NAD 3155 Restoration thread

niss_man · September 12, 2020

Just putting this thread up as promised to @DavidDoerfer who kindly paid forward some free gear a few weeks ago.

I received this amplifier for free from David. A NAD 3155 a bit of research shows they were built in 1985 and this one has been particularly well looked after. Nad specs. I wish all amps had proper specs like this (not many do anymore) instead of inflated power figures at high distortion figures this one shows both chanels driven from 20-20khz with no more than the rated distortion (which is nice and low).

I decided to not even power this amp up and just went into replacing a few capacitors and checking all of them with my meter. Mind you I would have expected this to have worked since most caps were well within spec. The board looked in good condition with what seemed to be original transistors as well.

You will notice that all the electrolytic capacitors on each board have their negative side all facing the same direction. Makes it easy to know which way to place the caps when soldering. (nice engineering thought).

I always found the heatsinks in these Nads interesting. It looks like they used an L piece of extruded aluminium and then get some sharp fins to scrape down the edges to create the fins for the heatsink as evident by the curly ends of the fins.

I haven't checked, but I think these are original transistors.

The caps I replaced. Note that 95% of these were still within spec but I replaced them anyway with new ones. I didn't replace the Power supply caps but they were well within spec and reused them.

I connect a bulb tester (have not got a variac). the bulb is connected in series with the 240v mains and the amp pluged in afterwards. When powering up the bulbs glows bright briefly (less than a few tenths of a second) before dimming down to barely just on. If it continues to glow brightly then there is something wrong with the amp blown transistor or what not that needs to be fault rectified. This amp was perfectly fine.

The bigger multimeter not connected here was used to show the dc offset on both chanels which was un der 0.5mV for both channels. The 2 smaller multimeters show the standby (quiescent) current through the power transistors. The manual states 20 - 25mV setting. Only a very minor tweak was needed for one channel once warmed up. It was still passable before adjusting though.

Put some sine wave through. Used a 7.3ohm resistor as load and just before clipping.

That was 36.8v peak...so for those who like math. 36.8 / root2 = 26v rms

V = IR so Current = 26/ 7.3 (7.3 ohm load resistors) = 3.56 A

Power = VI = 26 x 3.56 = 92Watts into the single channel being tested.

Running both channels at the same time produced 33.2V Peak so it has around 3V droop in supply voltage with both channels used as can be expected.

This equates to 75W per channel both channels flat out. This actually exceeded the NAD spec sheet although I didn't have any distortion meter on their so I could have been over driving it slighly even though the scope showed no flat spotting of the peaks of signal.

Finally some pics of the amp playing through some speakers. The amp still sounds great and hopefully has another good 20 years of life left in it.