Hi,
I need some experienced advise.
I'm restoring a NAD 2140 power amplifier and I've run into a problem that I don't understand, let alone find a fix.
The unit was blowing fuses and burning resistors with flames. Several transistors were out.
It is an early unit that had the extra stage before the output transistors, and has R639 1,2k and R641 180R.
I applied SB035 removing the -1 output stage.
I started by getting the regulator voltages in balance. Voltages were out of balance +45V and -41V.
This was done by tuning R813 to 24k (old version of amp has 22k, recent has 27kohm) and has now a clean +45V and -45.V, +25.9V and -25.7V.
The output transistors are fed with +51.1V and -51.1V, so within reasonable expectations.
I replaced all transistors, elco capacitors and resistances with quality items (Nichicon, Panasonic, Kemet, Vishay, ON, ...)
On powering up on DBT, all is OK, my 40W lamp goes to a very soft glow after the inrush current. Next was on 230VAC, no more fuses blowing, all resistors cool except R633 and R643, but within limits (I think).
The funny thing I don't understand is that all voltages towards the drivers are about 2V below what they should be. See attached scan with expected voltages in blue, actual voltages in red.
I have this issue on both channels, so clearly I made a mistake but after checking and double checking all R's, Q's and C's are according to the specs (except R639 1k2, R641 180R, old unit values).
Consequence is that I can't do anything that changes the idle current (VR603, I have 0 mV over the output emitter resistors), while I can change the output offset voltage to 0V (VR601).
Did anyone face this issue and how did you fix this?
A big thank you for your reply!
Chris
I need some experienced advise.
I'm restoring a NAD 2140 power amplifier and I've run into a problem that I don't understand, let alone find a fix.
The unit was blowing fuses and burning resistors with flames. Several transistors were out.
It is an early unit that had the extra stage before the output transistors, and has R639 1,2k and R641 180R.
I applied SB035 removing the -1 output stage.
I started by getting the regulator voltages in balance. Voltages were out of balance +45V and -41V.
This was done by tuning R813 to 24k (old version of amp has 22k, recent has 27kohm) and has now a clean +45V and -45.V, +25.9V and -25.7V.
The output transistors are fed with +51.1V and -51.1V, so within reasonable expectations.
I replaced all transistors, elco capacitors and resistances with quality items (Nichicon, Panasonic, Kemet, Vishay, ON, ...)
On powering up on DBT, all is OK, my 40W lamp goes to a very soft glow after the inrush current. Next was on 230VAC, no more fuses blowing, all resistors cool except R633 and R643, but within limits (I think).
The funny thing I don't understand is that all voltages towards the drivers are about 2V below what they should be. See attached scan with expected voltages in blue, actual voltages in red.
I have this issue on both channels, so clearly I made a mistake but after checking and double checking all R's, Q's and C's are according to the specs (except R639 1k2, R641 180R, old unit values).
Consequence is that I can't do anything that changes the idle current (VR603, I have 0 mV over the output emitter resistors), while I can change the output offset voltage to 0V (VR601).
Did anyone face this issue and how did you fix this?
A big thank you for your reply!
Chris
@BSST You were spot on with your advice👍. I indeed messed up the insertion of these four drivers (other way around).
After correction and check of all other components I was able to align the DC offset and idle current of the unit without any issues.
In an 8 Ohm resistive load, the unit pumps out a clean 1 kHz at 30V on the on the speaker output before the transformers start to make some noise.
More testing to follow and then finally some listening!
So 2 important troubleshooting lessons I learned here: Pay attention to what you put on the PCB and double check insertion. And the second one for sure helping me in my next troubleshooting: Vbe should be around 0.6V (in most cases I presume).
A very big thank you for helping me out with this problem and the great lessons learned! 🙂
After correction and check of all other components I was able to align the DC offset and idle current of the unit without any issues.
In an 8 Ohm resistive load, the unit pumps out a clean 1 kHz at 30V on the on the speaker output before the transformers start to make some noise.
More testing to follow and then finally some listening!
So 2 important troubleshooting lessons I learned here: Pay attention to what you put on the PCB and double check insertion. And the second one for sure helping me in my next troubleshooting: Vbe should be around 0.6V (in most cases I presume).
A very big thank you for helping me out with this problem and the great lessons learned! 🙂