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Old 4th April 2012, 04:22 PM   #1
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Default NAD 304 protect mode, very strange behaviour

Okay so I have here a NAD 304, in protect mode. One channel is good, the other not so good. Most of the time, there is 11 volts DC on the output, occasionally as much as 30 and sometimes only 100 mV.
After some basic testing, I decided to replace the 3.9V zener diodes as I wasn't reading the correct voltage drop over them. After that, the amplifier would click out of protect mode but the very strange thing is if I moved my hand over the pcb and close to the components, then it would click back into protect mode, so it may be picking up a tiny bit of EMI noise, just enough to upset things? When its not in protect, a quite hum can be heard from the speaker.
Voltages throughout the amp are all over the place and seem to change without anything else changing. I removed and did the diode test on each transistor individually and all transistors appeared to be fine, and I replaced the electrolytic caps in the bad side as well, but nothing seemed to make a difference. I also lifted a leg of each diode and did a voltage drop test on each and they all seem ok. Should I just replace all of the small transistors anyway? Or do I need to start checking resistors? This amplifier seems to have a very complex design compared to other amps, not sure what they were trying to achieve but it makes troubleshooting a tad more difficult.

Anyone who has any suggestions or ideas, your input is appreciated and I thank you for the time taken to help me with this amp.
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Old 4th April 2012, 04:26 PM   #2
Mooly is offline Mooly  United Kingdom
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I'm going to jump straight in and say it's the drivers going OC intermitently. Are they 2SD668 ish variety ?
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Old 4th April 2012, 04:42 PM   #3
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The drivers are 2SB649 and 2SD669. The pre-drivers are 2SC3467 and 2SA1370 if that helps
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Old 4th April 2012, 04:51 PM   #4
Mooly is offline Mooly  United Kingdom
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The Japanese (Hitachi) 2SB/2SD drivers have a peculiar failure mode of going intermitently open circuit base to emitter. Not NAD specific... I encountered this time and again in all type of gear and over similar devices.

The real answer is to try and be sure. I wouldn't use speakers for testing as a real fault could damage them.

The thing to check if you can is measuring across the base and emitter junctions on DC volts with it on. The reading should be in the 0.6 to 0.7v range for all the transistors. If one is faulty the reading will increase over that.

Obviously check for dry joints and so on too (on those transistors too). Make sure the solder "takes" cleanly to the leads. They have a habit of being oxidised.
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Old 4th April 2012, 05:18 PM   #5
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I'll give that a try. Regarding testing with speakers, I have a few old speakers I use for testing amps that I wouldn't miss if they blew
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Old 4th April 2012, 10:10 PM   #6
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I've had trouble with the 47k resistor R333 feeding the biasing zeners going bad.
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Old 4th April 2012, 10:53 PM   #7
rss388 is offline rss388  United States
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I just got done debugging a Harman Kardon amp with kind of the same problem you described. Going in and out of protection mode and later just plain blowing fuses depending if the output transistor heatsink was touching the chassis or not and if my hand goes touched the heat sink or not. Finally, narrowed it down to a 2SA1306 driver transistor. Out of circuit, the base emitter junction would measure 0.85V with a Fluke DMM on the diode scale, while its counterparts in the other side and the other channels all read 0.55V. I think it was on its way out to complete failure, in circuit it would bias the power devices on full and blow the fuses. Replaced with 2SB649A/D669A and all is fine now.
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Old 4th April 2012, 11:11 PM   #8
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I don't have any and my local supplies don't carry any of the 2SB649 and 2SD669 transistors. Am I able to get away with using BD139 and BD140?
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Old 4th April 2012, 11:33 PM   #9
rss388 is offline rss388  United States
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A quick scan of a couple on line data sheet and I could not figure out the cut off frequency of the BD140 pair. The 2SB649 I suspect is much faster than them so I am not sure what would happen if you stick them in. At least you would have to tweak the Miller cap to ensure stability. 2SB649 type is quite common in receiver designs. I harvested mine from a non-functioning Insignia amp.
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Old 4th April 2012, 11:34 PM   #10
rss388 is offline rss388  United States
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I would add before you think about replacement of the driver transistors you may want to ensure they are bad first.
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