NAD Power Supply Question

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I am in the process of restoring an NAD 304 amplifier and the schematics online are pretty blurry in the supply section. The common R333 and R334 are fine but are to be replaced with Dales.

The main issue was early protection mode due to a blown capacitor at C804. All 4 associated caps have been changed, but I am concerned that I am seeing 58V across the 50V caps and want to avoid destroying the new ones if possible.

According to the diagram, the test points would be at the caps. Am I wrong in thinking this is abnormal voltage? Could it be that the diodes aren't clamping as the should?

I'm pretty green when it comes to this sort of thing so any input would be greatly appreciated.
 
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This is a "stacked supply, so the caps actually have less voltage across each than the total.
Have you seen this? http://www.condoraudio.com/wp-content/uploads/Projects/NAD-304-Restoration-Repair-Upgrade.pdf

Thanks for replying! I do have that document and used his recommended supply caps. I did some poking around and checked the preamp voltages at R131 and R132 and they seem to be inline with what the schematic says. I really wish there was a better scan in the wild.

I am currently consulting the oracle o' Google to try and get a better grasp on stacked supplies.

I'm hoping someone out there knows a definitive test point and voltage I can reference to put this bed. I'm probably being overly paranoid, but judging by the cap that was removed it was over stressed. It was more exploded than just leaky.
 
Thanks for replying! I do have that document and used his recommended supply caps. I did some poking around
and checked the preamp voltages at R131 and R132 and they seem to be inline with what the schematic says.

This supply uses a secondary with 5 taps instead of 3. The center tap is grounded as usual.
The middle pair of taps each use a pair of diodes to output each polarity of the low voltage.
The outer pair of taps each use a pair of diodes to output each polarity of a higher voltage.

The trick is that instead of the outer diodes going to a capacitor which connects to ground,
that capacitor goes to the top of the low voltage capacitor. This allows two smaller, lower
voltage, cheaper capacitors to be used for the higher voltage, instead of two larger, higher
voltage, more expensive capacitors. The drawback is that the capacitance of all 4 parts
needs to be higher than it would otherwise be.

In any event, the voltage across each capacitor at its terminals should be less than the rated voltage.
 
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Thanks for all the input guys. I know its important about the voltages, thats why I'm concerned.

I've not had a chance to pull the diodes out of circuit to test but by my limited knowledge and looking at the schematic I really cant see much else that would cause the higher than normal voltage across the rails.

Would it be somewhat plausible it could be an issue with one of the diodes D801-804?
 
Thanks for the input rayma. I'm going to take a deeper look later this evening to see what I can find. The last time I checked my mains voltage it was pretty spot on.

I am trying to avoid having to clean up so much electrolytic the next time one of these large caps goes. This was supposed to be a "works great" e-bay purchase.... We all know how that goes sometimes.

I very well may wind up putting 63V caps in if I can't find the issue. The service manual doesn't state what the transformer voltages are so I can't reference anything there.
 
OK, so it was two shorted diodes at D806 and D807. Once those were replaced the voltage was pulled back down which I would expect per the schematic. Since these are unidirectional TVS's I assume they are functioning as a zener would in circuit.

Now off to set bias and idle and load test on the scope.

Thanks for the info guys!
 
TVS are interference attenuators.
The spikes on the mains can go to very high transient voltages for very short periods.
The TVS job is to short when the high voltage spike arrives.
These generally wear out, but can last for decades.
I think you have been unlucky.
They are usually rated for the highest AC mains voltage you will see, i.e. for >253Vac and the next value would probably be 275Vac.
They actually work on the peak voltage where a 275VAC TVS will not pass @ 389Vpk and then go into some form of avalanche mode.

I wonder if your (US import) NAD had 125Vac versions, rather than 150Vac versions?
 
TVS are interference attenuators.
The spikes on the mains can go to very high transient voltages for very short periods.
The TVS job is to short when the high voltage spike arrives.
These generally wear out, but can last for decades.
I think you have been unlucky.
They are usually rated for the highest AC mains voltage you will see, i.e. for >253Vac and the next value would probably be 275Vac.
They actually work on the peak voltage where a 275VAC TVS will not pass @ 389Vpk and then go into some form of avalanche mode.

I wonder if your (US import) NAD had 125Vac versions, rather than 150Vac versions?

These diodes aren't on the mains at all. They are post transformer and have a working voltage of ~28 volts. The shorted diodes caused two of the rails to sum. It shouldn't have been doing that. The higher than usual voltage eventually caused one of the filter caps to go.

All voltages now are within +- a few percent of the schematic. Idle current and offset were still well within spec and have been adjusted further.

It did a great job on the bench and look forward to giving it a listening test soon.
 
Could they be using the low voltage TVS as a crowbar to prevent a faulty too high voltage getting downstream?

Odd that the TVS failing should cause a worse voltage than what it may have been intended to prevent.

That would be my guess. I'm still pretty new to all this, but I do know those two shorted diodes were the cause of all the madness. I've gone through most of the components and everything else looks good. I suppose it's possible this unit saw a strong mains voltage spike that caused them to fail short.

From my reading it seems these can be used the same way a zener would be used.
 
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