Advice on repairing NAD 7150 Receiver

[[email protected]]

Hi all!

I am trying to repair an old NAD 7150 Stereo Receiver. The problem is
that there is no sound on the speaker output. Does anyone have a
service manual for this one?

Anyway, I have performed basic troubleshooting and found out the
following:

Input stage seems to work fine. Audio source connected to AUX as well
as tuner comes through to Tape 1 output when selected.
There is no sound on the headphone output.
When turning volume to max there is a faint sound of the correct audio
source on both speaker outputs and headphone output.

It seems to me that the output stage is broken. How could I proceed if
I don't find the service manual?

Regards,
Rasmus Bonnedal

 

[Arfa Daily]

Hi all!

I am trying to repair an old NAD 7150 Stereo Receiver. The problem is
that there is no sound on the speaker output. Does anyone have a
service manual for this one?

Anyway, I have performed basic troubleshooting and found out the
following:

Input stage seems to work fine. Audio source connected to AUX as well
as tuner comes through to Tape 1 output when selected.
There is no sound on the headphone output.
When turning volume to max there is a faint sound of the correct audio
source on both speaker outputs and headphone output.

It seems to me that the output stage is broken. How could I proceed if
I don't find the service manual?

Regards,
Rasmus Bonnedal

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

 

[[email protected]]

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

 

[N Cook]

news:79f24abe-ef6e-496e-84d0-4f4d85beab25@n20g2000hsh.googlegroups.com...


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

Shorting plugs added to unused inputs ?

 

[Arfa Daily]

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

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

 

[[email protected]]

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

 

[William Sommerwerck]

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.

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

 

[Arfa Daily]

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 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

 

[William Sommerwerck]

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.

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

 

[Arfa Daily]

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