I recently picked up a vintage NAD Integrated amp (7250pe) on craigslist.
Before agreeing to purchase I brought a long a set of speakers and ran it through its paces, testing all inputs, testing all knobs/pots/switches making sure they are smooth and noiseless and listening to make sure nothing obvious was wrong.
Once I got it home - however - I started having minor problems.
In its stock configuration this amp, while the receiver/integrated portion is in the same cabinet, is configured such that the pre-amp/receiver section has stereo RCA outputs, and the power amp-section has stereo RCA inputs. From the factory these were bridged, but those bridges disappeared at some point in the last 35 years.
At first, I hooked it up by using RCA splitters
connected to the pre-amp section output, so I could connect my subwoofer, at the same time as I am feeding my signal back in to the power-amp inputs.
This sounded good, but I had intermittent problems on the right channel, where it would go from sounding good, to starting to distort (even at low volumes) and then the right channel would go down to extremely low volume/go out all together. Powering the amp down checking the connections and bringing it back up again, corrected the problem for a while, until it recurred.
As art of my troubleshooting efforts, I remembered that I have had issues with weird intermittent shorts when using firm RCA adapters/splitters in the past, so I decided to test if these might be the issue. I disconnected the splitters, and instead ran an RCA cable from the pre-amp out to the subwoofers in, and then another cable back from the subwoofers outs, into the power-amp inputs on the NAD amp.
In this configuration I never had the right channel drop out (or at least not yet, I only tried it last night for about an hour)
I have - however - noticed that there is a not insignificant difference in volume output between the right and left channels.
Doing some rudimentary measurements using a ruler to keep the distance equal at 12" and a cheap SPL meter I got on Amazon
I have measured that at 12" and normal listening levels (for me, about 70-80db) the right channel is consistently about 3db quieter than the left channel. I can compensate for this using the amps balance knob and it sounds pretty good, but I am still concerned.
How would you guys recommend going about troubleshooting and potentially fixing this issue?
My thoughts are as follows:
1.) All knobs/pots are smooth and there is no noise over the speakers when they are turned, so I don't suspect that pot degradation is the issue, but I'm open to hearing your thoughts.
2.) I remember reading that pots can produce unequal volumes across the channels at low volumes. In my near-field listening setup, the pot is usually in the 20-30% range. Is this considered "low volume"? Could this be the cause?
3.) Could this be due to the 35 year old caps finally giving out? Is it even worth having the caps replaced on an old amp like this? (I have no idea what this would cost me to have this done at a repair shop, and I have tried by hand at board soldering in the past and found that I really don't have steady enough hands for the job)
4.) Would taking it to a shop to have cleaning and "tune up" done make sense? What should I expect this to cost approximately?
5.) Things I plan on trying when I have some time, but haven,t gotten around to yet:
- Switching the speakers left to right to see if the problem persists. (I don't think this is speaker related, as I don't recall having this issue on my Emotiva UPA-2, which I am moving to a different application, but it is worth checking out I guess.)
- Switching the speakers from Speaker A to Speaker B to see if the problem persists.
Other than this, I am at a loss right now. I'd be interested in hearing ANY comments you might have that could be helpful!