Advice on repairing NAD 7150 Receiver

Before doing anything else, check the rear panel for a set of loop thru' connectors for going out to a sound processor of some description. They are usually 4 x RCA phono type sockets, and may be marked either "Processor" or "Pre out - Main in" or something very similar. There is often a thick bar silk-screened between them to indicate that they should be linked. Very often, an amplifier will arrive on the bench with exactly the symptoms you describe, and the original factory-fitted linking bars missing. Usually, the person is a 'new' owner who has picked the item up second hand, and doesn't realise that the sockets need to be linked. If you find it to be the case on this one, all you need is a short stereo RCA to RCA lead, to patch them back together.

Arfa

Thank you for your quick reply.

Your guess was indeed spot on. It works very well now :)

Now there is two smaller problems. First the balance controller is noisy. I'm going to try to clean/replace/bypass it. The phono input, however, gives a large amount of noise out even with volume turned way down and nothing connected to the input. I guess this means that the phono preamp is broken. Would just disconnecting the preamp circuit be a reasonable course of action?

Thanks again for the quick and precise answer. It saved me a lot of time!

/Rasmus

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Shorting plugs added to unused inputs ?

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

If you're not intending using it, it might be. Is the noise on both channels, and is it controlled by the volume control at all ? Can you get the noise to completely disappear, with the volume right down ? Bear in mind that phono preamps do generate considerably more 'hiss' than the other inputs, due to a very high gain stage being switched in ahead of the main preamp, when "phono" is selected.

As far as cleaning pots and switches goes, these older units usually respond quite well to a squib of good quality switch cleaner / lubricant. Make sure that you find a hole in the pot casing that you can get plenty of juice into, then scrub the control by rotating vigourously from end to end, for at least 10 seconds.

Arfa

Thanks for the advice on cleaning pots!

The noise on the phono is present on both channel and affected by volume. However, it is disturbingly high even with volume turned all the way down. It also does not sound like a high noise floor, but rather more like an uneven rumble.

/Rasmus

That suggests bad transistors in the phono section. But that isn't your basic problem, is it?

That is a confusing set of symptoms. Because the volume control affects the noise, that must mean that it is getting in *before* the volume control, so the control *should*, in theory, be able to turn the noise all the way down. If it is only present when the phono preamp is switched in, then again, that would suggest that it is getting in back at the preamp, so again, it should be able to be completely turned down by the volume control. If it was a 'normal' problem, such as a noisy transistor or cap, or even resistor, you would only expect the problem to be on one channel.

About the only scenario that I can come up with, which might cover all bases, is if it is a power supply problem. If power to the phono preamp is switched, as part of bringing that function on line, then it's possible that something in the phono preamp circuitry, is putting noise onto that supply line, which may also be used by later stages. If this is the case, then the phono preamp will output corresponding noise into the rest of the amplifier chain, which will be the component of your noise that *is* controlled by the volume control, and also, the noise on the supply rail will cause noise to come out of later stages as well, which will be your noise component that is

*not* controllable by the volume control. A supply rail problem would also account for the noise being on both channels.

As a first move, I would try disconnecting the supply rail to the phono stage. If that stops it, and you're happy with that, then just leave it. If you want to try to get a fix, I would be looking at rail decoupling caps, and transistors.

Arfa

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This is testable without disconnecting the supply rail. Switch the scope to AC, and look at the rails while listening to the amp.

This is quite true, but assumes that he has a 'scope, and knows what to be looking for ...

I went down the route of suggesting just disconnecting the supply to the phono preamp, because he had implied that he was not particularly interested in phono functionality, just in stopping the noise. A 'sledgehammer' fix if you will.

Arfa