NAD 216 issues

[Kamuy]

Hi, my name is Justin and I'm a long-time lurker of diyAudio.

I have a degree in electronic engineering, and quite a bit of experience repairing audio electronics, but today I've come across an old NAD 216 power amp that has me totally stumped.

This amplifier, sometimes works, sometimes emits white noise, sometimes just buzzes and pops, sometimes only plays through one channel, and more recent troubleshooting has shown a 1KHz sine wave emitting from the right channel amp block output as nearly triple that of the left block output.

As far as I can tell, both the amp blocks and the power transformer are functioning properly, all my troubleshooting has lead me straight to the power supply block, but seeing as how the problems are so sporadic, I'm at a loss as to what to look for.

Both amp blocks are getting a totally even voltage of 61/0/61, at all times, even when these strange things happen.

I have observed that while both channels output the sine wave, right higher then left, after going through the power supply/filtering block again, the speaker outputs on the back measure as left outputting properly, right outputting next to nothing.

Any help on this matter, from schematics to advice would be greatly appreciated.

[anatech]

Hi Justin,
Well, straight forward troubleshooting should have you looking at the feedback network. Sometimes there is a large cap from the inverting input to ground in series with a resistor. It's either open or the solder joint has failed. That would be my crystal ball guess with no schematic to help. The power supply would not seem to be involved in your fault.

-Chris

[AKN]

Hi Justin,

Also, have you checked the speaker relays?
This is a common problem on NAD216.
Tap the relays while you are watching/listening to the output.

[Kamuy]

From what I can tell, the power supply, feedback circuitry, soft clipping and crossover circuitry are all on the board marked "power supply ( NAD 216/214 )". I know the problem is emitting from this board, however, it's in a very tight spot and I just haven't been able to get in with my scope or DMM to test anything. The wiring doesn't allow much movement without re-soldering and I don't have a soldering iron here. :/

[anatech]

Hi Justin,
If you feed a signal into the amplifier section, does it come out amplified or the same level?

Have you tried 100 Hz and 10 KHz to see if the loss of gain is frequency dependant?

From what I can tell, the power supply, feedback circuitry, soft clipping and crossover circuitry are all on the board marked "power supply ( NAD 216/214 )". I know the problem is emitting from this board,

I'm sorry, but people tend to think circuit, not board location.

The wiring doesn't allow much movement without re-soldering and I don't have a soldering iron here. :/

Make some tests that you can get readings from with the amp in it's normal, together state. Then you figure out what circuit is involved and approach it that way. Can you access the bottom of the PCB?

-Chris

[Kamuy]

Originally posted by anatech
Hi Justin,
If you feed a signal into the amplifier section, does it come out amplified or the same level?

Have you tried 100 Hz and 10 KHz to see if the loss of gain is frequency dependant?


I'm sorry, but people tend to think circuit, not board location.

Make some tests that you can get readings from with the amp in it's normal, together state. Then you figure out what circuit is involved and approach it that way. Can you access the bottom of the PCB?

-Chris

The signal comes out amplified on the left speaker output, not the right. However, I know both channels do function properly because when inputting a 1KHz tone from my laptop, I can get a perfect and equal sine wave off both the left and right channel direct outputs, just not the speaker terminals.

I've tried 60Hz, 100Hz, 1KHz and 10Khz, and the problems seem to be totally independent on the frequency of the input signal.

[Kamuy]

Strangly enough, the right channel, the one that has no output whatsoever at the speaker terminals, actually outputs a louder signal then the left, straight out of the amplifier block, without filtering.

[anatech]

Hi Justin,
From what you are telling me, look for cracked solder connections at the speaker terminals and possibly all rear panel mounted parts. Also look at the speaker relay (previously suggested) and the solder joints there.

It's take it apart time.

-Chris

[Kamuy]

I believe the relay is bad, but the soldering below it is a mess as well, and I'm wondering if there is more going on here because when I wired it up to do some testing, I noticed the first two or three amplifying transistors on the right channel amp seem to heat up much more so then the others in the right, or left channels.

Significantly so as well. After playing music for about 15 minutes, the right heatsink is hot to the touch, hotter near the first stage transistors, and the left side is still completely cool to the touch.

[anatech]

Hi Justin,
At this point you need to stop and obtain the complete service manual or at least a schematic for it. Once we have that we can make some detailed suggestions for you.

Something has changed, those parts were not getting hot before. Therefore stop and think it through with the schematic. Also, check your work after a night's sleep.

-Chris