NAD 3125 left channel weak

Archived from groups: rec.audio.tech (More info?)

Sounds as if maybe something has changed the input impedance of one channel.
What are the very first components that the signal goes into, upon entering
the amplifier?

Archived from groups: rec.audio.tech (More info?)

Tim,

Thanks for this info. I have tried the CD and Tuner inputs, but I don't
think I have tried the Tape input, at least not recently. The CD signal
works fine on the CD and Tuner inputs, but not the tuner signal.

I will do a bit more testing and report back.

ps: Using a DMM in-circuit, I have found at least one pair of small
caps in the amp section that are not matched - ie: one is vastly
different from the other. The rest I was able to test were matched
closely, meaning either they are both good, or both bad I guess...

Archived from groups: rec.audio.tech (More info?)

Tim,

Ok, I cleaned the selector switches, since I had it on the bench, then
tested again. (the switches are sealed up, so I had to spray contact
cleaner liberally, but I could see that some had worked into the
semi-transparent housing when the switch moved. I let it dry a good 10
minutes.

The problem exists with the CD & tuner inputs, but not the Tape input.
The levels on the tape input (from the tuner) are nicely balanced, and
perhaps 3dB higher.

So how do I find the infrasonic filter stage? I wish I could find a
schematic...

Thanks

Archived from groups: rec.audio.tech (More info?)

Shawn,

I would start by seeing if it is one specific input that is the
problem. Electrically, the CD and Tuner inputs are the same. The tape
input is just after the infrasonic filter. So if the problem is not
there on the tape input, but with the CD or Tuner inputs then the
infrasonic filter stage could be the problem. Have you tried swapping
which item is connected to which input?

Next I would try cleaning the input switches. After that I'd look for
poor soldering at the input jacks or switches. I would not star with
changing caps unless the tape input is OK, but the CD and Tuner (as well
as phono) inputs have the problem. Even then, I'd only change a couple
of caps, not 'shotgun' the entire unit.

The mono button curing the problem show that the problem is not the
volume or tone controls, as the mono button is before them. Re-post,
e-mail me directly, or both after you find out if the problem is
dependent on which input is used.

Regards,
Tim Schwartz
Bristol Electronics



TDWesty wrote:
>
> I have an NAD 3125 integrated amp (1986 vintage) with a strange left
> channel problem. With a lower level input, such as my 4125 tuner, or a
> VCR, the left amp channel drops out to almost nothing. Switching the
> amp to mono equalizes both channels. I have swapped speakers, wires,
> patch cables, and tried all of the line level inputs on the amp, with
> the same results. I have swapped left&right, etc, but the problem is
> always the left channel of the amp. The tuner works fine on both
> channels with another amp.
>
> With a higher level input such as CD or DVD, both channels work and
> sound fine. The left channel behaviour was inconsistent for a while,
> but is now consistent, and has been for months.
>
> I have done some reading about replacing caps on older amps, along with
> checking for bad solder joints, but have found no visible problems in
> either case. I have had the amp for 20 years, and it has never been
> abused.
>
> I would consider replacing the caps myself, if I had some idea where to
> start (ie: which ones are the mostly likely culprits, or which ones
> should be replaced due to age), but I would prefer to have a schematic
> to work with. I have found the schematic for the NAD 3120, which has
> test voltage values, etc., but so far no luck on the 3125.
>
> Any ideas appreciated.
>
> Thanks, Shawn Wright

Archived from groups: rec.audio.tech (More info?)

I did some checking of caps, and it appears that one in the infrasonic
filter section (C108) is bad, according to a DMM test comparing with
the matching C208 cap. It is a 1uF 50V non-polarized cap, and appears
to be the only non-polar one in the amp. I don't know if this is
significant or not.

I will start by replacing this cap and see if the problem goes away.

I'm wondering if I should consider bypassing the tone controls and Bass
EQ while I'm in here. I never use tone controls, and I know this was a
common tweak performed by the local audiophile shop when I got this
amp. Is it worth the trouble? The tone controls are on a separate
circuit board with a few small caps, and connected by a ribbon cable.

Thanks

Archived from groups: rec.audio.tech (More info?)

This one doesn't have the low level function that some NAD amps have.
Just Mono and Bass EQ. Further testing has narrowed it down to just the
tuner and Aux inputs, since the tape input works fine. According to
Tim, this places the problem in the infrasonic filter stage, which is
bypassed by the tape input.

I am thinking of buying all the replacement components for this stage,
as there doesn't appear to be that many. A few caps, resistors, and a
transistor in each channel. The transistor (Q101) is marked C1845 FA43.
The only match I can find for this is a Fairchild KSC1845xxx series,
which has at least 7 different versions. How do I tell which version to
use if I want to replace it? They do have two that are FCxx - can I
assume these supercede the FA43 series?

I re-flowed all the suspect solder joints, and have cleaned all the
switches & pots, so I think it is now time for parts replacement.

Archived from groups: rec.audio.tech (More info?)

TDWesty wrote:
> I have an NAD 3125 integrated amp (1986 vintage) with a strange left
> channel problem. With a lower level input, such as my 4125 tuner, or a
> VCR, the left amp channel drops out to almost nothing. Switching the
> amp to mono equalizes both channels. I have swapped speakers, wires,
> patch cables, and tried all of the line level inputs on the amp, with
> the same results.

<snip>
>
> Thanks, Shawn Wright

Seems to me there was a "low level" or similar labeled button on the front
panel. This switch, when dirty, would cause similar problems. Try
manipulating this and any other front panel controls (other than the MONO
you've already tried) and see if any affect your problem.

Most of the things I've seen on NAD's that would cause a low channel are in
the amp and soft-clip circuits, and not the preamp area.

Mark Z.