So, I have a second NAD 2100 that was in a professional repair shop for more severe damage (just didn't have the time to work on that one), and they found similar issues to the one in this thread. Bad ceramic cap, but also a blown diode. It is D428 (1SS81 - glass switching 200V 1A). When I brought the unit home, I was checking their work and did an idling current check. Left channel adjusts perfectly at 15mV, but the right channel was static at around 8mV and the pot did nothing. Having a quick look at the diode they replaced, it looks like a 1N4148. Hmmm. Stole a 1SS81 out of my other unit, installed it and bingo, pot now adjusts current and set at 15mV.
So, I guess it's pretty irritating that the shop used an incorrect part and did not bother checking idling current.
Now I need another 1SS81 for my other unit. Looks like it's an obsolete part. Is there a modern replacement for this part?
So, I guess it's pretty irritating that the shop used an incorrect part and did not bother checking idling current.
Now I need another 1SS81 for my other unit. Looks like it's an obsolete part. Is there a modern replacement for this part?
Most unusual as they are practically identical, Vf wise. They have nothing to do with the quiescent current settings, they just clamp the base of the transistors to avoid incorrect polarity.
Having repaired many NAD amplifier as an Authorised NAD Service Centre, the 1N4148/1N914 is recommended in the service bulletins. Not an incorrect fitment at all.
Perhaps they didn't use a 4148? They did not mention on the invoice what part they used. I'm just going by a visual. Are you saying I can replace the 1SS81 with a 4148? If so, I've got lots of those in my own stock and would try it.
They differ by about 50mV in Vf looking at the graphs, and have a max current ratio of 6.25. 50mV represents a ratio of 7 in forwards current at room temperature which agrees nicely with the current ratio. Whether this matters depends on the circuit, but for biasing precise forward voltage match is the main game being played so I wouldn't call them identical.
But for clamping any diode in town would do - perhaps there's a subtlety here? Or the 1N4148 failed?
One interesting point is the 1SS81 is very low capacitance, perhaps there's a stability issue?
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Stupid Mistake
Back to the original unit, some parts came in and I was able to populate all the missing parts on the left channel. Fired it up on the dim-bulb-tester and unit seemed stable and came out of protection. Centre voltage at -60mV, adjusted bias to 15mV and let it burn in for a while. Decided to check for sound and hooked up an input and some speakers to the left channel. We have sound.
Disconnected everything, removed unit from dim bulb tester and tried a normal power up. BOOM. Q439, 435, 425, 433, 431, 429, 427 destroyed along with R443, 435 & 441. Nice flame show actually...
I had adjusted the bias while on the dim bulb tester, so under a current-limited situation. When full line power was applied to the unit, the pot was set too high and pow. I should have turned the pot full CCW before applying full line voltage and re-adjusted the bias. Live and learn...
Back to the original unit, some parts came in and I was able to populate all the missing parts on the left channel. Fired it up on the dim-bulb-tester and unit seemed stable and came out of protection. Centre voltage at -60mV, adjusted bias to 15mV and let it burn in for a while. Decided to check for sound and hooked up an input and some speakers to the left channel. We have sound.
Disconnected everything, removed unit from dim bulb tester and tried a normal power up. BOOM. Q439, 435, 425, 433, 431, 429, 427 destroyed along with R443, 435 & 441. Nice flame show actually...
I had adjusted the bias while on the dim bulb tester, so under a current-limited situation. When full line power was applied to the unit, the pot was set too high and pow. I should have turned the pot full CCW before applying full line voltage and re-adjusted the bias. Live and learn...
All parts came in and we have sound again. I have not powered up the amp off the DBT as of yet. Problem is that I cannot adjust bias via the pots on either left or right channel. They are at a static 5mV each. Checked all components of the bias circuit and all are good. There are two transistors that are physically joined together: 430 & 428 (2SA1370 & 2SC1815Y) and same for the left channel. The parts list asks for the 1370 to be Hfe 100-200, so my guess would be that the 1815 would also have to fall within these specs to allow for biasing?
Haven't got the circuit in front of me but I would check that the bias generator voltage (measured across it) does still vary as you turn the preset, even if it is out of range.
Circuits that specify certain gain groups for what are ultimately basic building blocks make me nervous
Why have you got 5mv bias (I assume you mean over a resistor that directly relates to current flow in the outputs such as an emitter resistor) and not zero volts (no bias current) ?
Circuits that specify certain gain groups for what are ultimately basic building blocks make me nervous
Why have you got 5mv bias (I assume you mean over a resistor that directly relates to current flow in the outputs such as an emitter resistor) and not zero volts (no bias current) ?
Just a follow up to end this thread. All issues have been resolved and the amp is singing nicely again. I did some transistor matching and replaced Q428 & Q430 and Q427 & Q429 (C1815 & A1370 respectively). Bias is now fully adjustable and set at 15mV. This was quite the learning experience and my thanks to all that chimed in.
I have an NAD 2100, worked fine, that I recapped. It now will not come out of protection mode. I'm fairly new to SS repair. Any thoughts on where to start with this? Also, the protection light does not stay lit. It blinks. If the speaker switches are pushed in, I can hear the relays working. Lastly, there is a dead short across c510, whether or not the cap is in the circuit. I don’t see why that would be. All the test voltages are good.
And just to make it more complicated, for me anyway, I have a second 2100 that stays solidly in protection. No relay click or blinks. It has 30v at r471 where it should have about 100mv. Thanks for reading! Hoping to get these things back into action.
And just to make it more complicated, for me anyway, I have a second 2100 that stays solidly in protection. No relay click or blinks. It has 30v at r471 where it should have about 100mv. Thanks for reading! Hoping to get these things back into action.
A dead short across c510 means something short in the circuit, you have to test all the possible components like output transistors.
I make the voltages reference for your amp, please check them. If one voltage abnormal, please check the components around.
Please connect a DBT (dim bulb tester) for the amp under testing.
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Please confirm which amp you make the measuring results.
suggest to do the test below
For first amp :
the amp in protection - to check a DC in the output , use DMM measure the output coil L401/402 voltage to the ground.
a dead short across c510, check if they are short D502, C445, C446
For second amp : It has 30v at r471, check voltages with the reference.
suggest to do the test below
For first amp :
the amp in protection - to check a DC in the output , use DMM measure the output coil L401/402 voltage to the ground.
a dead short across c510, check if they are short D502, C445, C446
For second amp : It has 30v at r471, check voltages with the reference.
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