A friend of mine bought a new amplifier but he wasn’t pleased with the sound. He likes the aesthetic of older things, so he called me and asked if I could recommend a good old amplifier that could drive smaller speakers. The first one that I could think of was the venerable NAD 3020B. When I was young, this was the first affordable amplifier that actually sounded good. It’s not a power house by any means, putting out a (very conservative estimate) 2 x 20 watts. But the phono stage which is switchable for both MM and MC pickups is good, and the sound quality is excellent. So he scoured the used market and finally found a NAD 3020B. Unfortunately when he got it home, there was a loud hum from the output. So I told him to send it to me so I could fix it.
Time to take the amplifier apart and start doing some troubleshooting.
It has a bottom plate that is hold in place by a few screws, so access to the components is easy. But there is a metal bracket on the underside that needs removing if you need to measure the main power capacitors. The bracket is held down by screws from the upper side and the front and the rear. After that is removed, it’s easy to start measuring.
A Humming NAD 3020B
The hum had a low frequency, so my guess was that the main power capacitors was dried out. If that happens, the power supply can’t filter the ripple from the bridge rectifier, giving the amp a low hum.
If your using a regular multimeter, you can’t measure the capacitors in-circuit because the voltage the multimeter puts out is enough to start components on the circuit board, screwing up the measurements. So I usually start with my Signstek MESR-100 V2 ESR meter, a cheap meter that uses a low voltage, high frequency sine wave to test the capacitors. This means that you can actually test components in-circuit without problems. Very handy! So how did one of the main power capacitors measure up? Not to well I would say, which wasn’t a surprise. I continued to measure a bunch of other capacitors as well, and the where even worse. It was clear that I needed to either just give up or replace all the capacitors. I opted for the latter. My curiosity on how the amplifier would sound was to great to just give up. Just to make sure, I desoldered a couple of caps from the power supply and the amplifier stage and used a high quality meter, my DER EE DE-5000 Handheld LCR Meter to check the state of the caps. This meter can measure θ – phase angle, D (aka tan δ) dissipation factor, ESR, Q, series and parallel inductance, series and parallel resistance.
By the way, the DER EE DE-5000 is exactly the same meter as the company IET Labs sells. But they will empty your wallet by taking three times the price for the same meter. Brand name and all…
Well, the measurements didn’t improve, lets leave it at that. Some of the larger ones measured ESR in the 1k Ω range. You don’t see that every day!
Time To Turn Shopaholic
So time to go through the schematic and check all the capacitors I needed. A long list emerged. I choose to use only high quality components, so the amplifier would last at least for thirty more years, if not longer. So only Panasonic FM, Nichicon and Nippon Chemi-Con capacitors for electrolytic capacitors and Wima for smaller caps. I opted to replace some of the smaller caps in the signal path with Wima capacitors. I also increased the capacitance and voltage rating on the main power supply caps. Instead of the 85˚C ratings of the old ones was replaced with 105˚C rated ones. So the amplifier should be able to take a lot of abuse.
Making Life Easier
The NAD 3020B schematic and the repair manual I got for the amplifier was scanned ones I found on the internet. Unfortunately the repair manual which shows where all the components are was not very well scanned, so I had to constantly flip the amplifier over to see where the components where. But I gave up and took a photo of the upper side and the lover side and used Photoshop, lining up the images and turning down the transparency of one of the layers, giving me a good image of the placements of the components. Click on the image for an enlargement.
Yes, I admit, I love to get quality components. Getting a big bag of quality capacitors delivered by my friendly DHL courier makes me smile. Call me a nerd.
Fortunately I bought a cheap Chinese vacuum desoldering station a couple of months ago, so replacing the capacitors was very easy. The NAD 3020B model is famous for having very sensitive copper planes on the PCB, which lifts easily, but I had no problem when using the desoldering station.
So Was It Worth It?
After measuring the transistors to make sure they where in good shape and adjusting the bias, I powered up the amplifier after connecting a pair of home built speakers and the hum was gone. And it sounded good! Really good! I spent a couple of hours just leaning back, enjoying the music. Success!
I went over to my friends apartment and we first listened to his newly bought Onkyo A-9010 amplifier connected to a pair of Bang & Olufsen speakers with a matching subwoofer. It sounded ok, but a bit recessed midrange. Time to connect the NAD 3020B. We tried CD first and it sounded much better! I couldn’t try out the RIAA-stage beforehand because of a broken vinyl player, but to my relief it worked and sounded perfect. So we spent a couple of hours enjoying the music and drinking a lot of wine.
So If you can get your hands on an old NAD 3020 and have the means to restore it, go for it.