Repairing the NAD 7100 Receiver

We’ll get to a problem in a few seconds but please read this first.  The NAD amplifier, model 7100 is a rather robust beast. NAD, like most amplifier manufacturers wanting to protect their reputation always over-spec their components. You know the Rolls Royce stand on how much horsepower their vehicles delivered and the rather snide reply was always “enough”, NAD seem to follow a rather similar train of thought. NAD amplifiers deliver enough! This 7100 amplifier/receiver is no different. OK, in hindsight I will say that the amplifier is over engineered – if I was to build an amplifier I wouldn’t worry about energy saving, voltage shift, class D amplifiers. I want an amplifier that synchronises and mirrors the output with the input, no distortion even at full volume, a total reduction of any form of harmonics / cross-over distortion and I would not abide to the warm sound of valve  (thermionic tube) rule. I would design an amplifier, a set of speakers and a to die for graphic user interface using computer geek speak. In fact NAD seems to have done this with a million and one components to derive a mirror image of the input signal and according to the listener, in many cases, unrivaled sound.  This amplifier is from yesteryear – yet, with yesteryear lies promise. To build an amplifier first and foremost you need to look at the target market. Then you build the power supply.  NAD gets this right with all their amplifiers – they don’t skimp on the mains transformer even in their home theater market where you get mostly promises and under delivery.  If it’s not a switched mode power supply a ‘real man’ power amplifier is damned heavy – this comes from the power supply. Modern semiconductors weigh just as much as the 1970s – cooling laws have stayed the same albeit with more emphasis placed on cooling fans.  NAD seem to do it with great mains transformers and high capacity smoothing. Although the 7100 is an early years amplifier they used a lot of technology which is pumped into modern circuit design. This was an amplifier from years back – it was possibly over-engineered but they wanted user satisfaction. The 3020 is a case in point.

Simple is better

Yes, test an amplifier with all the latest technology and compare it to a directly coupled amplifier like the makes of Kenwood and Pioneer from 30 years back and you’ll be surprised at the quality – then already. NAD doesn’t compromise on value for money, the mains transformer bears testament to this. They are very expensive.  Yet often a 5 transistor audio amplifier sometimes sounds better than a 100 transistor amplifier. With safety comes a compromise in audio fidelity.

So what happened with this amplifier?

In cleaning, the moisture from the compressed air caused a voltage offset from the more sensitive input circuitry in the pre-drive stages – directly coupled amplifiers are definitely not lovers of having their bias altered in their primary stages. Each change in parameter from input to output is amplified – try sticking your finger on the mirror amplifiers or Darlington’s or differential amplifiers at the input of an audio amplifier – while great for checking basic function can be disastrous to the end result – high input impedance circuits just don’t like to be dirty.

And now?

Many repair companies use compressed air to clean gear coming in for repair. I am no stranger to this, neither computer motherboard repair where dirty boards are a no-no. Some companies wash off boards which are deemed faulty – I’ll stay away from this subject because it’s a dodgy topic – do NOT attempt this at home.  Compressed air always carries a residue of water even after a drier but you do need to dry the components off using the proper equipment or a domestic oven at low temperature. 2 hours in sunlight or room ambiance may not be enough.  In my case, in the southern hemisphere during June it’s clearly fool hardy.  I have done this before – it worked. Not now though.

Some things to check on the 7100

I went through the schematic of the 7100 and if we stick to the audio section, here specifically the output stages there are some things to check but first and foremost be aware that this is not an amplifier which should be trifled with. The output transistors are very expensive – ensure that power is off, meaning unplugged and the reservoir capacitors are discharged. If you are not familiar with audio equipment or working on any electronics circuit for that matter don’t even think about going further. The output stage on this amplifier uses voltages which can hurt you seriously and under the incorrect (or correct) circumstances will kill. Semiconductors do not like ‘live’ checks with the incorrect test equipment and like my example proves, the slightest bit of negative influence will create havoc on the protection circuits.  The output stage here forms a very high energy circuit – this means pursuing any further tests carries high risk. Get the schematic from the link above before embarking on a project of this magnitude.

To know what voltages to look for is one thing but to get to test points is another story. Always use a high input impedance meter or oscilloscope – be very careful of shorting out the test components – standard meter probes are great for working quickly when checking resistances but never voltages. Tape the tips with insulation leaving a millimeter or so from the ends if you have to check voltages. Work backwards from output transistors to the pre-driver stage, usually low current voltage amplifiers.

This amplifier is large on discrete components and small on ICs – this makes it more difficult for the DIYer whom does know his way around circuits. Understand how the circuits works, what could cause the problem and when you do find the problem – why it occurred.  Do not even contemplate a self repair if you do not have the schematic.

Audio amplifiers don’t usually just stop working unless it’s a really suspect design and build  – this will always be found in the layout of components and how the ‘engineers’ piggy back resistors across other components . At a commercial level this should not be seen and this will not be the case with the 7100. What the 7100 does offer you is plenty of room to move – but plenty of room to make mistakes.  Get back to basics – switch on, do primary checks – heat, visual and smell – does the heatsink get hot quickly, where does it get hot, do capacitors look swollen, are there resistors burnt.

What do I do?

If the amplifier is dead: fuses; then check for shorts on the output and driver transistors.  I will do a check first, with suspect readings remove the component for further testing. Transistors become leaky – current tests are also a good thing. (most test equipment, even DMMs have this function – strangely enough it’s the cheaper meters not the more expensive which measure HFe).  Power transistors usually will possibly show a short circuit between collector and emitter but not always. (sounds pretty uneducated, doesn’t it).

The LOL test

We seldom check the spark plugs with modern cars because they seldom go wrong when the engine doesn’t start. Right? Like modern amplifiers are actually very robust and it’s usually the stupidest thing that causes the biggest stink. If all else fails read the instructions. The 7100 is no different.  Source input must be the number one headache for most technicians. Then again sometimes a problem is communicated through clicks and grunts – yes our forefathers used to repair stuff as well. Understand what the user is on about. Often we laugh behind the user’s back because the problem seems trivial – most likely end-user  stupidity but there are just too many instances of comebacks or returns because we didn’t listen properly.

Audio amplifiers are prone to failure if there is user abuse. Often we don’t read the symptom right, like your local GP makes incorrect diagnosis through your output. One thing is sure though, if you aren’t sure – it will bite you. Circuit freeze is a necessity in any electronic repair – it’s not the Messiah but for intermittent problems is a God send. Faulty capacitors, hairline cracks and even noisy resistors can be found through circuit freeze and hair dryer use.

Old electronic equipment, like the NAD 7100, will fail eventually. Intermittent problems can be a bastard to fix. Audio distortion will come down to a difficult fix if you don’t know what to look for.  Know your test points but before reaching for the ‘scope’ or DMM are you familiar with the circuit? Nine times out of ten it won’t be the pre-amplifier but a leaky transistor in the power amplifier – if both amplifiers are doing the same thing look at the common components, if not do a comparison check. Capacitors are great in that they all fail *simultaneously (old age in most cases) – if not check the transistors though cross checking. Resistors also change in value – when you get more experienced you’ll find that low value resistors burn (faulty semiconductor), high value resistors go high in value, especially with high voltage circuitry.

(*simultaneously – to a degree. Electrolytics mounted near a heat source are prone to drying out. – Ed)

I’ll continue with this tomorrow by looking at the circuitry. Make sure you have the schematic handy.

Sunday 16th June 2013 – continuation…