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CRATE TD 70 Blowing main fuse

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  • CRATE TD 70 Blowing main fuse

    Hi All. My old crate td70 blows the main fuse [1AMP 250v] the instant I turn the amp on.Theres no physical sign of anything gotten hot or anything,the wierd part is it didnt happen while using it,It was going fine I turned it off,next day turned it on...nothing?!. IN previous threads regarding this amp I had trouble with it fading out after a while of playing.I cleaned all the input/output jacks and that seemed to fix it .Now This??? Anyhow any ideas where to start greatly appreciated CHEERS SCOTT

  • #2
    If it's blowing fuses as soon as you turn on, then I would say that one (or both) of the output transistors has shorted. Check the TIP 142 and TIP 147 for shorts.

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    • #3
      td70

      I changed the TIP142 and TIP147. It still blows the fuse instantly. Wondering if is related to previous problem of amp fading out after about an hour or so.Plugging a lead into Line in/out socket would fix it for a while.Anyhow any ideas appreciated. Cheers Scott

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      • #4
        If your amp runs on 120 volts, my schematic calls for a 3 amp fuse, slow blow type, not a 1 amp.

        There is a 1 amp fuse inside for the tube heater, and a 1/2 amp fuse for the tube high voltage.

        If the odd fuse was not all the problem, then disconnect the speaker until we stop blowing fuses. Is there substantial DC voltage across the speaker wires?
        Education is what you're left with after you have forgotten what you have learned.

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        • #5
          Hi enzo Its 240v [australia].Is a 1amp sloblow that keeps blowing,located between power in /on off switch.Have been using a 3amp normal blo for testing since I replaced output transistors,they blow instantly so I cant measure anything. I recently replaced the crate 8 ohm speaker with a celestion g12k-85 of unknown impedance,could that have cooked something??

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          • #6
            Search for "light bulb limiter" on the forum. Make one. Use it. It is a 100w light bulb in series with the mains.

            Then just start at the start.

            Disconnect the speaker. Leave it off until we are done.

            Are the main rectifier diodes shorted? Are the main filter caps shorted? Note where they go and disconnect the power transformer secondary wires from the circuit and see if the transformer alone blows fuses.

            REmove the TIP142/147 pair and power up without them. Still blow fuses? If so, it isn't the power amp. If not then the power amp has issues.

            Did you insulate the tabs of the new transistors? There should have been a mica or silicone sheet under each transistor, and a plastic centering washer around each transistor mounting screw. Easiest test is to measure resistance from the chassis to the metal tab on each transistor. If the tab is grounded, there is your problem.

            SInce the TIPs may have been stresssed, make sure to check the small transistor screwed to the heatsink between the TIPs.
            Education is what you're left with after you have forgotten what you have learned.

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            • #7
              Thanks again ENZO,Looks like I got some work to do

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