NAD C300 amplifier failure - fuses repeatedly blow

Archived from groups: rec.audio.tech (More info?)

Since it's smaller fuses blown, I would look for a shorted diode or
capacitor in the low-current power supply area.

Mark Z.

--
Please reply only to Group. I regret this is necessary. Viruses and spam
have rendered my regular e-mail address useless.


"Jason Le Vaillant" <dont@spam.me> wrote in message
news:4083776d$1@news.iconz.co.nz...
> My NAD C300 (a cut down C320) amplifier has died. The saga began when I
> noticed that the power light was red, indicating "overload". I turned
> the amplifer off, and then back on sometime afterwards, and the power
> light remained off - my amp was now completely dead. A great amount of
> fiddling ensued, with no result.
>
> I took the top part of the amplifier casing off and discovered that two
> 500 mA fuses on the power board (connected to the white and peach wires
> from the transformer) were blown.
>
> I replaced the fuses, and turned the amp on. The power light flashed red
> for a second, then nothing. I discovered that the replacement fuses had
> blown immediately.
>
> Investigations continue, but suggestions (or schematics, which I
> presently lack) would be much appreciated.
>
> --
> Jason Le Vaillant

Archived from groups: rec.audio.tech (More info?)

I don't have a schematic to look at - you're going to need one, it seems. It
was just my thought that if it was smaller fuses blown, then the main
rectifiers / filter caps / output transistors were probably OK, therefore
the problem should be somewhere else. I've never worked on this exact model,
though.

Mark Z.

--
Please reply only to Group. I regret this is necessary. Viruses and spam
have rendered my regular e-mail address useless.


"Jason Le Vaillant" <dont@spam.me> wrote in message
news:409753ce@news.iconz.co.nz...
> Mark D. Zacharias wrote:
> > Since it's smaller fuses blown, I would look for a shorted diode or
> > capacitor in the low-current power supply area.
>
> I tested all components on the power board, and it seems fine.
>
> Where else could there be a problem?
>
> What is the function of the low current power supply?
>
> --
> Jason Le Vaillant

Archived from groups: rec.audio.tech (More info?)

jfaudio wrote:

> Could be a difficult task without a print.

Indeed. Any idea where I might obtain schematics for the C300?

> What i would do is see if by resistance measurements you can determine
> which supply (-5,12,-12) from that board is shorted to ground. If you
> get lucky you may be able to track down the shorted component on that
> line.

-5, 12, 12? Hmm. I'm not sure how these voltages correspond to the
outputs from the power board. Basically, from the powerboard come six
wires, carrying signals from two diode bridges.

The first set of wires are red, green and black. The red comes from the
positive end of the first diode bridge, the green is negative, and the
black is ground. The current through these wires is limited by the 0.5A
fuses, which have blown. The final resistance between the red terminal
and ground (once all capacitors are charged) is 1kOhm. The corresponding
resistance between the green terminal and ground is 8.21 kOhm.

The second set of wires are yellow, blue and white (white is ground in
this case). The current through these wires is limited by the 3A fuses,
and the resistances between the yellow/ground and blue/ground are
similarly positive and finite.

--
Jason Le Vaillant