Everyone -- I am trying to troubleshoot my NAD C740 stereo receiver, which either won't power on or won't consistently stay powered on.
I will start by saying that I am a hobbyist, and while I am decently handy with basic DC stuff, I am more hesitant when it comes to mains-powered equipment (that is, reluctant to poke around with a multimeter when it is powered on)
The central issue is that after a long period of non-use, the receiver powers on and all functions seem to behave normally, but after a couple of minutes, it powers itself off.
If I attempt to power the radio on again at this point, nothing happens. But if I allow it to "rest" for at least a few hours, I can power it on again. The amount of time it stays on after this "rest" seems to depend on how much "rest" it has had (more "rest" = stays on longer)
In some cases, the receiver can run quite happily for an hour or more, but if I manually turn it off, and then attempt to turn it on again right away, it simply won't turn on again.
At first I thought this was a power supply issue, specifically of a relay in the initial power supply stage. The service manual (with schematics) is at https://www.manualslib.com/manual/1254519/Nad-C740.html and in the lower right corner, the initial power supply stage is shown (also highlighted in the attached image). I replaced the relay notated as RL901 in an initial attempt to fix the radio. This doesn't seem to have changed anything.
It seems from the schematic that this relay is in fact energized (through the transistor DTC123JS -- not sure what type that is) by a signal coming from the socket labelled JL951A. I can confirm that socket connects to the "NAD Link" sub-board (presumably used for components to control each other). But that in turn is connected elsewhere in the radio, which I haven't explored.
Presumably that "elsewhere" in the radio turns the DTC123JS transistor on and keeps it on.
I'm really mystified as to what is going on -- the behaviour itself and why it is time-dependent (the "rest" periods). My guess would be that some component is failing as it "warms up" and turns that transistor off. Perhaps a bad cap? Is this the usual failure mode for a bad cap? None of the caps that I can see look obviously damaged or bulging, and the main power fuse is fine. But there are many dozens of capacitors in this receiver (some very large, some very small)
Has anyone seen behaviour like this in consumer electronics before? What are some other likely culprits?
Any advice would be appreciated, Happy New Year to all.
I will start by saying that I am a hobbyist, and while I am decently handy with basic DC stuff, I am more hesitant when it comes to mains-powered equipment (that is, reluctant to poke around with a multimeter when it is powered on)
The central issue is that after a long period of non-use, the receiver powers on and all functions seem to behave normally, but after a couple of minutes, it powers itself off.
If I attempt to power the radio on again at this point, nothing happens. But if I allow it to "rest" for at least a few hours, I can power it on again. The amount of time it stays on after this "rest" seems to depend on how much "rest" it has had (more "rest" = stays on longer)
In some cases, the receiver can run quite happily for an hour or more, but if I manually turn it off, and then attempt to turn it on again right away, it simply won't turn on again.
At first I thought this was a power supply issue, specifically of a relay in the initial power supply stage. The service manual (with schematics) is at https://www.manualslib.com/manual/1254519/Nad-C740.html and in the lower right corner, the initial power supply stage is shown (also highlighted in the attached image). I replaced the relay notated as RL901 in an initial attempt to fix the radio. This doesn't seem to have changed anything.
It seems from the schematic that this relay is in fact energized (through the transistor DTC123JS -- not sure what type that is) by a signal coming from the socket labelled JL951A. I can confirm that socket connects to the "NAD Link" sub-board (presumably used for components to control each other). But that in turn is connected elsewhere in the radio, which I haven't explored.
Presumably that "elsewhere" in the radio turns the DTC123JS transistor on and keeps it on.
I'm really mystified as to what is going on -- the behaviour itself and why it is time-dependent (the "rest" periods). My guess would be that some component is failing as it "warms up" and turns that transistor off. Perhaps a bad cap? Is this the usual failure mode for a bad cap? None of the caps that I can see look obviously damaged or bulging, and the main power fuse is fine. But there are many dozens of capacitors in this receiver (some very large, some very small)
Has anyone seen behaviour like this in consumer electronics before? What are some other likely culprits?
Any advice would be appreciated, Happy New Year to all.
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