[SOLVED]
Hello everyone..I am new to the forum. I have a problem with NAD 3150 amplifier to which all capacitors Pre and Main sections were replaced, because the originals were now in bad condition; new ones are Nichicon, Panasonic, Elna. Replaced transistors Q609-610, Q619-620, Q621. Also replaced the VR601-2-3-4 trimmers. Following the NAD service manual, I make the DC offset adjustment without any problems; I then switch to BIAS..I start with VR603 and stop at 5 mV stable, then move on to VR604..but as soon as I get to 2.2/2.4 mV a "jump" happens towards 30-40 mV and beyond, and the left channel power amps Q615-617 warm up; so I turn off and start again, but the same thing happens again. I then attempt to start with VR604..which this time gets settled steadily to 5 mV, without "jumps"; I then go to VR603 and it happens that it now jumps exactly as VR604 did before, and it heats up the right channel power amps Q616-618. In practice, the situation is mirrored, with "swapping" of settings-voltages. The power supply on the two channels is perfectly balanced, according to the voltages in the schematic. Balance at center, Bridging OFF, speakers impedance Normal. I have only a NIMEX NI8400 digital multimeter autorange.
Any suggestions? I thank in advance those who can help me.
Hello everyone..I am new to the forum. I have a problem with NAD 3150 amplifier to which all capacitors Pre and Main sections were replaced, because the originals were now in bad condition; new ones are Nichicon, Panasonic, Elna. Replaced transistors Q609-610, Q619-620, Q621. Also replaced the VR601-2-3-4 trimmers. Following the NAD service manual, I make the DC offset adjustment without any problems; I then switch to BIAS..I start with VR603 and stop at 5 mV stable, then move on to VR604..but as soon as I get to 2.2/2.4 mV a "jump" happens towards 30-40 mV and beyond, and the left channel power amps Q615-617 warm up; so I turn off and start again, but the same thing happens again. I then attempt to start with VR604..which this time gets settled steadily to 5 mV, without "jumps"; I then go to VR603 and it happens that it now jumps exactly as VR604 did before, and it heats up the right channel power amps Q616-618. In practice, the situation is mirrored, with "swapping" of settings-voltages. The power supply on the two channels is perfectly balanced, according to the voltages in the schematic. Balance at center, Bridging OFF, speakers impedance Normal. I have only a NIMEX NI8400 digital multimeter autorange.
Any suggestions? I thank in advance those who can help me.
Last edited:
Welcome to the forum!
I assume you are setting bias without any attached load?
I'm afraid you really need a scope, as this may be a tough problem. I am suspicious of oscillation, but you would need a scope to investigate. A scope might also offer clues about what is causing this odd interaction between channels.
I've attached a service manual I found at manualmachine.com for other members. Legibility is not wonderful.
Good luck!
I assume you are setting bias without any attached load?
I'm afraid you really need a scope, as this may be a tough problem. I am suspicious of oscillation, but you would need a scope to investigate. A scope might also offer clues about what is causing this odd interaction between channels.
I've attached a service manual I found at manualmachine.com for other members. Legibility is not wonderful.
Good luck!
Hello, and thank you for your response.
Unfortunately, I don't have oscilloscope. But I wonder one thing..why do the two trimmers of the BIAS (VR603-604) alternately make the same "jump" from 2.4mV to 30mV and more? Each separately gets nicely adjusted, but if I switch from one to the other this defect happens. I could think of the old trimmers, but these are brand new.
Thank you for your patience. (I am attaching a schematic of the NAD)
Unfortunately, I don't have oscilloscope. But I wonder one thing..why do the two trimmers of the BIAS (VR603-604) alternately make the same "jump" from 2.4mV to 30mV and more? Each separately gets nicely adjusted, but if I switch from one to the other this defect happens. I could think of the old trimmers, but these are brand new.
Thank you for your patience. (I am attaching a schematic of the NAD)
Without a scope, I can only guess--- entirely not a good a good idea.
Can you tell us history of this amp? Is it newly acquired? Have you replaced any transistors? I gather you've replaced caps. Did you also replace small values also, eg. compensation caps?
What you describe sounds like oscillation--- an abrupt change in behavior, and associated dramatic increase in heatsink temperature. Often the oscillation causes both output transistors to conduct simultaneously, and they "totem pole" current from one supply to its opposite supply. It can lead to destructive failure. You should confirm the Zobel network is intact--- R561 and the two series caps. But I emphasize, this is pure guesswork.
Can you get help from a friend with some equipment?
Can you tell us history of this amp? Is it newly acquired? Have you replaced any transistors? I gather you've replaced caps. Did you also replace small values also, eg. compensation caps?
What you describe sounds like oscillation--- an abrupt change in behavior, and associated dramatic increase in heatsink temperature. Often the oscillation causes both output transistors to conduct simultaneously, and they "totem pole" current from one supply to its opposite supply. It can lead to destructive failure. You should confirm the Zobel network is intact--- R561 and the two series caps. But I emphasize, this is pure guesswork.
Can you get help from a friend with some equipment?
Hello, and thank you for the explanation. So I will answer your questions, and first the Zobel circuit..it is fully functional, I desoldered the components from the PCB and tested them with the multimeter, they are all within the values in the schematic. This NAD has been mine since 1985, but not to be too long I'll tell you that it has always sounded good; it was repaired about 10 years ago, but since the sound was no longer good, last December 2023 I took it over to a general electronic repair service, not for hi-fi. And I gave them schematic and service manual; at my request they installed quality components (Panasonic, Elna, Nichicon). They then returned it to me, but the NAD was turning on in protection mode; called the service they told me it was normal (???) and would fix itself. Absolute lie. Meanwhile, the service now no longer exists, but I was able to know that they replaced all the capacitors in the Main section, the relay with the diode, the IC701 protection integrated (TA7317P) and all the capacitors/resistors around it, then transistors Q609-610, Q619-620, Q621.
In order to solve it, I set out to look at the schematic (I'm a computer programmer, not an electronics technician) and following the protection IC i replaced Q619-620 again..and the NAD came out of the protection..it turns on regularly after the relay clicks, and it hasn't been going into protection for weeks; I also replaced the trimmers 601-2-3-4 myself, with Vishay single turn ones like the old ones. Also checked with the multimeter all the capacitors by the service replaced, one by one outside the PCB, they are perfect. However, it does give me the BIAS defect, as I've already explained.
Thank you for your attention
In order to solve it, I set out to look at the schematic (I'm a computer programmer, not an electronics technician) and following the protection IC i replaced Q619-620 again..and the NAD came out of the protection..it turns on regularly after the relay clicks, and it hasn't been going into protection for weeks; I also replaced the trimmers 601-2-3-4 myself, with Vishay single turn ones like the old ones. Also checked with the multimeter all the capacitors by the service replaced, one by one outside the PCB, they are perfect. However, it does give me the BIAS defect, as I've already explained.
Thank you for your attention
Wow, you've had a tortuous experience! Perhaps the repair shop is gone with good reason.
I'm still suspicious of oscillation, for want of any better theory.
Had you attempted bias adjustment yourself before taking it to the shop? I'm trying to determine if this problem might have been present before, or if the shop introduced the problem when they replaced Q609-610. Do you know if they used the same type replacement? I surmise the power output transistors are still original? I will also note that counterfeit transistors are a rampant problem when not purchased from mainstream distributors.
Of the transistors mentioned, only Q609-610 might provoke oscillation. The others are all protection related and not in the signal path.
To elaborate a bit, the Q609-610 circuit is a "bias spreader", aka a Vbe multiplier, and it senses/tracks temperature changes in the power transistors. It's a widely used circuit. However, these's an unusual detail in your NAD that I've never seen before. Universally in my experience, the collector of Q609 would be connected at R637 and C623; the bandwidth of the spreader would is relatively slow, restricted by C623. But in your amp, Q609 is connected into the base of Q613 and becomes a bit more involved in the amp's signal path--- unusual, but I'm not sure it's relevant.
I'm still suspicious of oscillation, for want of any better theory.
Had you attempted bias adjustment yourself before taking it to the shop? I'm trying to determine if this problem might have been present before, or if the shop introduced the problem when they replaced Q609-610. Do you know if they used the same type replacement? I surmise the power output transistors are still original? I will also note that counterfeit transistors are a rampant problem when not purchased from mainstream distributors.
Of the transistors mentioned, only Q609-610 might provoke oscillation. The others are all protection related and not in the signal path.
To elaborate a bit, the Q609-610 circuit is a "bias spreader", aka a Vbe multiplier, and it senses/tracks temperature changes in the power transistors. It's a widely used circuit. However, these's an unusual detail in your NAD that I've never seen before. Universally in my experience, the collector of Q609 would be connected at R637 and C623; the bandwidth of the spreader would is relatively slow, restricted by C623. But in your amp, Q609 is connected into the base of Q613 and becomes a bit more involved in the amp's signal path--- unusual, but I'm not sure it's relevant.
I own a 3150 as well, for 30 years or so now. At one point it developped rather substantial DC offset in one channel, that I traced back to some resistors in the power amp that had gone way beyond specs. Iirc they were 22k measuring over twice that value. Upon further investigation I found a lot more resistors that had drifted 20% or more so I ended up replacing quite some parts. That would not directly explain the jump you see but worth checking some basic values here. And try to get hold of a scope to check for oscillations and make sure the replaced transistors are genuine as already pointed out here.
Also the +/-24V regulators are always suspect in these old NAD amps. They run incredible hot.
Also the +/-24V regulators are always suspect in these old NAD amps. They run incredible hot.
Thanks for the answers,
maudio..unfortunately I don't have an oscilloscope, but still I have already checked many components, including resistors...and they are all as per the schematic. But given the right observation of BSST, on the Q609-Q613 wiring...there are also Q611-612-614...if one or more of these are faulty or shorted, could channel "swapping" occur? Better to replace them?
maudio..unfortunately I don't have an oscilloscope, but still I have already checked many components, including resistors...and they are all as per the schematic. But given the right observation of BSST, on the Q609-Q613 wiring...there are also Q611-612-614...if one or more of these are faulty or shorted, could channel "swapping" occur? Better to replace them?
checked the Q619/620 and Q609/610 transistors again, all working and with voltages E,B,C, as per the diagram.
Perhaps one of the power output transistors Q615-16-17-18? How can I check these? (Multimeter only, in Ohm or diode test).
Thanks
Perhaps one of the power output transistors Q615-16-17-18? How can I check these? (Multimeter only, in Ohm or diode test).
Thanks
..the BIAS problem remains, but I have a small update. I should mention that I have now replaced all the transistors in the Main section of the both channels, except the power output transitors and Q607/608.
I did a test at the simple..set the left BIAS to 4.8.mV stable with VR630, left the multimeter leads on the left and went to turn VR604 (right channel)..and the left BIAS jumped to 80-100 mV !!! That is, the two trimmers "adjust" each other's opposite channel, and thus I cannot adjust the single channel's Bias. So it is obvious that there is an unwanted link between the two trimmers (channels), but how to find it? Do I disconnect the power output transitors from the PCB from one channel and test the other, and vice versa? Or any other ideas? Thanks to all
I did a test at the simple..set the left BIAS to 4.8.mV stable with VR630, left the multimeter leads on the left and went to turn VR604 (right channel)..and the left BIAS jumped to 80-100 mV !!! That is, the two trimmers "adjust" each other's opposite channel, and thus I cannot adjust the single channel's Bias. So it is obvious that there is an unwanted link between the two trimmers (channels), but how to find it? Do I disconnect the power output transitors from the PCB from one channel and test the other, and vice versa? Or any other ideas? Thanks to all
Hi,
I have an important update...among many measurements and checks, I came up with the idea of desoldering all the output transistors (including the new Q609/610 ones) from the PCB, I then tested them with the MM and they were found to be working. I then mounted the ones for the right channel, adjusted the offset (a bit swinging between 0-0.5-1.5 mV, going up and down..maybe due to the old transformers) then adjusted the BIAS to 5 mV, no "jumps" and very stable, and played the NAD..all ok. I then put in the left channel transistors, adjusted offset (like the right, a bit unstable) and BIAS to 5 mV also without "jumps" nor "swaps" with the other channel. Maybe it was some bad soldering. So I did the listening test with the two channels, sounded great..maybe a little too warm the transistors, but it could be my impression. Only a background "white noise" remains, audible with volume at quarter turn and above..but I think it is normal even if audible. I am left with the question of transistor temperature. And I sincerely thank everyone for all your suggestions and patience.
I have an important update...among many measurements and checks, I came up with the idea of desoldering all the output transistors (including the new Q609/610 ones) from the PCB, I then tested them with the MM and they were found to be working. I then mounted the ones for the right channel, adjusted the offset (a bit swinging between 0-0.5-1.5 mV, going up and down..maybe due to the old transformers) then adjusted the BIAS to 5 mV, no "jumps" and very stable, and played the NAD..all ok. I then put in the left channel transistors, adjusted offset (like the right, a bit unstable) and BIAS to 5 mV also without "jumps" nor "swaps" with the other channel. Maybe it was some bad soldering. So I did the listening test with the two channels, sounded great..maybe a little too warm the transistors, but it could be my impression. Only a background "white noise" remains, audible with volume at quarter turn and above..but I think it is normal even if audible. I am left with the question of transistor temperature. And I sincerely thank everyone for all your suggestions and patience.