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NAD 7175 Left Channel No Sound

meako

meako

New Member
Preamp section seems to working. All connections and pots cleaned with contact cleaner. Power section seems to be the problem. I used a NAD 7150 preamp section to test. Right now the right channel sounds clear with no distortion. Left channel is just completely silent, even on headphones. I hear the relay click quickly when I turn on the amp. Visual inspection of fuses, all four seems to be intact. After moving the tuner board to expose the power amp board, I don't see any burnt or leaking parts. It actually looks very clean. From the Main IN to power amp to just before the speaker relay, all wires seems intact. My gut points me to the speaker relay as the culprit. Visually, I see the audio signal passes through it (duh?) before going to the headphone out and speakers out. Now how do I check if the relay is faulty? Also Can I check if there is a L channel signal before the relay? Thank you very much for any ideas.
 

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If you know how, check ohms readings between the left and right channel output transistors.

Relay is not the first thing I would suspect. All switches and buttons should also be deox'ed especially the tape monitor.

This this just start acting up or did you get it this way?
 
cademan, thanks for the reply. I got it this way. Actually, the seller I got it from thought the the amp has no sound at all. But it was missing the pre out/main in jumpers. When I put spare jumpers I got sound but only in the Right channel. The preamp/tuner section works except for the R channel from the preamp has a lower volume the the Left. The preamp problem might be just the pots needing deoxit though.

Anyway, I would like to try to fix the power amp section's missing Left channel first. What would kill the Left channel while still keeping the rest of the power amp functional?

I have a high suspicion on the relay but I don't know where to begin testing. I've traced out the L and R signals up to just before the relay using the 7175 schematics. I'm thinking that if there is no L signal at this point, I'll trace back to the power amp board and start testing the L output transistors.
 
If you have any junk speakers, you can use it to test just on the other side of the relay contacts. Shouldn't be any DC at the speaker ouputs as the relay is kicking in and one channel is working.

Could be pitted contacts or dirty switches or buttons. Does this NAD have a tape monitor button/switch?
 
Any way to test L channel output before the relay using a DMM?

Btw, shouldn't the buttons like the tape monitor, not be an issue since I'm bypassing the preamp section by using another preamp?
 
meako said:
Any way to test L channel output before the relay using a DMM?
Click to expand...
Better with a scope or toy scope or audio probe. With a DMM you could try setting to mV AC, black probe connected to chassis and red probe to the left channel test point. With no audio playing expect near zero mV, with audio playing expect to dance around abit, increase volume should result in increased mV... not very systematic...

upload_2022-2-5_20-4-27.png
 
meako said:
Any way to test L channel output before the relay using a DMM?

Btw, shouldn't the buttons like the tape monitor, not be an issue since I'm bypassing the preamp section by using another preamp?
Click to expand...
Right.....was just checking.

Checking before the relay contacts is pretty simple. There are usually just two connections for the coil only. There should be four contact connections just for the speakers. With the unit off and unplugged, two left and right should check the same with reference to ground.

Being very careful with your probes, you can have it powered up and check voltages between right and left output transistors. DO NOT SLIP.

Did this just start acting up or did you get it this way?
 
Last edited:
Guido57 said:
Excuse my ignorance, but what is a "toy scope"?
Click to expand...
Something like,
https://www.ebay.com/itm/194778495063
The above unit require diy assembly, completed units can be had for a few dollars more.
It really is just a toy, suitable for binary low voltage yes/no testing. I've got 2 tektronix CRO's, both playing up, probably need to recap the psu? Anyway, currently using the toy to troubleshoot lost channel (only tape 3) in a yamaha CX-1 pre. You could use an audio probe...
 
So I did a test with the red probe to the Left and Right channel test points. One at a time. And the black probe to chassis. Test with amp ON, with and without music playing through the power amp section main IN. Here are my readings on mv AC. Any ideas on what we're up against? Thanks.
 

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The DMM method suggested is a bit of a punt, trying to do things on the cheap. I'd focus on the OL, right channel playing music, you need to shift to a higher scale, next down is Vac. So expecting a few volts AC on the right while the left is mV pointing to an issue before the relay. Left channel, no music showed xxxmV, this may be noise/hum. When doing these measurements you should increase the volume and confirm you are "measuring music", expect voltage to increase. No guarantee with this method, just a suggestion, better to use an oscilloscope or audio probe(google)
 
mbz, are the reading s showing anything about why there is no sound coming out of the left channel?

cademan, is it looking like I need to track more to the power amp are and output transistors?

Thanks again guys.
 
So it looks like it really is not the relay. Here's my reading on the Left amd Right channel test point on V AC on my DMM.

Left - constant 0.8+ regardless if there is or no music playing. No change with volume.

Right - 0.000 with no music. Then fluctuates and variable with music playing, acting like a numeric music visualizer. Also changes with volume.

So no Left signal is coming to the relay. Do I check the output transistors now? Can you guys guide me on how to do that? Thanks.
 

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meako said:
Left - constant 0.8+ regardless if there is or no music playing. No change with volume.

Right - 0.000 with no music. Then fluctuates and variable with music playing, acting like a numeric music visualizer. Also changes with volume.
Click to expand...
Indicates a power amp problem, could be "anywhere". Normal technique would be to test for audio at various stages using an oscilloscope, a DMM wont do this job since it's low level signals. You could try with an audio probe(google). If you have another basic amp available then prefer using an old RCA cable with series (DC) blocking cap (0.1uf-1uf-xuf/60-400V film, basically any film) with a probe (or bare wire) connected to the other side of the cap, ie, probe-cap-cap-RCA cable-into AUX of amp with spkrs connected. GND cable between (phono GNDs) amp under test and amp with probes connected. If you don't have another amp then connect probes across disposable speaker driver (with series cap to block DC) but low level signals means sound from driver will be low.
 
mbz said:
You could try with an audio probe(google). If you have another basic amp available then prefer using an old RCA cable with series (DC) blocking cap (0.1uf-1uf-xuf/60-400V film, basically any film) with a probe (or bare wire) connected to the other side of the cap, ie, probe-cap-cap-RCA cable-into AUX of amp with spkrs connected. GND cable between (phono GNDs) amp under test and amp with probes connected. If you don't have another amp then connect probes across disposable speaker driver (with series cap to block DC) but low level signals means sound from driver will be low.
Click to expand...
Thanks! I will try to build an audio probe using speakers. I don't want to risk my other NAD amps since this will be my fist time building one. Is it Speaker ground/black wire to NAD 7175 speaker ground terminal, then speaker red wire to cap then cap to probe?
 
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