NAD 3020 (1982/3) - loud pop when turning on

[hrp1000]

I have a NAD3020 (from new, bought in 1982 or 1983) that has recently developed a loud pop when turning on. This is the original, not the 3020A or later, as far as I can tell, but s/n A3257885 might reveal something else!

It makes no difference where the volume is set prior to turning on, but having the "Audio Muting" button in makes the pop much quieter.

Before I open it up and start checking things with a multimeter, which are the likely components that have failed?

 

[BSST]

Hi hrp1000,

Welcome to the forum!

Can you post the service manual or a link to an appropriate version? Even a correct schematic may help.

Thanks and good luck!

 

[hrp1000]

Hi BSST

I found this set of circuit diagrams on the Web a while ago - I believe they're for the original NAD3020, but I could well be wrong

 

[anibal]

If pressing the mute switch lowers the noise, this shows you the problem is on the pre-amplifier, very likely some electrolytic capacitor is bad.
I think you can start replacing them, maybe "all of them" is a good idea because of the age of the unit.
By the way, I also keep my old 3020.

 

[luckythedog]

I believe they might all have a certain turn on sound that resolves within a second or so? But if you're sure it's a new thing maybe pull out the links on the back that seperate pre from power amps? Then you'll know if it's the power stage or pre?

 

[luckythedog]

Also check for no dc on outputs ASAP to avoid speaker damage

 

[BSST]

Thanks for schematics!

Anibal may be right.

Is the problem in both channels? I couldn't spot the the mute switch in the schematics. Can you help me locate?

There is a switch or relay symbol TB1/TB2 near L601/L602. Might be thermal switch, but is there any time delay relay present? Haven't found a coil yet...

 

[Johan_vd_W]

I would agree with replacing the caps. On later revisions, there's a muting circuit with a JFET in the signal path (J111) between the pre-amp and poweramp (Q509 / Q510), just before the balance pod. You should easily be able to see if this is present in your amp. There's a little delay circuit when powering on the amp which can cause trouble. I've read somewhere of other people who had issues with the muting circuit.

 

[anibal]

Although the switch is not labeled in the schematics you provided, I think this is the mute:

 

[anibal]

I suggest to replace all of them, but if you want to reduce the search for the bad cap, you may test if the noise is produced when phono is selected or also when aux is selected.
Checking this will show you if the problem is at the phono stage or the line stage of the pre.
The line stage only has 3 caps per channel of 47uF/6V3, the specified voltage looks too low and you can replace them with higher voltage units that fit at their space in the PCB.

 

[BSST]

Thanks for SW6 ID. It's consistent with description of interaction with mute switch. I'm also optimistic that Johan's mention of JFET muting in post 8 could be the culprit, especially if onset of problem was sudden and if problem is present in both channels.

 

[hrp1000]

Many thanks for the extremely rapid replies.

Same pop with either Aux or Phono selected, so I guess that points to the line stage of the pre-amp. Also, it's only on the right channel.

Having to replace 40 year old electrolytics is something that wouldn't surprise me (while I've got the top off I will replace the caps on both channels)- it looks to me as if there are 4 x 47uF/6V3 caps per channel - C514, C516, C518 and C524 (but my eyesight isn't what it was when I bought the amp). Any recommended brands to buy or to avoid?

 

[anibal]

I would go for any of the well known brands (Nichicon, Panasonic, etc.) but please purchase them from a company you can trust, not eBay which is flooded with fake parts. The cost of these part is low anyway.

 

[BSST]

And check if your amp has the JFET muting circuit mentioned in post 8.

 

[hrp1000]

@Johan_vd_W & @BSST - do you have a circuit diagram showing the JFET circuit? I note that the component numbers on the diag I supplied don't include J111, Q509 or Q510 (you'll have to forgive my circuit blindness/ignorance - the last time I built a circuit with bare components was in the 1960s, and I haven't really looked at circuit diagrams much since then).

 

[Johan_vd_W]

Sure! See attached PDF for a nice high resolution. Here you can see that piece of circuit in detail. It’s a bit of a strange circuit that does ac detection as well as a soft power-on I think.

 

[BSST]

Let us know if your amp uses this circuit. C533 and especially C531 appear to be terribly under rated for applied voltage.

 

[Johan_vd_W]

Well.... I think you might actually have the first generation since you said you did have a 'mute' button. I believe the newer versions are missing that button as it was changed out for a 'mono' switch 😛. Sorry about that lol. So think you can skip the search for a J111 and so fort.

 

[hrp1000]

I finally got round to going inside the amp and couldn't actually see any 47uF 6V3 caps (or at least, not in the pre-amp), and the count of electrolytics didn't match up with the circuit diagram I posted before. After a lot of searching on the web I found a scanned service manual for an amp that looks like mine and the values (and number...) of the caps matches up (so when I said I might be wrong about having the right manual before, I was right ;-)).

So for information, here's an extract of the manual with the pre-amp highlighted.

 

[BSST]

Do both channels have the same pop behavior?

Have you tried luckythedog's suggestions in posts 5 and 6?

Would you post the entire service manual? I'm curious about bias adjustment and specs about expected output voltage.