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NAD 3020A - ceramic disc cap replacement in pre amp

Thanks for the reference. I just gave the power switch a cleaning with a little contact cleaner and it's ended the sparking, guessing it was a carbon build up.

Hera are the values from what I can see.

R419 21 Kohms

R421 120 Kohms

R423 10 Kohms

R431 180 ohms

R411 can’t see this one on the board

R417 can’t see this one on the board

A capacitor in parallel with C413 may be R415 (but it’s hard to see) which is 680 ohms.
 
@Hifibythekg - if you look just behind where the MM/MC switch wiring attaches to the main pcb, there's 6 resistors in a row - R411 & R412 are in that row (the middle 2 I think).
R417 & R418 are mounted on the small vertical pcb attached to the rear panel MM/MC switch.
 
I don't have a good suggestion for your volume pot. question - it's a dual (stereo) ganged volume potentiometer with loudness 'tap', which you'll find difficult to replace today, so I'd suggest treat with caution until you've had time to study your options.

If you search the forum you will find an example where Leesonic fitted a better quality ALPS RK27 pot to one of his 3020's. He may be able to add something re. fitting it, but I doubt it's a simple drop in replacement. Hificollective https://www.hificollective.co.uk/potentiometer/alps-blue-beauty.html would be a good UK source for RK27 pots - but note, the RK27 has no loudness tap, so you'd lose the 'loudness' function on the amp entirely.
 
Hifibythekg said:
Thanks for the reference. I just gave the power switch a cleaning with a little contact cleaner and it's ended the sparking, guessing it was a carbon build up...
Click to expand...

The "carbon build-up" is due to sparking across the contacts and will eventually damage them; a snubber (capacitor + resistor) is the best long-term solution.
 
Thanks Leestereo.
Actually, the sparking sound upon power off has now recommenced..
I was just about to write the following, so here goes..

The amp is now sounding ‘great’ but there’s a couple of things I want to attend to.


1. When switching off the power, there’s a single ‘sparky' crackle sound but only through the right channel speaker.
There are a pair of blue caps at the power switch marked KC 472 P, could one of them be at fault, or is the power switch itself more likely?
If the caps, would these be an appropriate replacement?
https://uk.farnell.com/vishay/vy147...f-500v-20-y5u/dp/1902221?MER=sy-me-pd-mi-alte
Or could the crackle be caused by anything else?

2. The sound through the right channel is louder and more forward in character than the left channel.
It's definetely something within the pre-amp as I've plugged a headphone DAC into the 'Normal In' and the balance is perfect.
Could the volume pot be a cause of this (certainly it cuts out the volume completely on the left channel before the right channel when turning down the volume close to zero.) Is there a way to bypass the volume pot to test this?
 
Hifibythekg said:
Correction, R419 is 27 Kohms.
Also, a full list is provided here:
https://audiokarma.org/forums/index...rebuild-troubleshooting.757428/#post-10367391
Click to expand...

The RIAA resistor values in the 3020A are the same as that in the 3020B. The resulting RIAA equalization can be seen here:

3020A MM Stock.JPG

Decreasing the R419+R421 value (147kohm) to 122kohm improves the low-end accuracy. In general, the RIAA accuracy/performance benefits from upgrading the resistors to 1% metal film types and the capacitors to polypropylene film or C0G types.

3020A RIAA mod.JPG
 
Thanks, that's very interesting. Most of the caps in the preamp section are now film or C0G types.
Any thoughts on how best to tackle the volume imbalance?
 
The 'crack' that's audible in the right channel when you turn it off is due to arcing in the power switch, as the contacts open. I've had the same issue in the past.

You won't be able to clean the ON/OFF switch without stripping it down, it's impossible to get to the contacts otherwise. The following thread shows a similar switch repair, to give you an idea of what's involved: https://www.audiokarma.org/forums/index.php?threads/almost-smoked-a-nad.497407/ - it's 'fiddly' but was sufficient to fix mine. Changing the suppression caps is unlikely to fix it.

@Leestereo - interesting RIAA comparison. Are you using a SPICE model to calculate ? or ?
 
Well... In trying to disassemble the switch I managed to damage it... and replacements don’t seem to be readily available. Aaagh.
Did discover that contacts were heavily carbonised though.
Original is an Alps / Sanyo SDU 3 PE. Commonly used on older CRT televisions and other vintage audio gear I believe.
Anyone got a tip on an excellent replacement?
Also, the volume pot.. given the efforts and wonderful, worthwhile improvements of replacing all caps with high grade caps and films it seems that the original volume pot compromises the amp’s potential. Fair to say that there’s no way of installing an excellent volume pot without losing the ‘loudness’ selection switch?
Any advice on replacements and workarounds gratefully accepted..
 
Also, regarding my previous comment,
“2. The sound through the right channel is louder and more forward in character than the left channel.”
Managed to locate and cure this... two solder blobs (beneath two ceramic discs) were touching.. separated them and all was good.
 
Hifibythekg said:
Is this power switch a suitable replacement?
Click to expand...

Unfortunately not. The original NAD switch has 2 switch sections, the main AC supply switch at the rear, as well as a smaller switch at the front. The switch you cited only has the main AC section, but for a 3020A you need both. Apart from any old stock, or a salvaged part, the only new switch option for a UK model that I'm aware of is the one suggested by Leesonic somewhere on the forum - https://www2.mouser.com/ProductDetail/CK/NE1839EE?qs=U2%2BV/zDE79GIQgtl4FQ0uQ==
 
Goldie99 said:
Unfortunately not. The original NAD switch has 2 switch sections, the main AC supply switch at the rear, as well as a smaller switch at the front. The switch you cited only has the main AC section, but for a 3020A you need both. Apart from any old stock, or a salvaged part, the only new switch option for a UK model that I'm aware of is the one suggested by Leesonic somewhere on the forum
Click to expand...

Yup, you need the Mouser part 611-NE1839EE.
 
Phew, thanks Lee, was just about to press 'buy' on the other one!
Also, for the two caps attached to the power switch, what would be the best replacements?
Anything other than Quencharc, which is v pricey over here.
They're labelled KC 472 P.
Photo attached.
 
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