Like

NAD 3125 left channel weak

 

New member
Username: Tdwesty

Post Number: 2
Registered: Jun-05
My NAD 3125 amp has been flawless for 20 years or so, then recently the left channel started to get weak, but only with tuner or VCR inputs, not with CD or DVD higher level inputs. Now the left channel is almost gone with tuner/VCR, but just fine with CD/DVD. I have swapped input cables, used different inputs, swapped speakers and cables, and have isolated it to the left channel on the amp. Switching the mono switch on balances both channels. There is a slight signal on the left channel - it is not totally dead. Switching to CD/DVD brings it back every time.
I've read about dried up caps. Could this be the cause? I see four 4700uF 50V and four 1000uF 50V caps. Can I just replace them with same size caps from the electronics store? I have no problem soldering in components, but am not sure where to start. I've checked for cold solder joints, and they all look good. I cleaned to volume/balance knobs, which removed some scratchiness, but did nothing for the left channel. I didn't clean the tone controls, since I never use them - should I clean them also?

Thanks
 

New member
Username: Tdwesty

Post Number: 4
Registered: Jun-05
I'm going to post a followup, since nobody's weighed in on this one. I dropped by Sound Hounds in Victoria to have my woofer surrounds redone (PSB 70Rs), and asked about the amp. Upon describing the problem, they had no idea, but said I could bring it in for repair. They will do it for up to $85, or call me if it will cost more. They also had a used 7140 reciever for $200. So it seems that much over $100 on my amp is probably not worth it?
 

Silver Member
Username: Sun_king

Leeds, West Yorkshire UK

Post Number: 249
Registered: Mar-04
Depends how much you like the sound of your amp as to whether it's worth a big repair bill. Just because you can buy a different or new amp for similar money doesn't mean it will sound as good as the old amp once repaired. Maybe it's something simple though like the volume pot? These can cause channel volume dips.
 

Silver Member
Username: Sun_king

Leeds, West Yorkshire UK

Post Number: 250
Registered: Mar-04
Hi, I've just re-read your original post and notice that you cleaned the volume pot. This will indeed remove dust particles that cause scratchiness etc but won't solve the problem if the pot itself is actually on its way out. I'm not saying this is definitely the cause of your balance problems but it can be. Capacitors do deteriorate but I wouldn't have thought it is these that are causing your particular problem.
 

New member
Username: Tdwesty

Post Number: 6
Registered: Jun-05
Thanks for the response - your comment about the sound rings true. I do like the sound of this amp, and suspect that it will still sound better than most newer amps when fixed, but I'd like to try my hand at solving it before taking it to a shop. I have narrowed down the problem a bit further thanks to some help from rec.audio.tech.
The problem only occurs for the aux & tuner inputs, not the tape input. It turns out my earlier testing was flawed, as the tuner works fine on the tape input. According to a schematic (which I haven't seen, but have been told is so), this means the problem lies in the infrasonic filter section for the left channel. The tape input bypasses this filter apparently.
I thought I had located a suspect cap in that area, but now it checks out fine...
However, I've found two cracked solder joints, which could be the problem, or could just be from me disturbing things with the meter - I don't know. I need to get more butane for my iron, and will try to reflow all the suspect joints next.

 

Silver Member
Username: Sun_king

Leeds, West Yorkshire UK

Post Number: 251
Registered: Mar-04
OK, good luck! Let me know how you get on.
 

Giuseppe Zarbo
Unregistered guest
Hi Shawn, I've just received my 3125 purchased on Ebay, and I found the same problem, on the right channel. Have you come up with a solution? This could be very helpful to me. Thanks and Regards
Giuseppe, Bremen, Deutschland
 

Gold Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 6250
Registered: May-04


If the problem occurs only on the two inputs (tuner and aux) it is obviously within the shared circuitry for those two inputs and not in the common to all input areas such as the volume control. I have no idea why a tuner input would need an infrasonic filter since most line level sources such as tuner have no infrasonic output. (Subsonic filters are often used on phono inputs to filter the noise of record warps or poorly suspended turntables.) More than likely what was meant would be the DC blocking capacitors in the signal path. Possibly the cap was placed there to block DC from entering the amplifier's circuitry from defective or poorly designed source equipment. After 20+ years most caps should be replaced, most particularly electrolytic capacitors that would be used for power supply or blocking functions. Caps can be going out and not show any outward signs of problems. As the cap ages, its values change and the result is the performance of the circuit changes along with the changing capacitor's values. Applying heat to the leads of a cap might temporarily solve a problem, but the capacitors need to be changed. If you are unfamiliar with how to swap capacitors, this is a job for a technician as there are voltages inside the amplifier which could make it impossible for you to hear whether you've fixed the problem. Do not stick your hands inside an amplifier, even if it is unplugged, if you do not know where not to put them. Never place both hands on the amplifier if you ignore the first warning; your heart is then in the path between hot (your right hand) and ground (your left hand). If you don't know what you are poking at inside an amplifier, never do any work that you cannot accomplish with one hand in your pocket.





 

New member
Username: Javmeister

London, London UK

Post Number: 1
Registered: Mar-06
Hi Shawn, any luck with the channel problem? i have a NAD 3150 with the same problem....
 

New member
Username: Tdwesty

Post Number: 7
Registered: Jun-05
Hi Javed,

I believe I traced the problem to a transistor used in the infrasonic filter circuit on the one channel. This was suggested by someone who looked at the schematic and suspected the transistor. He then sent me a copy of the schematics.
I have had trouble locating the transistor or a suitable replacement with similar values. In the meantime, I picked up a 3020 on ebay that I have been using instead.
Since yours is a different model, it could be something quite different. But if, like mine, the problem only exists on certain inputs, then you can look for components that are specific to those inputs. Some day I will revisit this, but I have too many projects right now, and the 3020 sounds good enough for now.
« Previous Thread | Next Thread »

Facebook

Directory

Main Forums

Today's Posts

Forum Help

Follow Us