Hi
Newbie here so go please go slow
My NAD 3020A has developed a fault. When I turn it on it's okay for about two minutes. Then the sound begins to fade, turns gritty and cuts out almost totally. When I turn it off then back on again it will do the same. I have a couple of Nakamichi decks, a CD player and a turntable hooked up to it. It's the same on all. Nu hum or any other unwanted noises. I am not a pro but have some limited electronic knowledge, a DMM and some soldering capabilities. I wonder if it's something I can fix or considering its age it's bust or it needs a pro. It has been working perfectly over ten years. I appreciate any help. thanks.
peterbt
Newbie here so go please go slow
My NAD 3020A has developed a fault. When I turn it on it's okay for about two minutes. Then the sound begins to fade, turns gritty and cuts out almost totally. When I turn it off then back on again it will do the same. I have a couple of Nakamichi decks, a CD player and a turntable hooked up to it. It's the same on all. Nu hum or any other unwanted noises. I am not a pro but have some limited electronic knowledge, a DMM and some soldering capabilities. I wonder if it's something I can fix or considering its age it's bust or it needs a pro. It has been working perfectly over ten years. I appreciate any help. thanks.
peterbt
Welcome to diyAudio Peter 
If both channels are doing the same then this could be something really simple like a loose fuseholder or an intermittent mains switch.
For both channels to fade it would suggest a possible supply problem.
Also it might be the 3020 that uses a FET muting arrangement between pre and power amp sections and that could give these symptoms. You would need to do some very basic voltage measurements.
From memory if it does have that arrangement then it could also again be mains switch related (auxiliary contact on switch).
There is another thread on that somewhere and I remember simulating the mute circuit for someone.
If both channels are doing the same then this could be something really simple like a loose fuseholder or an intermittent mains switch.
For both channels to fade it would suggest a possible supply problem.
Also it might be the 3020 that uses a FET muting arrangement between pre and power amp sections and that could give these symptoms. You would need to do some very basic voltage measurements.
From memory if it does have that arrangement then it could also again be mains switch related (auxiliary contact on switch).
There is another thread on that somewhere and I remember simulating the mute circuit for someone.
The gritty sound could well be a symptom of the FET muting (we are assuming at this point it does have that) and occurs as the FET slowly becomes non linear as the gate voltage drifts ever lower (more negative). It would be a classic symptom.
You will need the correct circuit diagram to refer to but that other thread should give you a good start. If it is around that area then small caps related to the generation of the FET gate voltage could be at fault
I'll look in again tomorrow (out of time today)
You will need the correct circuit diagram to refer to but that other thread should give you a good start. If it is around that area then small caps related to the generation of the FET gate voltage could be at fault
I'll look in again tomorrow (out of time today)
Here is a NAD circuit. The switch (I think is a 3rd contact on the mains switch) could be the culprit.
You need to check the voltage at the point arrowed when the fault occurs, If this is negative then the FET's are cut off and the unit is in mute. If the voltage drifts across that threshold the sound will become very distorted as the FET is neither on nor fully off.
You need to check the voltage at the point arrowed when the fault occurs, If this is negative then the FET's are cut off and the unit is in mute. If the voltage drifts across that threshold the sound will become very distorted as the FET is neither on nor fully off.
Attachments
The schematic posted says C531 is a 47 uf 63 v cap. Confirm your unit follows that pattern.
Small electrolytics are often the first to fail. Small ratio of volume to rubber seal length. To prevent units from being broken all the time as caps fail one by one, once there is a e-cap problem I change them all. A manufacturer tends to use all the same service life rated e-caps in a product. However, I do two at a time then sound check to make sure I didn't inject a problem with a bad solder joint or incorrect part. If sound gets worse, I pretty much know where the problem would be after changing only two caps.
I buy the longest service life caps I can find so I don't have to do this job over again every 8 years, as I did when I was only allowed to buy unrated life e-caps from local TV parts stores. For example, sprague atomlytic had no service life rating. I can buy 47 uf caps rated 10000 hours service life these days. Farnell & digikey show the service life in the selector table. Many other vendors make you download the datasheet on each product and read it. Others don't tell purchasers what brand & line the cap is. I never buy e-caps from ebay amazon or alibaba. Only authorized distributors. Too much geriatric junk out there. Rubber ages sitting on the shelf or powered up in the unit.
Small electrolytics are often the first to fail. Small ratio of volume to rubber seal length. To prevent units from being broken all the time as caps fail one by one, once there is a e-cap problem I change them all. A manufacturer tends to use all the same service life rated e-caps in a product. However, I do two at a time then sound check to make sure I didn't inject a problem with a bad solder joint or incorrect part. If sound gets worse, I pretty much know where the problem would be after changing only two caps.
I buy the longest service life caps I can find so I don't have to do this job over again every 8 years, as I did when I was only allowed to buy unrated life e-caps from local TV parts stores. For example, sprague atomlytic had no service life rating. I can buy 47 uf caps rated 10000 hours service life these days. Farnell & digikey show the service life in the selector table. Many other vendors make you download the datasheet on each product and read it. Others don't tell purchasers what brand & line the cap is. I never buy e-caps from ebay amazon or alibaba. Only authorized distributors. Too much geriatric junk out there. Rubber ages sitting on the shelf or powered up in the unit.
Last edited:
A lot of caps to replace. I have a 47 uf cap so I'll replace the C 531 but the rest I'll have to source. The problem with that the Digikey or Farnell and the others sell caps in larger quantities as a rule. Hard to get just 3 or 4 of each value. I've also noticed that on the flip side of this amp the item numbers are not marked so de-soldering is harder. Thank you all for the help so far
Peter
Peter
Thanks for that advice. I wasn't going to do that anyway. It would have taken me weeks. I've found something else though. While the amp was connected to a sound source I touched the C 531 positive side with the red probe of the meter while with the black probe I touched ground on the chassis and the sound came right back. Otherwise there is no voltage shown if I just measure positive/negative on the cap. How should I proceed? I've also found out unfortunately that all the e caps I have don't have the same value and voltage so if I have to change this cap I'll need to find one.
Peter
Peter
Sounds as if you have a bad solder joint. Joints that make things better when you touch them with the meter are often defective. I've found many of those in products that are not dusty, caps show no sign of use, were dogs from the factory that happened to pass the QA test once.
Farnell here will sell you one of anything, but they have a minimum buy below which they charge you a handling fee. I never buy less than $40 worth. A roll of solder is $40, a DVM is $30, a bag of alligator clip leads is $10, 6 fuses are $10, should not be a problem unless are too cheap. So buy all caps for project at once, which cuts the freight cost too, but only replace one or two at a time. With sound checks in between to prove you didn't inject a mistake.
Farnell here will sell you one of anything, but they have a minimum buy below which they charge you a handling fee. I never buy less than $40 worth. A roll of solder is $40, a DVM is $30, a bag of alligator clip leads is $10, 6 fuses are $10, should not be a problem unless are too cheap. So buy all caps for project at once, which cuts the freight cost too, but only replace one or two at a time. With sound checks in between to prove you didn't inject a mistake.
Last edited:
sorry i should have said i didnt replace all of them only the main power ones and any others that i considered very close to the applied voltages, so i uprated those, but no i didnt replace the entire set.
Last edited:
That is a big clue and confirms the FET muting theory. The internal resistance of your meter is pulling the FET gates to ground and turning them on.
Look at the diagram above.
You need to check that R547 and R548 are OK. R548 would be the one to check first. These two resistors develop a positive bias voltage for the FET''s.
With the amp OFF and unplugged can you make sure that the switch shown as SW-1C that I circled is open.
It is odd that you say there is no voltage on either end of the cap C531. There should be a high negative voltage on the negative lead of that cap.
If that is still the same then can you measure the voltage on D505. Look at the diagram to see the expected sort of voltages.
Make sure your meter is on DC volts and nothing else
R549 sees a high impedance at its positive end and so no significant voltage could develop across the resistor. It would see negative 60 on each end if the switch contact remained closed.
I am not sure if the switch contact is a normal open/close type or whether it is a momentary wiping contact that makes only while the switch is being pressed.
If R549 is isolated (lift one end to disconnect it) then the circuit should all work and the voltage on the pos end of C531 should rise to around positive 12 to 15 volts.
If it doesn't then either the 150k is open or the cap is leaky.
I am not sure if the switch contact is a normal open/close type or whether it is a momentary wiping contact that makes only while the switch is being pressed.
If R549 is isolated (lift one end to disconnect it) then the circuit should all work and the voltage on the pos end of C531 should rise to around positive 12 to 15 volts.
If it doesn't then either the 150k is open or the cap is leaky.
I'm travelling at the moment so don't have my notes, but I can confirm SW-1C is a 'normal' on/off switch, not momentary. It's a small lower voltage switch incorporated as part of the main on/off power switch, so it works in tandem.
I think you'll find we covered a lot of this in an earlier thread re. the (same) 3120 muting circuit.
DC offset on NAD 3120 preamp output
I think you'll find we covered a lot of this in an earlier thread re. the (same) 3120 muting circuit.
DC offset on NAD 3120 preamp output
Yes, I had observe this on various old NAD models and also on Luxman's L190/L210 integrated amps.The gritty sound could well be a symptom of the FET muting (we are assuming at this point it does have that) and occurs as the FET slowly becomes non linear as the gate voltage drifts ever lower (more negative). It would be a classic symptom.
You will need the correct circuit diagram to refer to but that other thread should give you a good start. If it is around that area then small caps related to the generation of the FET gate voltage could be at fault
I'll look in again tomorrow (out of time today)
If I had such old amp on desk without any repair service before, troubleshooting isn't first step.
First steps before starting troubleshooting are follow:
- Removing all transition resistances occur through the age (switches pots, solder-joints etc).
- Replace electrolytics (not all, but the usual suspects)
- remove all FET's for mute functions - i. e. shorting D-S in serial operating, interrupt D-S in parallel operating
(Such MUTE FET parts are only for user's, who don't want to get into the habit of keeping the volume control in "zero" position before switching on and off (to avoid unwanted crackling while start-up the operation).
If now the same unwanted effect is to observe, one can start usual troubleshooting.
Last edited: