NAD 3020A Loud pop on switch off

I've recapped / rebuild several 3020's, including the A/B models, and can confirm that your 3020A does have the muting circuit (that's where the red/white wires on the front section of the power switch go to), although I doubt that the FET muting is the problem.

I suspect you probably have a worn/dirty mains switch, which is likely arcing when you turn the unit off. Have you tried just turning the amp off at the mains socket where it's plugged in ? If you don't get the loud pop, then it's switch related. Solution would be to replace the switch (if possible), or more likely to completely remove, strip, and clean / replace the switch contacts - it's quite fiddly, but can be done. I think there's a detailed procedure somewhere on the Audiokarma forum. Replace the 2 suppression caps in your photos at the same time - as Ian said, they're to suppress arcing within the switch, and are likely to be worn after 23 - 30 years use.

Do you also experience any loud 'thump' (or other noises) when you turn the unit on ? if so, it sounds like it's due for recapping and a good overhaul generally.

Finally, if you need detailed schematics or help with it, feel free to pm me (I'm in S. Lincs.).
 
Thanks for the reply. I've tried switching on and off via the wall socket and there are no noises. Yay :). When I switch on with the power button there is a buzz which is not overly loud and sounds similar to a Star Wars light sabre/sword. This decays to nothing after about 12 seconds. There is also a quick buzz which lasts about a second and moves from right to left and happens after about 3 seconds of switching on. There are no noises when switching on or off at the wall socket. This has been checked only with headphones. Not sure if thats relevant.

I'll search the Audiokarma site for more info and take the lid off the amp to see how viable this is for me. If you've got time could you advise what to buy from Maplins to replace the suppression caps. Thanks.

I'll update any progress. Thanks again.

http://www.maplin.co.uk/
 
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Yes, that's the thread I had in mind - it is fiddly, but the original switches are all but unobtainable now, except maybe from donor units.

Are you saying that if you turn the unit on at the wall (amp on/off switch already set to on), it makes no 'unusual' sounds at all through headphones ? I find that a bit strange, as the 'light sabre'/buzz at turn on is normally capacitor related in my experience, not switch related - and while it's common, it does suggest aging caps. You might want to check the service manual, and at least measure the dc-offsets before hooking any speakers up.

Anyway, if you do decide to tackle the switch - I'd suggest to avoid Maplins, and use either CPC, Farnell, or RS-Online, for (small order) parts, they have far better product ranges. I use Mouser (US based) for larger orders, and detailed component specs. The following link should be a suitable replacement for the switch caps - but that's unlikely to help unless the contacts are cleaned or replaced as well.

http://http://cpc.farnell.com/1/1/12611-capacitor-x1y2-4-7nf-de2e3kh472ma3b-murata.html

Just switching at the mains, defeats the muting circuit - the power switch is used to switch the main supply to the transformer, but it also has a second switch section at the front, which is used to operate the muting circuit. So, it's not an approach I'd personally use or recommend, but I know others do.
 
Just checked again and the light sabre buzz is still there but very quiet. Its impossible not to hear it on headphones when using the power switch but almost inaudible when switching at the wall socket. The left right buzz does not happen at all when using the wall switch.

As the caps are easy to replace I would like to try replacing just them. I've found online a receipt of a 3020 service/repair which states "Mains switch capacitors to remedy arcing". Presumably this solved the problem as there is no mention of switch repairs. I think its worth trying as if it works it saves repairing or replacing the switch.

I'm assuming the switch problem is a form of spark erosion which leaves a layer of crud on the contacts which builds up with use rather than time. If a 3020 is low mileage is it possible this fault hasn't developed yet?.

I live near an RS Online and can collect from there. If you could advise which capacitors to buy (part numbers) it would be appreciated. Thanks.

RS Components | Electronic and Electrical Components

No idea how to check the dc offset or what it is. I've used the amp in the past with speaker without problem.
 
I somehow managed to get the CPC link wrong - so here it is again, (I hope) working:

CAPACITOR, X1Y2, 4.7NF - DE2E3KH472MA3B - MURATA

The RS-Online site is down at the moment, but if you search the same manufacturer part number later, they probably carry the same part. If not, they're X1 / Y2, rated caps (for mains safety use), 4700 pF (= 4.7nF), for at least 250 Vac (for UK wired units).

Good description of what happens with the switch - it's indeed a use phenomenon, not age, so you may be lucky. The offsets, which I assume are mV, are fine - the reason to check was simply that if they're too high, you risk your speaker coils.
 
Replaced capacitors with those above and no change :(. Exactly the same as before. Worth trying for the small cost and relatively easy work (although a smaller soldering iron would have helped).
I'm going to use it as it is and hopefully remember to turn the balance to the left before switching off.

Thanks for your help.
 
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The real issue is the switch condition, isn't it. You've already proven that with using the outlet switch where there are no caps at all because a couple of ceramic caps isn't going to fix loose fitting, arced out contacts. The caps will work fine with nice new switches though.

Here are some assorted listings of what's available: 4 Pcs DPST Spring Loaded Locking Push Button Switch AC 250V 8A 128A KDC A11 2 | eBay
No doubt they are copies of old standard power switches for TVs, Stereos etc but the hard part will be matching dimensions like the mounting hole spacing, shaft extension etc so that it mounts with the button in approximately the right position. The hole spacings are fairly standard but length might require some spacing washers, stand-offs etc.

I realise these don't include the little switch fiitted in front of the mains switch but is this essential? If this is the mute switch, I can understand how the odd noises might arise.
 
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Completely agree Ian. It's a common fault with the 3020, especially after ca. 30 years service. I've successfully cleaned & polished a couple of these switches now, but it's certainly not my idea of fun... New replacements are hard to find, but the following 2 links may help:

Sanyo JCX2900K new power switch (and others) - AudioKarma.org Home Audio Stereo Discussion Forums (Mouser 611-NE1839EE)
Switch is physically smaller, more difficult to fit, but higher ampere rated than the original, so should last. I have a couple, but haven't needed to fit any yet.

http://ebay.co.uk/itm/Concept-16-5-...pt=Vintage_Electronics_R2&hash=item58b4a91f75
Switch is similar to the early NAD 3020 type, but I'm not sure of the ampere rating on these, maybe lower than the originals.

The 3020 A & B models have JFET's directly in the signal path, just before the pre-amp output sockets - the front portion of the main power switch is used to control the circuit that's opening/closing them. It basically cut's the signal path for a few seconds at start-up, and immediately at shut down, so should remove (most) start-up / shutdown sounds. It's certainly possible to remove that part of the circuit - the original 3020 didn't have it at all - but to completely remove it, I'd take the JFETs out of the signal path as well.
 
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O/T: For some time now Ebay links as they are shown, will not work when copied elsewhere. Ebay's advice, and it works for me, is to shorten the link to just at the end of the item number excluding the ? mark. Simply, you only highlight that first part of the link address and voila - just like it used to be.
 
I have a similar issue with my 3220pe. Power on thumbs first and then another noise moves from left to right. Power off gives pops as well. Both 3020 and 3220 share the similar design even the schematics are placed together in one manual. I checked my board and schematic I bet that C602 and/or C603 in the muting circuit are dead. I will find out later.
 
Hi,

I had the same problem: NAD 3020A with popping in the right channel when switched off by SW1. Not so when switching the unit off by the wall socket switch.

Measuring revealed serious bouncing on SW-1c.

Solution: I soldered a 10n ceramic capacitor of the terminals of SW-1c. Debouncing almost perfect. Not even a hint of popping anymore.

Cheers