I'm looking at a very first generation 3020 Series 20 with the 4 main caps near the headphone socket (not in the centre of the board) and having great difficulty trying to ascertain what's wrong with it at the point of trying to set the bias.
I cannot get any reading measuring across 0.22 ohm resistors installed at R653 and R654.
Measuring in the collector feeds with a 1 ohm resistor, I cannot get past around 12mV bias without the power transistors starting to overheat and the bias increasing on its own. I found this out the hard way as one channel blew up completely the first time around, but the fault is still occurring on both channels with all new outputs installed.
The unit is otherwise powered on perfectly fine with with both channels reading very minimal DC offset.
The power transistors were tested before installation and should be working perfectly.
The driver stage has been replaced with new ONSemi BD237G/ 238G.
New BD139s have been installed in the power amp section (Q609/ 610 and Q607/ 608)
New KSC1845F have been installed in place of 2SC1400 (Q601/ 602)
The RX resistors have been replaced with Bourns trimmers with 1.2k ohm fixed resistors in parallel. Capacitors C627/ 628 were replaced with Kemet R82 series film capacitors due to the fact the originals were too large to reinstall after the trimmer modification.
All electrolytic capacitors have been replaced (verified correct with untouched NAD 3020 side by side)
I've checked the orientation of the transistors multiple times and checked all resistors and diodes in circuit.
Oddly, I have about 33v on the collectors of the power transistors and not 28.
As far as I can see, there is either something shorting both channels, I've made the same mistake in each channel or it's something to do with the power supply, which I find incredibly unlikely given I have good DCV values on the power supply daughter board.
I have good experience in vintage audio repair and have done one of these in the past. I am at a complete loss at this point as to what to do next. Can anybody advise some useful checks?
I cannot get any reading measuring across 0.22 ohm resistors installed at R653 and R654.
Measuring in the collector feeds with a 1 ohm resistor, I cannot get past around 12mV bias without the power transistors starting to overheat and the bias increasing on its own. I found this out the hard way as one channel blew up completely the first time around, but the fault is still occurring on both channels with all new outputs installed.
The unit is otherwise powered on perfectly fine with with both channels reading very minimal DC offset.
The power transistors were tested before installation and should be working perfectly.
The driver stage has been replaced with new ONSemi BD237G/ 238G.
New BD139s have been installed in the power amp section (Q609/ 610 and Q607/ 608)
New KSC1845F have been installed in place of 2SC1400 (Q601/ 602)
The RX resistors have been replaced with Bourns trimmers with 1.2k ohm fixed resistors in parallel. Capacitors C627/ 628 were replaced with Kemet R82 series film capacitors due to the fact the originals were too large to reinstall after the trimmer modification.
All electrolytic capacitors have been replaced (verified correct with untouched NAD 3020 side by side)
I've checked the orientation of the transistors multiple times and checked all resistors and diodes in circuit.
Oddly, I have about 33v on the collectors of the power transistors and not 28.
As far as I can see, there is either something shorting both channels, I've made the same mistake in each channel or it's something to do with the power supply, which I find incredibly unlikely given I have good DCV values on the power supply daughter board.
I have good experience in vintage audio repair and have done one of these in the past. I am at a complete loss at this point as to what to do next. Can anybody advise some useful checks?
You're not going to get ANY ohm reading across the R653/54, only a dead short due to the jumper wire across them, which is only clipped out for bias trimming.
Since you've replaced numerous transistors already, and added in parts, it's now impossible to diagnose over the internet.
It would have to be on my (or another technician's) bench.
Since you've replaced numerous transistors already, and added in parts, it's now impossible to diagnose over the internet.
It would have to be on my (or another technician's) bench.
To determine where the problem is, disconnect both audio channels from the power supply.
Test the power supply. If it is bad, fix it.
If the power supply is ok, then connect only one channel to it. If it is bad, fix that channel.
If it is ok, disconnect it and connect the other channel instead. If that channel is bad, fix it.
Test the power supply. If it is bad, fix it.
If the power supply is ok, then connect only one channel to it. If it is bad, fix that channel.
If it is ok, disconnect it and connect the other channel instead. If that channel is bad, fix it.
The manual specifies to measure DC mV across R653/ 654, set between 5 and 11mV. I am not trying to measure resistance.
There is another method measuring across resistors in the collector feeds between either the NPN or PNP outputs and drivers, where I am actually getting a DC mV reading with the bias going out of control way before the ideal value of 30mV is reached, given the 1R resistor.
The jumper was removed prior to installing the resistors and will be put back after removing the resistors as they are not required in normal operation, only for setting the bias.
I appreciate your perspective given many parts have been changed, but these are all brand new fully working parts and known good substitutes. I have rebuilt another 3020 and not had this problem. However, I did not have to modify the bias circuit in that unit.
Hopefully somebody who's diagnosed a similar issue or knows these units well enough can chime in with some suggestions that will lead me down the right path.