I am working on this for a friend, the left channel was out. I went through the amp and replaced the outputs/drivers and a couple of resistors, on power up the right channel was pulling too much current. I went through the amp again, the soft clipping and power supply. In the power supply one diode was bad D510, which I replaced.
I powered up with a DBT and everything looked good, I made sure the idle current was low enough so it wouldn't blow without the DBT.
I powered up without the DBT, voltages looked good, Per the service manual I adjusted the DC offset to 4 or 5mv, adjusted the idle current to 27-28mv (spec is 26-30mv). I replaced the solder bridge and powered it up. A sine wave looked OK, square wave showed some ringing (8ohm speaker load).
I hooked up a CD player and let it play at low volume for about 10 minutes and all was well. I slowly turned the volume up and at about 9:30-10 oclock on the potentiometer the left channel blew the outputs, some drivers and R454.
Obviously I am missing something, any ideas?
This is the amp/power supply schematic
This is a closer view with the blown components marked
I powered up with a DBT and everything looked good, I made sure the idle current was low enough so it wouldn't blow without the DBT.
I powered up without the DBT, voltages looked good, Per the service manual I adjusted the DC offset to 4 or 5mv, adjusted the idle current to 27-28mv (spec is 26-30mv). I replaced the solder bridge and powered it up. A sine wave looked OK, square wave showed some ringing (8ohm speaker load).
I hooked up a CD player and let it play at low volume for about 10 minutes and all was well. I slowly turned the volume up and at about 9:30-10 oclock on the potentiometer the left channel blew the outputs, some drivers and R454.
Obviously I am missing something, any ideas?
This is the amp/power supply schematic
This is a closer view with the blown components marked
Weird, its only one channel. Maybe try some changes in the powersupply to avoid the voltage/load selection
So new parts should be ok.
It blew at higher output, that could indicate a problem in the rail switching part.
It blew at higher output, that could indicate a problem in the rail switching part.
I did notice that the transformer makes more noise than most I have encountered. I wonder if that could be the issue?
2N3055 is a 60V BJT, it's on +/- 40V rails?
Were the originals some H variant?
Obviously I don't understand the PE circuit, I'm guessing it adjusts the rails somehow, so the transistors don't actually see so many volts?
It's a lot of nominal rail voltage for 20Wpc?, even allowing for the concept of coping with short peaks above 20Wrms.
Here in England, I've noticed that the mains can vary more than you'd hope, I've seen bigger than nominal voltages on amplifier rails.
Maybe the transformer noise is related to harmonics on the mains? Maybe the transformer is starting to fail somehow?
Were the originals some H variant?
Obviously I don't understand the PE circuit, I'm guessing it adjusts the rails somehow, so the transistors don't actually see so many volts?
It's a lot of nominal rail voltage for 20Wpc?, even allowing for the concept of coping with short peaks above 20Wrms.
Here in England, I've noticed that the mains can vary more than you'd hope, I've seen bigger than nominal voltages on amplifier rails.
Maybe the transformer noise is related to harmonics on the mains? Maybe the transformer is starting to fail somehow?
Did you replace the mica washers & heat sink compound? Those can develop cracks after 30 years. Silicon rubber requires no heat sink compound.
Try with an 8 ohm resistor load. Like ohmmite or dale 225 w wirewound logs or something. Could be your speaker is scraping the coils.
No reason to buy 2n2955 2n3055 these days. All On TO3 parts cost about the same. Use MJ15024/25 or MJ21193/94 .
Try with an 8 ohm resistor load. Like ohmmite or dale 225 w wirewound logs or something. Could be your speaker is scraping the coils.
No reason to buy 2n2955 2n3055 these days. All On TO3 parts cost about the same. Use MJ15024/25 or MJ21193/94 .
Original 1970 2n3055 had bad gain at higher currents like 3 amps. Like 5 or less. Designers specified higher rails then to cope, also heat sinks on drivers.
An additional possibility of shorts is thermal expansion causing a screw holding down the transistor, to contact the heatsink. If the PCB does not positively locate the screws in the center of the holes in the heatsink, the mica kits from newark have plastic ferrules that center the screws in the larger holes of 1970's TO3 transistors.
How unusual is the lack of emitter resistors?
Does this make the design prone to variations in transistor characteristics as manufacturing processes have evolved and the old parts have aged?
Does this make the design prone to variations in transistor characteristics as manufacturing processes have evolved and the old parts have aged?
An option is to remove the higher rail voltage function by lifting the leg of these diodes. I think the lower rail voltages should be around 30v.
Original RCA 2n3055 were homotaxial and had some built in emitter resistance. Similar parts are available now only from surplus houses, or outright liars.
MJ2955 was always epitaxial, so I really do not know how NAD got away with no emitter resistors. Perhaps their 20 w rating included a lot of cheap transformer resistance.
Certainly with higher gain modern output transistors, lowering the rail voltage per post # 11 is not a bad idea. I do not know how good or bad the heat sinks on this model are. O.P. bamavette has discovered some heat soak vulnerability. The mechanical problem possibilities outlined in post # 8 #9 should certainly be eliminated.
It's not uncommon to add 0.22 ohm emitter resistors when replacing the outputs on these with modern transistors that have lower built in Re. It greatly improves thermal stability, perhaps at the cost of slightly higher distortion but my guess is you'll never notice.
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The left channel is likely experiencing instability when driven harder, possibly due to a thermal or bias issue. Check the bias current and thermal stability closely, as overheating components could lead to failure. Also, verify the driver transistors for proper function and ensure they're not damaged. The ringing on the square wave suggests potential oscillations, which could be a sign of instability in the feedback loop or inadequate compensation. Finally, inspect the power supply once more to ensure it can handle the load without sagging under higher volumes.