NAD C165BEE Wont turn on

[andreaspedersen]

Hello.
I have a NAD-C165BEE preamp that wont turn on, but if I fiddle alot with the main powerswitch on the back sometimes the lights come on and I can use it as normal.
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The problem as far as I can see is that the 5.6V on the top left is only at about 1.2v - 2.3V

Could anyone of you explain how the switching begins around T51 ?
I understand that when the voltage exceeds 5.6v the optocoupler shuts off the MOSFET. But I cant understand how the voltage on the secodary side gets to that point in the first place.
I have attached the servicemanual, Please advice.

[wasedadoc]

IC51 is a switching PSU chip.  Have a look at a datasheet for that.

The standby supply (IC52) is used to activate the relay which switches input mains to the bridge rectifier.

[andreaspedersen]

Ok, thank you. What would be the appropriate steps to diagnose why the voltage level is only 1.2v and not 5.5/5.6v?
I will try my battery scope and see if the switching is working.

[wasedadoc]

I'm a bit reluctant to offer advice on testing and diagnosing that as the output of the bridge rectifier into that chip is about 340 Volts (assuming 230 Volt AC supply) and is NOT isolated from the mains.  As such it is dangerous to anyone not having the necessary knowledge and experience.  Unfortunately from your posts so far I am of the opinion that you are such a person.  Apologies if that is not the case but I am erring on the side of caution as I much prefer a live you and dead equipment instead of vice versa.

[andreaspedersen]

Thank you for your consideration! I am fully aware of the danger. Its been a few years since i last played with high voltage electronics, but i know the basics. I know that there is 230v * 1.41 after the rectifier.  I also have a set of isolating transformers that i used to drive my mains driven scope. I will rhis time use my fluke scopemeter that is battery operated, just to avoid needing to wire up my transformers.

In the datasheet i got the impression that this mosfet switch was off until it got enable signal. That confuses me, as i cant understand the first switching cycle.

[wasedadoc]

1.  You should NOT use your isolating transformer to power an oscilloscope.  You should use the transformer to power the equipment you are testing.

2.  The IC is a lot more than a MOSFET.  It is a complex circuit including an oscillator, pulse width modulator, driver and switching device which pulses the primary of the transformer.  The opto provides feedback to control the IC so that the rectified secondary is the desired voltage.

First check is that there is about 340 Volts across the capacitor at the output of the bridge.

Second is to disconnect the 5 volt dc output from the rest of the equipment.  If the 5 volt recovers there may be fault in the circuits it is powering, not in the power supply itself.

[andreaspedersen]

Thank you for your reply. I will not use the transformer to power the scope, but the equipment

I have checked that there is 323V over C53 after the rectifier. I have also tried to disconnect the main board from the powerboard, it had no effect.

I will next try to look at the waveform, and see if the switching is running. I will also check the D58 and D59 diodes. Next I guess are the caps?

Thanks!

[james_s]

With switchmode power supplies the first thing I always check is electrolytic capacitors. It is very common for them to fail.

[wasedadoc]

The switcher must be running.  If it was not there would be no voltage at all on the secondary side.

In addition to the electrolytics on the secondary side (especially C59) also check C52 and the Zener diode.

[andreaspedersen]

You got it right!
It was C59 that was bad, also C54 and C58 was bad. Replaced all of them, and
now the amplifier starts right up, and plays nice. On thing I am worried about though..
I can see on the thermal camera that the +27v regulator Q101 at idle gets hot. +60degrees, but the negative side does not(a bit over ambient temp).

Any advice here?

[wasedadoc]

Measure the voltage drops across R169 and R177 and calculate the currents. If they are significantly different, investigate why.

[james_s]

Does the +27V output power other things? Illumination? Protection circuits? Daisy chained regulator that provides a lower voltage? There are valid reasons the load could be higher on the positive rail, but it could also be a fault.

[Kim Christensen]

I can see on the thermal camera that the +27v regulator Q101 at idle gets hot. +60degrees, but the negative side does not(a bit over ambient temp).

I wouldn't worry about it. +60C should be OK. That transistor is dropping 11 volts or more across it.

[james_s]

The temperature isn't the issue, it's the fact that one is running considerably hotter than the other that is questionable. There may be a perfectly good reason for the load to be higher on that rail but I don't know.

[coromonadalix]

hum  i would say it could have a problem, getting more resistive,  check it base voltage and maybe its driver   who form a darlington ....

q101 should dissipate the same as q108  unless it is destabilized by something, unless the zenner   or the 27v  line consumes way more  as written earlier

check the c165 board   next to the circuit in the SV manual   


normally  the circuit should be well balanced and symmetrical for the power lines

[Kim Christensen]

The temperature isn't the issue, it's the fact that one is running considerably hotter than the other that is questionable. There may be a perfectly good reason for the load to be higher on that rail but I don't know.

Looks like there's more circuitry connected to the +27V rail than the -27V rail. Most of the negative rail just feeds the negative supplies of OP-Amps.
One thing he could check is the difference in temperature between IC108 and IC109 since these derive a +15V rail and -15V rail from the +27V & -27V rails... That could narrow the scope of the search if IC108 is the hotter one.
But I think he's just chasing ghosts if the pre-amp is now working 100% like he stated. (Assuming he has tested every function/input)

[coromonadalix]

and yes  if it works well now   

[JDestuyver]

Hi Guys,
I've read the comments and suggestions to fix a C165.
My C165 is dead. What happened is the following:
I pushed the power on, the front panel one (switch at the back is always on) but main power in the house was out.
Unfortunately main power came up at the very same moment I was pushing the power on (front panel).
Now the C165 doesn't respond anymore. No amber, red, blue light.
When I disconnect the main board with the small plug and white 3 cable ribbon cable 5,6v is available at C54 on poweboard.
When connecting the main board voltage drops below 1v. I replaced IC51 already...
As you've been digging somewhat in these circuits is there any clue that could help me and point me to the defect component ?
MANY thanks !