Right Channel of Amplifier Much lower than Left

[brada220]

I have been repairing a Vintage NAD 7150 receiver - Original problem was that when playing a music source the right channel would abruptly quit - the time span before it would quit became shorter and shorter until it would never go on - I took a look and found multiple caps had leaked - I replaced all that looked bad - thinking I was done - I adjusted the DC offset and idle current alignment to spec - put it back into service and the right channel still is bad - one can barely hear sound coming from the amp on the Right channel - I need advice on how to trouble shoot the issue - my main problem is that I cannot tell what components are for the right channel - I have schmatics but my ability to understand is poor
If anyone can be of help would be greatly appreciated - Please remember I am a newbie and will need ample guidance and patience
Thanks

 

[Binkman]

Greetings;
Just be patient for a while for a 'hit' response... but initially you have swapped input source channels? Have you cleaned any pots or switches? Checked for obvious cable loose etc.. like take a good look with a magnifying glass for solder pads repaired?

SM(s) here for interested.
https://www.hifiengine.com/manual_library/nad/7150.shtml

 

[brada220]

Greetings;
Just be patient for a while for a 'hit' response... but initially you have swapped input source channels? Have you cleaned any pots or switches? Checked for obvious cable loose etc.. like take a good look with a magnifying glass for solder pads repaired?

SM(s) here for interested.
https://www.hifiengine.com/manual_library/nad/7150.shtml

I swapped input channels, swithched cables to speakers and right channel of amp misbehaves - I didclen the pots and switches and the"noise" when rotating the knobs are gone
What should I be looking for re the solder pads

 

[northpaw]

You want to be sure to clean and exercise all the switches and pots. Some might have escaped you, such as the ones on the back panel. Also, the balance control is likely integrated into the volume control, concentrically, so that will have multiple pots behind the panel. The balance pot being "hidden" like that is easy to overlook (don't ask me why), so make sure you hit each one you see coming off that shaft. Usually if you operate the control, you can see its wiper move (as I recall, my 72xx NADs have open pots and I can see them move).

I've had problems with a channel cutting out too, but a cleaning always fixed it. For me, the balance control was common culprit, but it can be the speaker switch, tape monitor, etc. Exercising the controls well while/after cleaning does usually help.

It is usually easier to address the cleaning before chasing other things. Bad solder joints can be identified sometimes by dull appearance, and more obviously by movement of the lead in the hole in the PCB if you can wiggle the component a little, but probably the most common clue is the appearance of circular (or annular) cracks in the solder bead on the underside of the board. Best looked for with a strong light and some magnification. The evidence can be quite faint, so if the cleaning step does not solve the problem, be prepared to search carefully.

 

[ConradH]

That unit looks very similar to the 7175 I just worked on. The push button switches were terrible and did not get better with Deoxit. No effect at all. First time I've seen that. If you have a -20 dB button and the contacts are bad, that could do it. What did seem to work was plain alcohol- a few good drops down where the wire latch mechanism is, then work the switch a lot. Then use Deoxit. Alcohol will short things out! Don't power the unit until completely dry. Canned air will help. I still don't have my switches as intermittent-free as I'd like, but they're good enough to use. Don't forget to clean the impedance slide switch. If you had physically leaky caps (the big filters are glued down and that's not leakage) replacing those is good, but my guess is you could have bad caps with no visual indication.

I can't imagine trying to get one of these units working well without a signal generator and oscilloscope. Some of the switch issues are subtle. Putting a 400 Hz square wave through the thing and watching the output as you switch switches makes everything clear in a hurry. I didn't even realize how bad mine was until doing this.

 

[brada220]

You want to be sure to clean and exercise all the switches and pots. Some might have escaped you, such as the ones on the back panel. Also, the balance control is likely integrated into the volume control, concentrically, so that will have multiple pots behind the panel. The balance pot being "hidden" like that is easy to overlook (don't ask me why), so make sure you hit each one you see coming off that shaft. Usually if you operate the control, you can see its wiper move (as I recall, my 72xx NADs have open pots and I can see them move).

I've had problems with a channel cutting out too, but a cleaning always fixed it. For me, the balance control was common culprit, but it can be the speaker switch, tape monitor, etc. Exercising the controls well while/after cleaning does usually help.

It is usually easier to address the cleaning before chasing other things. Bad solder joints can be identified sometimes by dull appearance, and more obviously by movement of the lead in the hole in the PCB if you can wiggle the component a little, but probably the most common clue is the appearance of circular (or annular) cracks in the solder bead on the underside of the board. Best looked for with a strong light and some magnification. The evidence can be quite faint, so if the cleaning step does not solve the problem, be prepared to search carefully.

I am taking your suggestion and recleaning all of the pots and swithches - My suspicion is that there is a bad connection somewhere that's is not all that evident - will start looking atthe board more carefully now that you described what I am looking for - I just wish I could distinguish right amp components to make he job somewhat simpler.

 

[ConradH]

If it's like the 7175, the right channel schematic isn't given. For that one you just add 50 to all the component numbers to get the right channel. See if yours is similar.

 

[gadget73]

The NAD integrated I had with serious imbalance problems turned out to be a couple of bad caps in the feedback circuit. I found it by switching parts channel to channel. When I moved the problem, I replaced both of those caps to verify all was happy.

 

[brada220]

That unit looks very similar to the 7175 I just worked on. The push button switches were terrible and did not get better with Deoxit. No effect at all. First time I've seen that. If you have a -20 dB button and the contacts are bad, that could do it. What did seem to work was plain alcohol- a few good drops down where the wire latch mechanism is, then work the switch a lot. Then use Deoxit. Alcohol will short things out! Don't power the unit until completely dry. Canned air will help. I still don't have my switches as intermittent-free as I'd like, but they're good enough to use. Don't forget to clean the impedance slide switch. If you had physically leaky caps (the big filters are glued down and that's not leakage) replacing those is good, but my guess is you could have bad caps with no visual indication.

I can't imagine trying to get one of these units working well without a signal generator and oscilloscope. Some of the switch issues are subtle. Putting a 400 Hz square wave through the thing and watching the output as you switch switches makes everything clear in a hurry. I didn't even realize how bad mine was until doing this.

I replaced the big filter caps as I mistook the glue for leakage though there was some shrinkage around the underside perimeter. - LOL I am using this NAD unit as a "project" - but purchasing an osclloscope is out of the question

The NAD integrated I had with serious imbalance problems turned out to be a couple of bad caps in the feedback circuit. I found it by switching parts channel to channel. When I moved the problem, I replaced both of those caps to verify all was happy.

Where does one look for the feedback circuit on the board? I don't see anything on the board that looks like a mirror image of any other section of the board to start swithching parts channel to channel - I guess my problem is tha tI can't identify what belongs to what channel

 

[brada220]

If it's like the 7175, the right channel schematic isn't given. For that one you just add 50 to all the component numbers to get the right channel. See if yours is similar.

Not sure if I am following your advice - can you provide an example?

 

[northpaw]

I just wish I could distinguish right amp components to make he job somewhat simpler.

In your first post, you said you adjusted DC offset and idle. Just look at the location of the pots you adjusted: one of each will be for right channel, one of each for left channel. The respective parts of the power amp will be localized around those pot locations, as they are in the heart of the power amp section. That said, things like volume, balance, tone controls, etc., are in the preamp section, with wires/connectors going to the controls on the panel. If you download a SM using the link Binkman provided, you can use the layout diagrams to help locate things (the 2nd file on HiFiEngine, "alternate scan" is the better one, at least for the layout diagrams, as they were scanned with lower contrast and captured the circuit traces). Conrad is right that some NAD schematics don't show both channels, but the ones on HiFiEngine for the 7150 do (at least for the power amp part, as that is all I looked at). The power amp part is at the far right in the circuit schematics (once you orient them as they are intended to be read), just to the left of where the L-R speaker connectors are.

I see the schematic shows pre-out/main-in jumpers. This can be helpful if you have other equipment you can connect to, temporarily. If you hook the pre-outs to another power amp, you will see if your issue is in the 7150 preamp. If you hook the main-ins of the 7150 to another preamp (or similar source), you can tell if the 7150 power amp is working correctly. This kind of info will shorten your search if you end up looking for bad solder joints. But you have gotten other good advice above.

 

[ConradH]

Just looked at the 7150 circuit and they give you both channels. Nothing to figure out. The part designators should be printed on the board. Look at the schematic and look at them. The right channel stuff will generally be grouped on the right. I assume this is the same miserable arrangement where you have to pull the tuner board out of the way to get at the signal/power board? IMO, swapping parts from one side to the other is really bad practice. You'll just end up with two non-functioning channels. Here's a quick way to divide the problem, easiest with headphones. At a low volume, touch your finger to the main-out pre-in jumpers on the back. If you get equal buzz from each one, left and right, the problem is ahead of the power amp. If not equal, the problem is downstream- the power amp. My guess is you have dirty switches in the preamp section. In all honesty, if you have more than that you need to get the thing on a test bench. It's going to be way too difficult to troubleshoot without test equipment, and you're likely to do more damage than good. There's always the shot-gun recap approach, but the odds of that fixing anything that way are probably less than 50%, plus it tends to introduce new problems. Try the alcohol trick on the pushbutton switches. The "low level" -20 dB switch is a good candidate for trouble, though all of them are suspect. The sonic effect of the bass boost and the infrasonic filters are hard to detect most of the time, so you can't tell if every button press has had the desired effect.

 

[brada220]

Thank you all for the valuable advice.
Not so sure that my problem is dirty switches - although they needed cleaning
I suspect that I still have a bad cap or transistor - Ican't think of anything else to explain the significant sound degradation in the right channel
Anyway - I am replacing several additional suspect caps and looking for bad solder joints - if its not that then I think I'll have to give up my project as being too advanced for me

 

[Binkman]

Got to admit I can start a fire in DIY.. Just to the responders? wow.. must be some parallax in orbit?
'Give us (diy) your poor.. tired.. this is a good thread!
..............
I'm not on ;leesonics' member customer list but you can try to pm him or maybe dig search up his old posts on gloogle... use 'audiokarma' and model and his moniker.. might get some hits. (probably a shit load of other models..)

 

[ConradH]

Don't underestimate how much misery those switches can cause. Lots of units are known for switches that need cleaning, but the NAD I just did was so resistant to cleaning I couldn't believe it. Even when I thought I had it working, the THD would go way up until a switch was wiggled. More cleaning fixed it. I'll also say that it had a couple bad caps. Change out the lowest voltage ones first- look for any caps rated at 10 volts or less. Replace them with 25 volt caps if they'll fit, which they probably will. Really low voltage caps tend to get electrically leaky and drop in value before the rest. Make sure your main-in / pre-out jumpers are clean.

 

[northpaw]

but the NAD I just did was so resistant to cleaning I couldn't believe it

My experience is that the NAD units I've had (all 72xxPE) were more susceptible to dirty switches/pots than other gear. I've had passing thoughts that they used lower-cost components with alloys that were less resistant to oxidation. Like maybe even pot metal (sort of kidding; I think...). If you keep after it, the units otherwise work really well. But 10 twists of the knobs or flips of the switches is usually woefully inadequate for exercising.

 

[pskaudio]

I had problems with the rotary speaker switch on a 7240 PE. A speakers, no right channel. Took a good bit of spraying and working back and forth.

 

[ConradH]

Not all switches are created equal, and even if they are, lubricants might be an issue. If they chose poorly, the lube could eventually turn into an impervious film. It surprised me that Deoxit didn't work but alcohol did. If it was purely an oxidation problem, I'd have expected Deoxit to cut right through it unless the switches were plated with something unusual.

 

[brada220]

Don't underestimate how much misery those switches can cause. Lots of units are known for switches that need cleaning, but the NAD I just did was so resistant to cleaning I couldn't believe it. Even when I thought I had it working, the THD would go way up until a switch was wiggled. More cleaning fixed it. I'll also say that it had a couple bad caps. Change out the lowest voltage ones first- look for any caps rated at 10 volts or less. Replace them with 25 volt caps if they'll fit, which they probably will. Really low voltage caps tend to get electrically leaky and drop in value before the rest. Make sure your main-in / pre-out jumpers are clean.

I ordered caps that might be swollen - at this stage I can't find any caps that have leaked - but some of the tops seem to have expanded past the top of the cap

Just to reiterate, the problem I have is no sound from the right channel - all I hear is some crosstalk at very high volume - after caps, what other type of component should I be looking at - transistors, diodes etc and how do I go about testing

 

[avionic]

but some of the tops seem to have expanded past the top of the cap

Or the plastic sleeves have shrunken from excessive heat.