I was working on a NAD 3140, general recap as there was lots of leaking. I had everything working, and was using a DBT to power up. I brought the bias up to around 11-12mv (14 is recommended).
I believe the mistake I made was getting ready to set the final bias and offset I plugged it into the regular socket, I let it warm up and after about 15 minutes heard a pop/fizz and one channel went down. I checked the bias quickly and it was very high.
Nothing obvious blew that I can see, and the voltages on the output stage are lower on both channels. Any thoughts on what might have been damaged?
Attaching voltages:
I believe the mistake I made was getting ready to set the final bias and offset I plugged it into the regular socket, I let it warm up and after about 15 minutes heard a pop/fizz and one channel went down. I checked the bias quickly and it was very high.
Nothing obvious blew that I can see, and the voltages on the output stage are lower on both channels. Any thoughts on what might have been damaged?
Attaching voltages:
Last edited:
All the electronic failures relating to components that go--"pop-fizz" are usually related to capacitors .
Did you replace the dodgey old open frame bias trimmers ?
You should've done that before attempting to reset the Bias.
Edit.
As you said you Raised the bias while it was powered through a dim bulb.
I'm not sure increasing the bias was the best idea.
Might the lower voltages through the circuit also affect the bias ?
You should've done that before attempting to reset the Bias.
Edit.
As you said you Raised the bias while it was powered through a dim bulb.
I'm not sure increasing the bias was the best idea.
Might the lower voltages through the circuit also affect the bias ?
Last edited:
I was working on a NAD 3140 a few years ago. A bias problem on the left channel smoked metal oxide resistor R629 ( or R630 right channel). Check to see if they are open.
Not clear exactly what you have done. A big no-no is setting the bias (anywhere near its recommended value) while on DBT. OK to check that the bias responds to trimmer adj while on DBT but then reset bias to near zero, connect to mains then set bias.
The "bad voltages" appear to be on DBT, mention if not the case. Voltages should be to 1 decimal place so can estimate Vbe, Vce etc... Q607c is plainly wrong. Check suggestions from UserAbuser and omega man
That was 100% my mistake, didn't occur to me and thought I was leaving myself enough room.
629 and 630 both check out, 630 was a bit out of spec at 94 ohms and had a cracked exterior. Will replace to be safe.
I believe the before and after were both on the DBT but I can recheck and get you better numbers. I was surprised the voltages dropped on both channels.
I am by no means an expert. My limited experience has been that the DBT will warn you if you are adjusting the bias too high. If it seemed OK through the DBT, would it be that much different through the mains? Perhaps the bias wasn't the problem at all but rather a consequence? If all of the resistors in the area are OK, then I think I would be looking for a problem in the regulated power supply. The low voltage problem is in both channels. All of those transistors are fed by the regulated supply.
I agree with what was said about replacing those trim pots. I had one bias trim pot on a 3140 that did nothing at all. I also had dc offset problem of 120mV that i couldn't track down on a NAD 2140. Replaced the dc offset pot and I was able to zero it with no problem.
I agree with what was said about replacing those trim pots. I had one bias trim pot on a 3140 that did nothing at all. I also had dc offset problem of 120mV that i couldn't track down on a NAD 2140. Replaced the dc offset pot and I was able to zero it with no problem.