NAD 7100 Repair – Continuation
As mentioned in a previous article we will break down the 7100 here and explain it’s workings, hopefully to make trouble-shooting easier.
Caution: This column has been written for those savvy in electronic repair, those knowing the difference between voltage, current and resistance. High voltages, such as used in the 7100 audio amplifier/receiver are dangerous. The 7100 series amplifier/receiver uses two supply rails +/- 45V or 90V in the audio stage and in the PE stage +/- 72V or 144V DC. In an environment which has not been designed for proper diagnostic checking, proper supervision and/or diagnostics and are not carried out by suitably qualified personnel these voltages can be lethal.
Like most amplifiers or electronic equipment the simplest repair is usually a dead set – this is not necessarily the cheapest repair however because the mains transformer is usually the costliest item on the parts list (usually!).
The NAD 7100 is going to be no different. The NAD 7100 has extensive safety circuitry built in and as long as the user doesn’t put 110 / 220V onto a loudspeaker terminal things should rarely go wrong with the components except for old age taking it’s toll.
NAD 7100 Repair: The Power Supply
NAD 7100 Power Supply
- The NAD 7100 – Power Supply
The +/- 45V feeds the output stages. Or does it? Power Envelope and Power on Demand.
NAD 7100 Repair – How to get sufficient head room and beat the competition.
What is the potential power output of this amplifier? (( (Vcc * 0.707) / Load R ) ^ 2) * Load R or Vcc ^ 2 / 8 * Load R. In this case 253W into a 4 Ohm pureley resistive load. Power supply limitations and circuit configuration which includes drive to the output transistors play a role here. According to specification this amplifier can deliver 60W RMS into an 8 Ohm load.
This power supply uses pretty hefty reservoir capacitors (10 000 uFD) of very good quality in the power supply. A loud hum and/or fuses blowing could be cuased by faulty electrolytics or a faulty diode in the bridge rectifiers D501 / D502. Often it is necessary to disconnect the power supply output from the load if trouble-shooting in this area. It will also prevent further damage from occuring. Make sure that you have disconnected properly and not only part of the circuit – do a resistance check.