A sorry tale of woe and stupidity: NAD C370

[JamFool]

Hi,

Background….

I have owned the NAD C370 for about 18 happy years. The only problem I ever had was with the protection circuit.
This I managed to fix by swapping out the capacitors on the protection board and moving the Zener diode to the other side.

The amp continued to work ever since without problems.

Recently I invested in some new studio gear – audio interface, patch bays etc. After reorganizing the room I hooked up the amp to the system and found that only the left speaker had sound.

I checked the signal coming from the audio interface on both channels – no problem there. I checked the signal being sent from the patch bay – no problem there. I swapped the speaker cables on the back of the amp – the sound now came through the right speaker so the speakers are OK and I concluded that the problem must be the right channel on the amp.

I opened up the amp and had a look around. I noticed that one of the four large capacitors (10 000uf) had a little bulge on the top. To check I switched the left and right pairs over… no change so I guess they were OK.

I swapped the left and right pairs of output relays… no change again so I guess these were OK too.

Next I removed and inspected the right power amp board – I found the two large white resistors on each end had cold/loose solder joints, in one case the track had lifted up from the board. A few more solder joints looked suspect so I reflowed them.

Sadly, still no sound from the right channel.

Then an idea came to me – a really stupid idea! I thought I could isolate if the problem was on the power amp board by swapping the left and right boards. I took out both boards and went to connect the molex leads. All of the interconnects were the same apart from one small lead in the middle. The right side had a connection with 3 pins and the left just two.

Right side…..

Left side….

Without thinking I hooked them up so that all the connectors apart from these two were from the same side – so these ones were crossed over – what a stupid thing to do ☹

I powered on the amp and the green lights flashed on for a moment and then nothing – just a bad smell as something had burnt out. I cursed myself and swapped the boards back. Inspected the fuse and found it was blown.

I replaced the fuse and the amp came on – green lights but now there was no sound from either channel. I noticed that when I touched the two pinned cable on the left board that it made a humming sound.

In despair I looked to Finn.no and found someone was selling a C370 quite close by. I bought it and had it back home the same evening.

I hooked it up to my system and was stunned to find that sound was only in the left speaker!! OMG, this meant that there had been nothing wrong with the amp all along!! Arghhh….

Sure enough the real problem had been with the brand new cable from the patch bay to the amp inputs – can’t believe I did not check that. I checked the cable from audio interface to patch bay and the signal from the patch bay output but missed the cable from the patch bay to the amp – double idiot!

I repaired the cable and sound now came from both speakers.

So, I managed to destroy my perfectly good amp and buy a new identical one when all I needed to do was rewire a cable – I didn’t even think to check it as it was brand new. I could cry.


There is my sorry tale of woe. My question is – do you think there is any way to rescue the old amp? What I did to it is so unusually stupid that I will not find similar accounts or fixes online. Do you have any idea how I could start to find out what damage I did?

I would really appreciate any help anyone could offer on this.

 

[bowtie427ss]

ALWAYS ALWAYS ALWAYS start with the simplest possibilities and check them off by process of elimination.

First step should have been to swap the input cables to the amp. That would have eliminated the amp as a possible cause.

Be thankful that you were able to quickly find a replacement amp.

As for the damaged amp, if you can find a tech willing to work on it for less than market labor rates, have them give it a go. Otherwise, unless you need a doorstop that looks like an amplifier, i'd let it go and not look back. Chalk it up to the ever increasing cost of education.

Most important of all, don't beat yourself up about it. Anyone who has dipped their feet into audio to any degree has made similar mistakes. It happens to the best of us, we've all let the magic smoke out at one time or another when we had only good intentions.

 

[JamFool]

ALWAYS ALWAYS ALWAYS start with the simplest possibilities and check them off by process of elimination.

First step should have been to swap the input cables to the amp. That would have eliminated the amp as a possible cause.

Yeah, so very true - normally It would have been the first thing I would have tried. I'm not sure why but on this occasion my brain was working without the slightest bit of common sense. Utterly embarrassing.

Most important of all, don't beat yourself up about it. Anyone who has dipped their feet into audio to any degree has made similar mistakes. It happens to the best of us, we've all let the magic smoke out at one time or another when we had only good intentions.

You're very kind and yes - I was very lucky to find a replacement, a costly lesson that I won't forget anytime soon. I'll try to find a friendly tech like you suggest - it would be really nice to have 2 up and running as I believe they can be run bridged.

 

[bowtie427ss]

I'll try to find a friendly tech like you suggest

There are some technically savvy folks here many of whom tend to work on/and are more familiar with particular brands.

Be patient, often these folks actually have real lives that don't continuously revolve around internet communication and audio.

If you browse NAD related threads, you'll discover who some of these folks are @leesonic. Even if not available to work on your unit directly, many of these folks are quite generous with their experience, knowledge, and time. They can help guide you through your own DIY repair process saving you much time, frustration, and expense.

 

[JamFool]

That would be awesome! Thanks @bowtie427ss

 

[bowtie427ss]

If it's any consolation, i once long ago converted one of these to a doorstop by failing to unplug the unit while replacing a blown meter lamp, took out drivers and outputs on both channels. A tiny arc, a puff of smoke, instant doorstop!

 

[JamFool]

Ouch