Noisy relay now dead - help ~!

[Nick Mulder]

Hello,

My stereo amp (NAD 317) when I turned it on started making a ratcheting up buzzer noise which would last about half a second then 'start up' (some relay would click on and all was well)...  This lasted a few days until this weekend when I was going to have a look at it but today it no longer switches on or anything, power = nothing ...

Take a look at the photo - I'm getting AC past the fuses at 'C' (checked the fuses to be sure) - and 15v DC at both left and right amplifiers +VL ('B' is left) - I dont see any negative DC though on the -VL wires (is there meant to be -ve ?).

I figure the noise had a typical relay buzzer sound to it and suspect it could the relay marked 'A' (48V 250VAC) - there are relays on the audio inputs, but I just don't think its them, of course I'm open to any suggestions as to what is going on...

Has some cap failed in the AC to DC power doohickey ?

~What to check, and how to check it ?

any help appreciated,
N

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[Hero999]

You could replace the relay of course but I don't think that would solve the problem.

Do you have a schematic?

If not you could reverse engineer the part of the schematic around the relay.

My guess is the relay limits the inrush current to avoid tripping the breaker. There will be a high power resistor in series with the mains inlet before the transformer. This limits the current from the mains until the filter capacitors on the DC side are fully charged. When the DC voltage on the rectifier rises above a certain threshold, the relay will turn on and short circuit the power resistor, allowing the amplifier to work at full power. There's probably something wrong with the circuit responsible for activating the relay.

 

[Nick Mulder]

pdf is 3.3MB

hmmmm

here's the PSU pages though...

Too small ?? 

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