NAD C350 volume control problems !!!

[mprivitera1]

Hi guys,

I have a problem with the volume control, it works for a few seconds then nothing, after minutes even half an hour it starts working again, it goes up and down for a few soconds ... then nothing. After I can find a replacement as it is 8-pin ?

Thanks

 

[JonSnell Electronic]

Check the little transistors that supply the power to the motor.

 

[mprivitera1]

Check the little transistors that supply the power to the motor.

..you think one of the 4 transistors is broken .... I try to replace them ..
Is there any equivalent A1015?

 

[JonSnell Electronic]

What happens is that over time they can fail.
Place your DVM across the motor supply, (between C341+ and C 342+) measure the voltage. That will tell you if there is an issue with the transistors.
2SA1015 can be replaced with almost any PNP signal transistor with a reasonable hfe; 2N4403, BC558 etc etc etc.

 

[JonSnell Electronic]

DO NOT change transistors for the sake of it, it could be a dirty commutator in the motor!

 

[sgrossklass]

DO NOT change transistors for the sake of it, it could be a dirty commutator in the motor!

Seems a great deal more likely either way. 2SA1015s would probably sustain <5.6 V approximately forever.

That pot doesn't look super expensive in the first place (NAD parts quality...). It could very well be that the motor just needs a bit of running nonstop at full voltage to clean the brushes and all, at least that seems to help older DC motors. The downside is that you'd have to get the motor out first, or at least decouple it from the pot, so unsoldering and dismantling the thing would seem quite unavoidable.

Do look at motor voltage as suggested.

 

[Mark Tillotson]

Its rather a dodgy looking H-bridge circuit, there are no free-wheel diodes to suppress voltage spikes, only capacitive spike suppression, which causes high current spikes on switch-on whether or not it succeeds in snubbing high voltage spikes on switch-off. Perhaps the control lines are switched slowly. Hopefully with enough dead time to prevent shoot-through.


Basically check all those transistors, and definitely replace the capacitors before powering up again, if they've dried out inductive kick-back spikes will likely happen.


Also I'd apply a little machine oil to the bearings if accessible.

 

[mprivitera1]

... I measured the heads of the motor volume, how much I press the volume remote control up or down I measure 0V and of course the potentiometer doesn't move. how it works I measure 4.3 volts.
on the connection line in the +5volt / gnd I read 4.7volt .....

 

[Mark Tillotson]

Remove, re-seat them, and consider reflowing all the solder joints for the connectors.


(And replace those capacitors!)

 

[stocktrader200]

does it go either direction or nothing at all? 2SA1015 is 150ma max so it could be bad.
c-b pins are reversed on 2N3906 PN2907A so they won't drop in without pin twisting.