Hi,
Hope someone can help here, as I am about to give up on this amp
I had an blown output transistor on an NAD 350 I replaced both power transistors on the bad channel from an reliable source. I tested the pre-drivers and Q122 was replaced. I powered up with an current limitor and both channels work I have sound, I am able to set the DC off set one both channels No DC on the coils and amp turns on, but the bias on the bad channel remains at 0mV no matter what I adjust the pot to. The other channel is fine. I tested each resistor, transistor and Pot's twice and all seem fine, each time comparing values with the good channel.
I swapped IC104 and IC102 with the other channel, swapped R186 & R188, checked that the resistance changed from the pot to the base's From 10ohms to about 277 Ohms on good channel and 10 to 281 ohms on bad channel, similar on both channels, so pot seems good.
Here are some measurements, using current limitor.
Voltage on R158 = 0.65V at the point where it goes to R178 : R160 = -0.59
Getting same values on good channel (R157 & R159)
C118 and c120 seem ok same voltages on good and bad channel.
All in circuit measurements:
R178 = 10ohms
R174 = 10 ohms
R182 = 423 ohms
R181 = 424ohms
Bad channel: Collector slightly low, would this cause this issue?
Q124 E=0v, C=46.2, B= 0.7v
Q126 E=0v, c=-47.3, B = -0.5
Good channel
Q123 E=0v, C=47 , B= 0.7v
Q125 E=0v, c=-47, B = -0.5
I am running out of ideas here, thanks in advance for your help
Hope someone can help here, as I am about to give up on this amp
I had an blown output transistor on an NAD 350 I replaced both power transistors on the bad channel from an reliable source. I tested the pre-drivers and Q122 was replaced. I powered up with an current limitor and both channels work I have sound, I am able to set the DC off set one both channels No DC on the coils and amp turns on, but the bias on the bad channel remains at 0mV no matter what I adjust the pot to. The other channel is fine. I tested each resistor, transistor and Pot's twice and all seem fine, each time comparing values with the good channel.
I swapped IC104 and IC102 with the other channel, swapped R186 & R188, checked that the resistance changed from the pot to the base's From 10ohms to about 277 Ohms on good channel and 10 to 281 ohms on bad channel, similar on both channels, so pot seems good.
Here are some measurements, using current limitor.
Voltage on R158 = 0.65V at the point where it goes to R178 : R160 = -0.59
Getting same values on good channel (R157 & R159)
C118 and c120 seem ok same voltages on good and bad channel.
All in circuit measurements:
R178 = 10ohms
R174 = 10 ohms
R182 = 423 ohms
R181 = 424ohms
Bad channel: Collector slightly low, would this cause this issue?
Q124 E=0v, C=46.2, B= 0.7v
Q126 E=0v, c=-47.3, B = -0.5
Good channel
Q123 E=0v, C=47 , B= 0.7v
Q125 E=0v, c=-47, B = -0.5
I am running out of ideas here, thanks in advance for your help
Attachments
Hi mjf,
Thanks so much for responding, That is an great check indeed, Never thought about that.
I checked it anyway and it only changes a tiny bit from 47V to 46.7V and same on the negative side -47 and -46.7 when pot is turned from fully clockwise and anti clockwise.
On the good side it changes much more. What would happen if c122 & c124 was shorted to the power rail? I will take the voltages at Q122 & Q126 when I get home later today. Q122 was replaced already with the same P/N transistor, as it was shorted, I can not remember which pins, I will check that too later.
Thanks so much for responding, That is an great check indeed, Never thought about that.
I checked it anyway and it only changes a tiny bit from 47V to 46.7V and same on the negative side -47 and -46.7 when pot is turned from fully clockwise and anti clockwise.
On the good side it changes much more. What would happen if c122 & c124 was shorted to the power rail? I will take the voltages at Q122 & Q126 when I get home later today. Q122 was replaced already with the same P/N transistor, as it was shorted, I can not remember which pins, I will check that too later.
Sorry for delay in getting more info, Ok this is what I have:
Q122 : E=0V, C= -47.2, B= -0.5
Q126 : E=-0.5V, C = 47.9, B = 0V
Original Q122 was shorted between Emitter and base.
If anyone could please help?
Q122 : E=0V, C= -47.2, B= -0.5
Q126 : E=-0.5V, C = 47.9, B = 0V
Original Q122 was shorted between Emitter and base.
If anyone could please help?
No the part number for 122 is good, I just replaced Q124 with an BD139 and its working ok now, strange I had tested that previously. I will bias it now and run it for an good few hours on the current limitor first
Hi, I just tested it again and the middle pin / collector on Q122 is indeed spot on at 0.6V now, looks like Q124 was causing the wrong reading, either that or I got the pins mixed up reading this on, but Q124 out of circuit is not reading anything in diode mode. Bias is remaining stable, I will check it again when not on the current limitor and also when hot. Thanks for your input.
BD139 seem like an ok replacement, the Maximum Collector Current is slightly lower, original 1.5A and bd139 is 1A would this be an issue in the position its in?
I have no doubt it would be, but I don't have one to hand, as long as the 1A is ok, I am happy. This already had lots replaced on it, including output transistors, pre-drivers and a few blown resistors.
Yap I had all that done, Power rails was first inspection, then the power resistors, I always inspect my soldering under magnification and good light, Even though I have been soldering for over 30 odd years there can always be an dodgy connection, or an small ball of solder, or solder that comes out of the solder sucker short something. I always clean the flux with IPA and an hold tooth brush, then its easier to inspect the connections. I must say the tracks on these Nad's are fairly weak even with an good temperature controlled soldering iron its easy to damage pads. I presume these were all designed in UK and made in china.