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Old 25th April 2012, 07:11 PM   #11
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The two speaker terminals that are shorted together (0 ohms between them) are the right ones.

Do you have a current limiting resistor?
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Old 25th April 2012, 07:36 PM   #12
madHD is offline madHD  United States
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I'm not sure - where should I be looking? There is nothing on B+ supply to primary center but there are some others that look like they might be.
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Old 25th April 2012, 08:21 PM   #13
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I was referring to a 2 ohm 50-100 watt current limiting resistor that would go in the B+ line feeding the amplifier.
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Old 25th April 2012, 08:47 PM   #14
madHD is offline madHD  United States
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No, I didn't have a resistor on the B+ line. Could you explain the consequences of this or point me to a link? What does this mean for the amp?
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Old 25th April 2012, 09:12 PM   #15
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There's an easy way to find intermittent shorts in transformers but you need some way to limit current. Some people use a car headlamp.
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Old 25th April 2012, 09:27 PM   #16
madHD is offline madHD  United States
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Did I read something about this on bcae? I could get a resistor or lamp of this nature fairly easily. What is the technique for finding intermittent shorts?

Thanks - this is awesome stuff.
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Old 25th April 2012, 09:44 PM   #17
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I'm not 100% sure that it will work with a headlamp so if you can get the resistor, do so.

Connect the resistor in series with the B+ terminal of the amp. Don't connect anything to the amp ground or remote. Connect the ground (that would normally go to the ground terminal) to one of the non-bridging speaker terminals. When you twist the transformer and the short makes/breaks, there will be a tiny spark. Insulate between the windings.

They typically short where the terminal windings meet the rest of the windings. Inserting a small piece of cardboard coated on both sides with adhesive like Goop will keep them from shorting again.
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Old 30th April 2012, 06:22 PM   #18
madHD is offline madHD  United States
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Some followups:

1) I ordered a 100W 2 ohm from Mouser.

2) Since I am limiting current can I do the transformer diagnositcs with the board unbolted from the heat sink to see the bottom side - all transistors open to air or is this a bad idea even for a few seconds?

3) Assuming I find and fix my short, what do you recomend for the thermal grease between the transistors and the sink?

4) This is just a thought, but would a 1 or 2 amp battery charger work for a current limted power supply? Maybe need some filtering?

Thanks again.
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Old 30th April 2012, 10:30 PM   #19
madHD is offline madHD  United States
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Perry, I read though the bcae stuff on heat sinks so I have that figured out.

You can ignore points 1 and 4 since the resistor is on its way.

I also read the part about the semiconductions clamped to the heat sink but I suspect the short to be on the underside so I can check the top side first and if I have to pull it off I will to view the underside. Is there a particular transistor or diode pair I need to be mindful of regarding heat in the few seconds I have?
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Old 1st May 2012, 06:47 AM   #20
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2. The current should only flow through the transformer but I can't be 100% sure so if you don't clamp the transistors, monitor their temperature.

3. I recommend Dow Corning 340 but virtually any heatsink compound available from digikey or mouser will be OK.
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