NAD C370 input selector issue

[grolin]

Hi!

I have just bought a second hand NAD C370 amplifier but an issue appeared.
As default the CD input is active when the amp is switched on (this is correct from the factory). So far so good but when I try to switch input source the only audio that is coming out from the speakers is the CD input source. Only Tape Monitor affects this (and Mute). The amp shows that another input has been chosen by the lights on the front panel and a small click also happens for each input selection done.
The sound sent from the amp that is produced by the speakers is not distorted by any means.

I have tried to locate which relays that clicks when changing inputs but have not been able to. Also did a quick check inthe service manual but I am not that good that I can find out which relays that affects the input selection...

Does anyone know what can cause this issue?

Thanks in advance!

Magnus

 

[Chris Daly]

Hi
I just looked up the schematic for you
NAD C370 SM Service Manual free download, schematics, eeprom, repair info for electronics

and section I would check is what grounding occurs after
what looks to be a connection block called C8806
the relays are arranged with 12v+ primed to go
and what is likely to be the input selector just grounds
each relay coil relative to that input. see if
you are getting 11.5v or so, from each wire of C8806
that should be the contacts of the switch,
the pole of the switch would follow as being ground.
It is probably not that simple, as will no doubt involve
an integrated circuit.

I will have a closer look at the schematic to check

Cheers / Chris

 

[grolin]

Thanks Chris for the quick reply!

I´m checking the service manual but I cannot see the block called C8806.
Are you sure you do not mean CB806?
If CB806 is the one I should be looking for I cannot see that one either...
Is it on page 11: "Wiring Diagram" in the .pdf?

Magnus

 

[JonSnell Electronic]

Is the supply to IC905 OK? That IC drives the transistors that switch the relays. With no relay power you will have no sound, from any input so the supply for the relays I will assume is OK.

 

[Chris Daly]

Thanks Chris for the quick reply!

I´m checking the service manual but I cannot see the block called C8806.
Are you sure you do not mean CB806?
If CB806 is the one I should be looking for I cannot see that one either...
Is it on page 11: "Wiring Diagram" in the .pdf?

Magnus

Yes CB806 ( must get a brighter light here )
so IC905 which is a 8 bit shift register
as Jon points out provides a pulse
to bases of transistors q901 - q908

Check for 5v+ at C915, if you have a scope
check for a waveform at mid point of the two
4k7 resistors applying to each input selection

IC905 is supplying the base of each transistor
that then provides connection via the
transistor emitter to ground, which then engages
the relay coil.

Interesting you said before you can hear the relays
working. With an input selected measure the coil
voltage, and see what is there.


Cheers / Chris

 

[Mooly]

These kind of faults can be difficult. I haven't got the circuit in front of me but from memory you need to check that the 'keyscan' to the system control IC is OK. In practice that means checking that the switches (the one that its stuck on) isn't short or leaky and holding the processor in a non valid state. Go around all the pins on the IC and confirm that you see what is expected. Its quite possible the IC is faulty or 'spiked' and stuck in a non responsive state.

See if you can detect any change in logic levels on the two 4094 ?? IC's which are used as a kind of data expander. If you are not seeing any change on the inputs to these then that again points to a possible problem with the main system control chip.

And check the supplies to these chips. Make sure they are clean and of the correct value.