Hi!
Just wanted to share my experience from a few days ago when I found myself a rather nice (IMHO) looking NAD C 720BEE together with a C 525BEE CD player.
The amp was acquired as a replacement for the NAD 302 which I'm now a bit hesitant to leave running for longer periods due to it's lack of any protection circuit.
(More on that later).
The C 720BEE was cleaned and then tested without any issues whatsoever. However, a few days later when starting the 720BEE it remained in red (protected mode) regardless of speakers connected or not and I thought, bummer, time for a re-cap then.
(The SM only guided me to check the protection circuit to proceed though, not very helpful here).
However, before starting with the re-cap I really wanted to find the actual cause for this sudden regression - and soon found that the -18V supply was only at -1.16V when ON.
Ok, then tried to trace all possible caps involved in with the -18V supply without any success. Also, there was no obvious indication of abnormal current draw from the -37V either...
Strange I thought and then my wife suddenly appeared in the doorway and I quickly explained what I was doing and trying to find and then she out of the blue (she's very intuitive and smart) told me that I might be searching in the wrong place(s) and asked me about the circuit board and suggested that I should take a look at that instead:
...and she was right, the circuit board was to blame for this regression and the reason for the missing -18V was due to
that R414 didn't have any connection with the +18V side as the schematics below indicates:
The fix was a rather crude one with a link soldered to remedy the above and then it was back to the living!
(The soldering on this board didn't look too nice to start with, this unit could have been a lemon 😉)
Now I'm just awaiting new 2x15000uF caps for the power supply to arrive to finish the re-cap.
(Horrible that these 15000uF caps are live even when the AMP is in standby mode, it's really time since one of them has a small bulge...)
More to follow when AMP has been fixed up to specs!
Thanks for reading,
Per
Just wanted to share my experience from a few days ago when I found myself a rather nice (IMHO) looking NAD C 720BEE together with a C 525BEE CD player.
The amp was acquired as a replacement for the NAD 302 which I'm now a bit hesitant to leave running for longer periods due to it's lack of any protection circuit.
(More on that later).
The C 720BEE was cleaned and then tested without any issues whatsoever. However, a few days later when starting the 720BEE it remained in red (protected mode) regardless of speakers connected or not and I thought, bummer, time for a re-cap then.
(The SM only guided me to check the protection circuit to proceed though, not very helpful here).
However, before starting with the re-cap I really wanted to find the actual cause for this sudden regression - and soon found that the -18V supply was only at -1.16V when ON.
Ok, then tried to trace all possible caps involved in with the -18V supply without any success. Also, there was no obvious indication of abnormal current draw from the -37V either...
Strange I thought and then my wife suddenly appeared in the doorway and I quickly explained what I was doing and trying to find and then she out of the blue (she's very intuitive and smart) told me that I might be searching in the wrong place(s) and asked me about the circuit board and suggested that I should take a look at that instead:
...and she was right, the circuit board was to blame for this regression and the reason for the missing -18V was due to
that R414 didn't have any connection with the +18V side as the schematics below indicates:
The fix was a rather crude one with a link soldered to remedy the above and then it was back to the living!
(The soldering on this board didn't look too nice to start with, this unit could have been a lemon 😉)
Now I'm just awaiting new 2x15000uF caps for the power supply to arrive to finish the re-cap.
(Horrible that these 15000uF caps are live even when the AMP is in standby mode, it's really time since one of them has a small bulge...)
More to follow when AMP has been fixed up to specs!
Thanks for reading,
Per
The wire link seems to be bridging existing track - did you find the crack or was it just a case of bad soldering of the resistor lead?