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Need help understanding behavior of DBT

W

woostersoz

Member
Hi
I am trying to revive a NAD 7220PE. When I first powered it on via a Dim Bulb Tester, the bulb glowed dimly. Then I checked the 4 4A fuses and found that all of them had blown so I replaced them. Now, the bulb glows brightly indicating that the NAD has a short (and nothing sparked or blew, at least visibly, when I switched it back on)
Is this expected behavior? If yes, any tips on where to troubleshoot next would be very helpful.
thanks
 
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Bright bulb means you're drawing too much current. Do you know which fuses protect what? Service manual is a good place to start.

Process of elimination would suggest figuring out why the high amperage fuses blew to start with. I've seen lots of NAD's with bad caps in them.

I'd pull the output transistors and see if the bulb remains bright with them pulled.
 
Thanks. The service manual is too large to attach so I have attached the schematic only. Any pointers on what to start pulling out would be appreciated :)
 

Attachments

I'm not the guy to walk you through the repair, tbh. I'm not built for slogging through stuff.

Most guys would start by measuring the voltages at the transformer and check the voltages coming off of the power supply. If those are out of whack, nothing else can be right.

The output devices will draw a bunch of current which is why I pull them. If when they're pulled, the bulb starts bright then gets dim, you've eliminated them as a cause of the short. They are the most common user-generated failure point in most amps, and as much as I hate to say it, user error/carelessness is the first thing I look for unless I owned the piece when it worked.

I do a lot of this stuff in a different way than a trained tech might approach it. I take a triage type approach. Get the fault fixed and then go after the bruises and scrapes...
 
Measure the DC voltages between D509 and ground and then D 510 and ground. You should get -40 on one side and + 40 on the other. That's the output of the 4A fused power supply.
 
The bulb glowed bright with the fuses intact as the fuses completed a circuit which led directly to the blown component. Once the fuses had blown, there was no "short," so the DBT acted as it should have.

In an amp that's working fine - you may also get an initial bright glow as the protect circuit works, and the power supply caps charge up, then the glow should dim, and the protect circuit should click the speakers on (if all is working well).
 
No sense in replacing 4A fuses if they've blown already. Start pulling transistors from the outputs backwards, and see which ones are shorted. I don't have the schematic to hand, but I would hazard a guess like most NAD amplifiers, the earlier stages of the power amp are powered from the same regulated power supply as the preamp. The 4A fuses are for the less tightly regulated supply that power the outputs, the drivers, and a few other transistors in the power amp stage.

Lee.
 
What wattage bulb are you using in the DBT?

A small bulb can in some cases give a false indication of problem. That there are blown fuses speaks likely to a larger problem than just too small of bulb, but it is something to be aware of.
 
Pull the outputs and then try the amp on the DBT. If it starts bright and then dims down, you've found the problem. At that point replace the outputs and associated drivers.

If the DBT stays bright with the outputs pulled, the problem(short) lies elsewhere.

For me, If I'm pulling it, I'm probably replacing it unless it's really expensive or rare. Not a fan of going back in to replace something that I tested under less than real-world/ in-circuit conditions that decides to cause problems after I think I'm finished...
 
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