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Thread: Ashdown ABM 500 Blown Thermistor In Soft Start Circuit

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    Ashdown ABM 500 Blown Thermistor In Soft Start Circuit

    The soft start circuit consists of nothing more than a Thermistor in series with one leg of the primary of the transformer. This one was fried to a crisp after the owner claims was turned back on after a 1 set rest. It had been working without issue prior.

    Comparing measurements with another ABM 500, the primary matches at about 1.2 ohms, but I have no other way to measure other than a digital VOM. The thermistor on my good amp is stamped NTC 4.7D-20. Yet when I read across it at room temp I measure 5.7 ohms. The bad one is obviously blown wide open.

    Without tearing apart a perfectly good working unit, could I substitute a light bulb in it's place to verify the rest of the amp? What wattage? If it does check out, what do I look for in a replacement for the blown device. I understand the NTC part, just not sure about the 4.7D-20. Is that 20 ohms at inrush down to 4.7 ohms after?
    Thanks in advance!
    EDIT: Upon closer inspection, that number is 4.7D, not 4.70. I have corrected it in post.

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    Last edited by Relsom; 12-08-2018 at 03:09 PM.

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    Old Timer J M Fahey's Avatar
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    I bet "cold"/nominal is 4.7 ohm, since 5 ohm (nominal) NTC are very common in that range power.

    FWIW I use 5 ohm ones in my 300W Bass amplifiers , 220V mains.

    That you measure 5.7 ohm on your "good" one is reasonable, and may be due to more than one cause:

    * you have not substracted multimeter "zero ohm error" from your measurement, which would make your NTC actually a 5 ohm one.

    * nominal resistance is at 20C , sometimes 25C ... what is your ambient temperature?
    And even holding it in your hands can change that value, , so not big deal, 4.7 ohm is the value then.

    * I bet "20" means the diameter in mm, which is an indirect way to rate power dissipation ... for an accurate estimation we would need the pertinent datasheey, but I doubt we are too far from proper value.

    *Mouser stocks 4.7/5 ohm 20mm diameter NTCs, I buy mine there.

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    Juan Manuel Fahey

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    Thanks for your reply Juan.
    No I did not add in the zero offset of the meter.
    Ambient Temp is 21~22C.
    I measured 20.5 mm diameter.
    I looked at Mouser and I'm unclear on all of the variables that go in to ordering these things. I have emailed Ashdown and hope they respond with some helpful information.

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    Old Timer J M Fahey's Avatar
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    Yes, thereīs a zillion variables at play here, it took me a lot of time to calculate what *I* needed for my amps, Iīm designing a general purpose SMPS so I can replace iron EI transformers in my amps from 200 to 600W RMS.

    There is a ton of data ... but a lot of it is obscure, contradictory or can be "read" in different ways, thatīs why I am not calculating a suitable one for you, but you are not actually designing but repairing instead, so: if no good answer from Aswhdown, Iīd plain search Mouser for a 4.7 or 5 ohm one , 20 to 22 mm diameter ... and just use it.

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    Juan Manuel Fahey

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    I can't find the 4.7s available anywhere over 15mm (3A). Given a choice of 5D-20(7A) or 5D-25(8A) which would you choose? The amp is fused at 8A and has a max stated consumption of 700W @117vac. Space is not an issue. It seems the 25mm one would be more durable but I don't know the other variables that come in to play well enough to make my decision based on current rating alone.
    Thanks once again!
    ....Chris

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