• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

NAD 7120 - weird stuff going on

P

P. S.

Active Member
Hi,

I am having weird issues going on with this receiver. I can solder but have only basic understanding of electronics. Trying to figure out where the problem is. Maybe somebody ran into same issue and could help?

1) Most weird: with power button OFF - I have around 2v of DC offset on speaker terminals (!)
There's a 4700pf capacitor in the power supply unit that I suspect got shorted.

2) When I power the amp on - the DC offset in speaker terminals drops to 50mv and 80mv. Cannot trim it down from that point using the trimming pots.

3) Bias trimming does not do much- but I suspect I am measuring it at wrong points. The manual is not clear on this:
"Connect DC milivoltometer across R150 (output transistor's emitter resistor) and adjust VR102 for 2-4mV reading on meter"
I am attaching the photo of this section of amp PCB board.

Problem is that R150 is the transistor (not resistor). I think that I should be measuring the bias voltage across R145 (right next to "bias" mark)?

Any help would be appreciated. Apologize for dumb/basic questions but I just started my journey with fixing the amps and don't know much yet. (actually started electronics course just for that purpose but I am just a few weeks out. Still got lots to learn! Still lots of fun/frustration ahead of me ;-) ).


unnamed-2.jpg
 
Register to hide this ad
You say with the power switch off, you see 2 VDC offset on the speaker terminals. You are right, this is very strange. What do you see if the unit is unplugged?

Are the rail voltages right? You can measure at the collectors of Q110 and Q111 for the +/- rails respectively. Use caution and don't short anything out.

SoCal Sam has a good point about the trimmer pots. It's not only NAD, but many of these open carbon trimmers have high failure rates.
 
@Ylli @SoCal Sam thank you for tips!

Just to add a bit more mystery to the DC offset while power is off... @Ylli you are actually right - the vdc offset is also present when the cable is unplugged (!). Just to prove that I am not lying - attached the photo of my readings. You can see the receiver cable is not plugged and the offset is -123mV.

And yes - I checked if multimeter is not broken by reading voltage on AA battery and another receiver I have.

No clue what's going on.

IMG_3509.jpg
 
P. S. said:
1) Most weird: with power button OFF - I have around 2v of DC offset on speaker terminals (!)
There's a 4700pf capacitor in the power supply unit that I suspect got shorted.
Click to expand...
One of the 4700uf main filters (eg, the negative rail) is retaining some charge while the other (positive rail) is not retaining some charge
to counter balance the neg rail. Replace both. NAD skimped on a lot of things, trimmers... the power supply should be recapped for
peace of mind.
 
For peace of mind I would also recommend building a DBT, power the amp up thru that to ensure there isnt a short (ie at the output stage). However if you arent blowing fuses, this aspect may be OK.
 
  • Like
Reactions: mbz
Can all of you help me too gang ... just starting out too... have in my Collection a AKAI AA5810 , Rotal RA611 and a Sony TA1630 and have just got a big AKAI AA1175 dual Mono Amp do you gents have much experience with this amp . Look out for Reliving ... posts

Any how sorry PS back to the job in hand....
 
mbz said:
One of the 4700uf main filters (eg, the negative rail) is retaining some charge while the other (positive rail) is not retaining some charge
to counter balance the neg rail. Replace both. NAD skimped on a lot of things, trimmers... the power supply should be recapped for
peace of mind.
Click to expand...

Thank you so much for @mbz for solving the big mystery! To tell the truth it was quite weird to see voltage with cable unplugged (ghosts of previous owners - maybe...)
I will be ordering the caps and trimming pots later this week and post updates how the fixes go.

Thank you so much for your help!
 
https://www.audiokarma.org/forums/i...ial-please-feel-free-to-copy-and-post.875584/ Here is the info on the DBT. Dim Bulb Tester. You can plug the unit being checked into the DBT. Apply power. The light bulb limits current flow into the unit. Bulb lights up bright and stays bright then turn off the power. Hopefully preventing further damage to the unit. Shorted output devices will light the bulb bright. Reliving and P.S. Welcome to AK.
 
Also chip all of the old yellowed glue off the circuit boards inside the unit. It becomes conductive and also very corrosive. It will eat the leads right off of components and make them fall of the circuit boards.
 
NAD80 said:
https://www.audiokarma.org/forums/i...ial-please-feel-free-to-copy-and-post.875584/ Here is the info on the DBT. Dim Bulb Tester. You can plug the unit being checked into the DBT. Apply power. The light bulb limits current flow into the unit. Bulb lights up bright and stays bright then turn off the power. Hopefully preventing further damage to the unit. Shorted output devices will light the bulb bright. Reliving and P.S. Welcome to AK.
Click to expand...
Great stuff ... am in U.K. with 240AC will do my research .
Just about to restore or try too restore a 1977 AKAI AA1175 dual mono..... HELP....

How many hours would a Pro take to restore this Amp???? and hourly rate .

I restore classic E Type Jaguars for my sins.
 
Last edited:
I am trying to order replacement parts for my 7120 and have a bit of a problem with choosing the trimming pots. I need 2 resistances 1k and 5k. (the service manual does not mention wattage)

I browsed mouser and was kind of overwhelmed by the multitude of options. I for sure know I need a 'multi-turn' one but I am confused with regard to the potentiometer type etc.
Can you recommend what type of trimming pot is typically chosen as a replacement in vintage gear? (or it does not matter at all as long as it has proper electrical params?)
 
I've always been partial to Bourns. Their 3299Y series would probably be the most appropriate for your footprint.
Bourns 3299Y-1-102LF
Bourns 3299Y-1-502LF
 
Reliving. said:
Can all of you help me too gang ... just starting out too... have in my Collection a AKAI AA5810 , Rotal RA611 and a Sony TA1630 and have just got a big AKAI AA1175 dual Mono Amp do you gents have much experience with this amp . Look out for Reliving ... posts

Any how sorry PS back to the job in hand....
Click to expand...
Hi Reliving
Should start a new thread with your issues. this way, you'll get more attention to your post.
 
I Have A 7120 that sometimes gives full power on the F M Receiver. But mostly only about half Volume Tape & auxiliary are working But when I press buttons I get static And kickn down power if I wiggle them sometimes comes back full power to tape or f m auxiliary for c d plays Full power all the time. Is it the whole button switch board I guess that would need Replacement tired of wiggle them ✌️‍☠️☮️
 
You must log in or register to reply here.
Back
Top Bottom