is operating, though I don't fancy powering it up in this condition. A diode check, out of circuit, will be safer.
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Last edited by Ian Finch; 7th April 2013 at 03:30 PM.
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Old 7th April 2013, 04:20 PM   #5
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Getting the voltages reversed will have killed your amplifier especially if there was smoke.

You will need to check every transistor with DMM, also check the Hfe if your meter does that as I have seen transistors fail with an Hfe of 1.
You will also need to check any electrolytics for short circuit as they sometimes go short if reverse voltage is applied.
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Old 14th April 2013, 12:27 PM   #6
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When i first powered on i didn't have the resistors in the fuse holders for testing,
so the front end small transistors are gone? do you think the big ones are as well?

Yer i assume that alot is killed, i just don't know were to start, considering scraping and starting over, i think i built another one at home that i haven't finished yet.
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Old 15th April 2013, 01:09 PM   #7
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Quote:
Originally Posted by phoenix455 View Post
do you think the big ones are as well?
You said you did not have the resistors in the fuse holders. Does this mean you had nothing in the fuse holders? If this is the case you have probably saved the output/driver devices and the 470uF electros from destruction.
Your best bet may be to replace Q1 through Q7 and the 3 diodes.

Sometimes semiconductors subject to abuse can be leaky or noisy even though they have gain. Take a close look at all the resistors, particularly for tell tale signs such as charring of the board under them. Measure them all when you have the transistors and diodes removed. Good luck.

Keith
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