Hey guys, I have an NAD T955. There were originally two channels that were not working so I used the other three channels to run my LCR for a few months with no issues.
Recently I decided to troubleshoot the other two broken channels. I opened it up to find four of the fuses blown. There are 20 fuses in total, all the same type and sized fuses. So it appears there are 4 fuses per channel. Channels #2 & #3 were the broken/not working channels, but only the fuses from channel 3 were blown, channel #2 had all good fuses. I pulled those 4 good fuses from channel #2 and tested them on channel #3 and it worked right away with no issues. So channel #3 is working now and fine. I’m not sure what caused it to blow initially.
So as of now I have all of the channels running but one, which is channel #2. New fuses don’t seem to do anything. They don’t blow, but they don’t solve the problem either.
I am obviously not a professional here. Any thoughts/recommendations?
Recently I decided to troubleshoot the other two broken channels. I opened it up to find four of the fuses blown. There are 20 fuses in total, all the same type and sized fuses. So it appears there are 4 fuses per channel. Channels #2 & #3 were the broken/not working channels, but only the fuses from channel 3 were blown, channel #2 had all good fuses. I pulled those 4 good fuses from channel #2 and tested them on channel #3 and it worked right away with no issues. So channel #3 is working now and fine. I’m not sure what caused it to blow initially.
So as of now I have all of the channels running but one, which is channel #2. New fuses don’t seem to do anything. They don’t blow, but they don’t solve the problem either.
I am obviously not a professional here. Any thoughts/recommendations?
The LED,s should turn Blue are any Red ?
There is a protection circuit in this amp.
Check the voltages at the blown channel , if okay you have an open circuit component fault .
Luckily there is a good -full schematic at-
[url]https://www.vintageshifi.com/repertoire-pdf/pdf/telecharge.php?pdf=Nad-T-955-Service-Manual.pdf [/URL]
There is a protection circuit in this amp.
Check the voltages at the blown channel , if okay you have an open circuit component fault .
Luckily there is a good -full schematic at-
[url]https://www.vintageshifi.com/repertoire-pdf/pdf/telecharge.php?pdf=Nad-T-955-Service-Manual.pdf [/URL]
Yes, when powering on the working channels light up red, then turn to blue, but channel 2 stays blue throughout the power on cycle and throughout standard operating. No temporary red for channel 2 during power on.
I noticed this when change fuses as well. If you remove the fuses from a channel that channels led will stay blue throughout the power on cycle. As to indicate that it is not working I suppose
I noticed this when change fuses as well. If you remove the fuses from a channel that channels led will stay blue throughout the power on cycle. As to indicate that it is not working I suppose
Yes that's the operation of the power control circuit the LED,s when under initial surge when powered on are Red but change to Blue when the amp is stabilized .
If channel #2 stays Blue on switch on then the power supply senses there is no load to initialize therefore you have an open circuit as something has blown on that channel most likely an output device as I think the fuses are 6amps and you say the 2nd channel fuse was blown so it would take a large current device to blow that but now you replaced the fuse and it didn't blow .
Check the output transistors on that channel for open circuit or the connecting resistors feeding them.
If channel #2 stays Blue on switch on then the power supply senses there is no load to initialize therefore you have an open circuit as something has blown on that channel most likely an output device as I think the fuses are 6amps and you say the 2nd channel fuse was blown so it would take a large current device to blow that but now you replaced the fuse and it didn't blow .
Check the output transistors on that channel for open circuit or the connecting resistors feeding them.
NAD T-955
I have repaired four channels with this exact operation, with the same symptoms you are having.
You will need to remove Channel 2 amplifier board from the chassis, which is not complicated, but takes the removal of about 40 screws and there are three different (very similar) types of screws, so don't confuse where they came from (ziploc bags are your friend here)
Once you get the amplifier board out with the heatsink as a whole - you will want to locate R101, 102, 105, 106, 107 and 108 - these should all be 680R 1/4 watt resistors. I had one channel with a single open resistor, I also had two channels where all these resistors had been replaced and were the wrong resistance (3.2kR I think).
Next you will want to check R103 and 104 - there are 33R 1/2 watt resistors. I had two channels were this resistor was either open, or very high resistance due to heat stress.
As Duncan mentioned - you will know if the channel is working, because when you plug it back into the chassis and power-up, the LED will go RED and then turn BLUE after 3-4 seconds. If that does not happen, you have not found all the out-of-spec resistors.
Let me know how you get along - included pics of the board and schematic section to be looking at.
I have repaired four channels with this exact operation, with the same symptoms you are having.
You will need to remove Channel 2 amplifier board from the chassis, which is not complicated, but takes the removal of about 40 screws and there are three different (very similar) types of screws, so don't confuse where they came from (ziploc bags are your friend here)
Once you get the amplifier board out with the heatsink as a whole - you will want to locate R101, 102, 105, 106, 107 and 108 - these should all be 680R 1/4 watt resistors. I had one channel with a single open resistor, I also had two channels where all these resistors had been replaced and were the wrong resistance (3.2kR I think).
Next you will want to check R103 and 104 - there are 33R 1/2 watt resistors. I had two channels were this resistor was either open, or very high resistance due to heat stress.
As Duncan mentioned - you will know if the channel is working, because when you plug it back into the chassis and power-up, the LED will go RED and then turn BLUE after 3-4 seconds. If that does not happen, you have not found all the out-of-spec resistors.
Let me know how you get along - included pics of the board and schematic section to be looking at.
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Hi, I am having slightly different issue on this T955. 2 of the channels would cut in and out intermittently. Some times power cycle restores them but usually does't last long before it happens again. The lights do go red at first and turned blue after, even when no sound is coming out though. Would these be symptoms before channels go completely? Thanks.