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Nht subtwo blowing f1 fuse on ac input module. found shorted Q16 2SA1302 and Q15 2SC3281 transistors what was the reason for these parts failing and what else should i be checking.

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  • e_norin Mar 07, 2009

    For the NHT SUBTWO with the Q14,15,16 and17 transistors removed measured the three pads on both Q15 and Q16 and the voltages are as follows E= 0.5, C= 126, B= 126. I had replaced all the 2SA1302,2SC3281 transistors and all six IRF 640's before i got your post, powered the unit up without input signal and got a green light. but when i hooked up input it then poped the F1 fuse on the ac input module again.if the problem is in the driver circut could u walk me thru this.thank you

  • Anonymous Apr 15, 2009

    the drivers transistor 2SC4793 or 2SA1837 are likely defective too, My has same problem. found Q13, 14, 15 shorted out. Also 1 large electrolytic capacitor 2200mf/200 open, I think the capacitor opened, cause the output transistors failure.


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Try turning on the unit without the two transistors installed. Measure the DC voltages on the 3 pads to ground. These should read .6V,40V and 0, both +and -. If you read anything else, the driver circuit has a problem.


Posted on Mar 02, 2009

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Appreciate the direct "Ask Me". With reference to your post here;

Voltage checks are done with the power on whereas resistance/continuity check are performed with the power off and preferably the component out of circuit. The VOM would do.

You posted that the fuses have not blown and therefore you are assuming that the output transistors are not defective. Pls confirm this by performing a check on the base to collector to emitter of the output transistors. This page would illustrate how to check a transistor using a VOM. The output transistor once removed from the circuit (pls mark which goes to where), laying flat with the pins/feet pointing towards you would be base, collector and emitter. Although the real outputs if 2SA1302/2SC3281 would be larger.
Most NHT sub amps are standard design amplifiers. To my knowledge, NHT amps (but not all) uses a unique design to supply power to the output transistors. The fuses does not tie directly to the output transistors from the +/- of the power supply. Instead the fuse goes to several power MOSFETS (IRF640) that regulate/control/monitor the current/voltage supplied to the output transistors. If any of the IRF640 is blown, the output would have no voltage even if the fuses are good.

To be more specific and to confirm the above, would appreciate if you could post/share the schematic diagram.

May I also inform you that there are posts made by other members of the community (our co-experts, I am aware that you have provided solutions yourself in some other posts) that would be of additional help. ginko has posted the method/manner by which you could test the transistors and other components, while bunnydawg addressed the fuse not blowing. welcomejee has posted with an amp schematics of an NHT Subtwoi.There were however and I would expect more posts that are too general/generic to be of any real use to you. I would leave it to your discretion to filter through the flood of information.

As a site policy, once you rejected a solution or give it a "Thanks for Trying", that particular post/expert would no longer be viewable. For your benefit, may I suggest that you copy and paste the "workable solutions" to a document that you would have a record of previous or no longer viewable postings.

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