NAD 7100 Repair – continuation from part one

Power amplifier failure can be caused by it being dropped or physically mishandled without the owner knowing about it. I had a Sansui amplifier which had been dropped – it could not be repaired, the output and driver transistors were all blown.  (this was a unit which had been stolen and later retrieved before the new ‘owner’ could test it). Lots of egg on my face – what should have been a simple repair just took up too much time. Initial investigation showed hairline cracks around the input circuitry of the power amplifier stages, around the differential amplifier in fact, right next to a chassis mount. It was not a sophisticated circuit like the 7100 but try as I might I ended up blowing the power transistors again – the test voltages would be normal, bias current normal, temperature luke warm and suddenly the fuses would blow. I never found the cause of this – I just had to walk away and cut my losses. Power amplifiers that have been dropped usually show cracks around any part which has been fastened to the chassis, possibly mains transformer or other heavier components. Bridging with good quality wire is the best fix, insulated or uninsulated. Try to keep as short as possible, running insulated wire across the crack to form a link is the most popular method.

Electrolytic capacitors dry up. Often experienced repair personnel know the cheap capacitor makes from the more expensive and no, not all capacitors are created equal. Pay for the best. Use the correct voltage and temperature rating. SMPSU use 105 deg caps. Don’t put 85 deg C caps in a SMPSU unless it’s for testing purposes only The quickest means of testing amplifiers with low gain is by using an oscilloscope. I knew a technician whom boasted he had never used a scope to conduct a repair. Try that on high frequency video circuits which have low gain. At some stage or other you will find a scope is invariably useful – it is often quicker to use for diagnosis and in small signal circuits is worth it’s weight in gold. Scopes lend themselves to D.C. checks as well as A.C. – not having to make any changes to configuration makes this a very versatile piece of equipment. I’ll also go so far as to say that many DIYers have improved the quality of their cheaper home systems by using a scope. In some cases one can ONLY conduct a repair with a scope, especially when it comes to filters and a.c. measurement. A word of caution here: Be very careful of input grounds. A floating supply which gets grounded by your scope may cause a lot of damage. Know the limitations and if necessary follow the grounding rules applicable to any test equipment. See here – Tektronix information on floating measurements.

Power amplifiers are not usually difficult to fix if you know what to expect when certain components go faulty. Older amplifiers which ‘just fail suddenly’ are often a result of dry-joints, cracked boards and/or electrolytic capacitor failure. Newer amplifiers are in fact very reliable unless the user overdrives the output stage – e.g. into very low impedance loads. Modern equipment however does seem to veer away from using good quality transformers as a means to price cutting.

Cutting corners…

The 7100 designers and manufacturers did not cut corners. Most of the faults advertised on the web cover cracked boards and faulty electrolytics. The output transistors are very, very overated for 60W use. That’s a good thing – like any audio amplifier the output is derived from the VI plot (SOA or safe operating area)) – this is the maximum rating of the power transistors used to drive or output current into a known load. This rating is based on power, maximum voltage and maximum current and the SOA ratings by the manufacturer will show the secondary breakdown. Stable amplifiers (like the 7100) can dissipate power into a load of different values without degrading performance. To get this, first and foremost the engineers must ensure that power output devices are always operated within their SOA (safe operating area). The 7100 uses NPN transistor 2SC3281 in it’s output stage. This is a 150W device, peak Ic is 15 to 20A and peak voltage at collector is 200V. Higher impedance loudspeaker loads require higher output voltages to get maximum power. The 7100 uses a 90V power supply – +/- 45V DC.  It is rated at 60W continuous. This transistor way exceeds design criteria. However, they do fail. When output transistors fail invariably so does a driver transistor. In fact you need to check the transistor out of circuit – short circuits don’t tell the whole story. It is always advisable to replace all the power transistors and the drivers when a faulty output stage is found. Once these have been replaced you will need to set the bias current – this must be done on switch on as well as after a few minutes. Bias current will increase after a few minutes, causing more heat. More heat causes lowering resistance in bipolar devices. This feedback creates what is known as thermal runaway and most amplifier manufacturers have thermal protection built into their amplifiers to improve stability. In fact, it’s the thermal characteristics of bipolar transistors which can cause their early demise as well – their current handling capability drops as the temperature rises. The designers of the 7100 have not skimped here. (as an aside – if you are using a split rail power supply use resistors across the fuse holders to limit the current for testing purposes, usually 1W 10 Ohms will suffice. Non symmetrical supplies only use one resistor of 22 Ohms. Never couple speakers onto the output until you have checked DC offset – Ed)