NAD 7140 Preamp Section Not Working

[ScooterMcTav]

Hello All,

There were actually two threads similar to this already on AK, but I do know we like to have folks start their own threads. And really, thread #1 was full of folks who are way smarter than me, while thread #2 was a guy who just cleaned his controls and got things working.

Regardless, I picked up one of the cleanest NAD 7140 I've ever seen, and I was very pumped to have the MC phono stage. Unfortunately, as much as I like the Vintage Sony STR-5800 I have set up right now, it is missing some clarity on the top end I would like, and my outboard phono stage is meh. My tube amp is paired with a Proton pre with MC Phono stage - based on its sweet luscious sound, I think the NAD may be the bees knees for my second application.

So with jumpers in, I can only get the faintest input, which will change with the source. No control does anything to this faint sound, unless it is switching sources.

Amp section works well, as tested by my iPhone and an iPhone to RCA cable. Preamp not so much, though every preamp section seems to be working fine as I can switch to it, and get the faint output. It is not a fault with the Tape Monitor switch, as the tape in jacks do nothing other than the very faint sound when the monitor is switched.

Seller told me it had problems, but that they were able to get some sound out of the unit. I found one fuse on the tuner section had blown (100mA) but no other signs of heat, wear, leaking, or damaged components.

I do also have the service manual (thank you HFE), but this doesn't mean I really know what to do with it.

Unfortunately, my experience reading schematics is about 32 years in the rearview (with my eyesight), and I am a ham-fisted solderer at best. However, this does not mean I am unwilling to tackle this project.

I would greatly appreciate any help that my A-K friends could provide, especially those who are patient, and who could possibly help me with visual clues.

 

[mbz]

Start with basic health checks of preamp power supply. Will try and get test points.

 

[mbz]

Preamp uses a NJM2043 opamp, one of the +/- supplies could be down
taking out both channels. There is also a power on mute FET Q603
and Q653, maybe problems in this area.

Measure dc voltage to ground (black speaker post or phono GND or chassis)
at,

- Q604 collector (measure at R628 as highlighted), expect about +22Vdc
- Q604e or opamp pin 8, whichever you can safely do (probably Q604e), +15Vdc
- Q605 collector (measure at R630 as highlighted), -22Vdc
- Q605e or opamp pin 4, whichever you can safely do (probably Q605e), -15Vdc

 

[mbz]

Check the muting FETs, measure at either D651, D601 or R632
as shown, immediately at power ON it should be -xVdc and after
4-8 seconds it should drop to near 0Vdc. Hopefully you can
access these measurement points from underneath.

 

[ScooterMcTav]

Excellent MBZ - I hope to have some time to look at the unit this weekend. Appreciate the very clear instructions.

 

[mrwasabihead]

...I found one fuse on the tuner section had blown (100mA) but no other signs of heat, wear, leaking, or damaged components..

Hi "ScooterMcTav", I repaired one of these last spring. The one I have had a blown fuse in the tuner section as well. It would power up but no sound.
The problems were IC114: L78M12 [12V Positive voltage regulator]. And another Positive voltage regulator on the power supply / amp board. That was IC902: 78M24 [24V Positive voltage regulator]. Those may be places to check as well. Should you find a need to replace either of them remember to check the pin-outs of the replacements. Some are "in-out-common" and some are "in-common-out". Can't remember which the 7140 had but be careful whenever working on a live unit. Good luck.

 

[ScooterMcTav]

Thanks all - unfortunately didn't have time on the weekend to look at it, but will have some time this coming weekend.

my 100 mV fuse in the tuner section was blown, and I did replace it, but still no sound.

Hopefully will get to it next weekend.

 

[Qtronic]

Hello,
I have one thing for you to check out, which has been a problem on 3 NAD units I have worked on.
Old circuit board glue becoming corrosive AND conductive.
Look around on the circuit boards, especially around larger board mounted components.
To each his own abilities, but what I did was remove all of the old glue with metal picks, exacto knives, rubbing alcohol, scraping tools.
I had 2 CD player/receiver combo units and both had displays out, scraped all the glue away from the crystal oscillator on the circuit board behind the face, and both units came back to life.
Had a NAD amp and it just did not sound good and would falter when volume turned up, scraped all the glue out of that one, replaced a capacitor that had corroded leads from the glue destroying it and the unit has worked flawlessly ever since.
I was able to get resistance readings just probing the glue with a multimeter on the above units, it can create many different gremlins/ issues in these units!
So look for any and all glue on the circuit boards and remove it one way or another, even if it’s just preventative maintenance.
Also, as a start, deoxit clean all switches, controls and check boards for cracked cold solder joints, with a magnifying glass.
Safety first!
Cheers,
Q