Hi,
Wanting some help with a repair of this old NAD3020 amplifier that has blown the left channel.
I have hopefully attached the relevant schematic. Soft clipping is off and Pre-out / Nor in connected.
When I got it the right channel played fine, but the left had no output and a -10V DC offset (from memory). The MJ2955 output transistor Q617 was blown, I had one spare, so put it in. Still no dice. Noticed R651 between the emitters of the driver transistors Q611 and Q613 was black but still measured 180ohms, but I replaced. Also R629 1W was slightly discoloured and it's neighbour R631 was black. R647 was also slighly burned. The PCB was blackened under R629 and R631. I replaced R631 and R647, but R629 was still reading good and was 1W. I guess it gives a clue as to what part of the circuit where the current is flowing more than it should. The other side looked fine. Someone had replaced R642 on the right side, but no signs of thermal stress.
I then went back and looked at the transistors in the signal path. I pulled them and used a component tester. Q601 was faulty, but but Q603 and Q605 were fine. I replaced Q601 with an ebay one, but still nothing, and the R651 I had replaced was getting hot again. I pulled the drivers Q611 and Q613, and also the biasing transistor Q609, which were also faulty with the component tester. I checked Q605 and Q607, which tested fine. I had ordered some BD237 and BD238 at the same time from ebay, so put them in. I had checked most of the resistors in circuit, and the voltages up until Q601 to Q603 were fine, but thereafter they were way off. Unfortunately, as I was testing and then not one time I forgot to put the DBT in circuit, and I blew everything beyond Q607 again. I attributed the fact it wasn't working to perhaps less than satisfactory parts from ebay, but they were cheap and quick (from a UK seller). I have since ordered replacement transistors, and have everything except the 2n3055 and MJ2955s.
I think there are still problems with the circuit, and I took advantage of the fact I had isolated a lot of the circuit by not having the transistors in. I had also noticed problems with the +ve rail voltage.
The first thing is I don't get the diode drop from D603, except the the cathode end of the diode - that screws up the voltages for the emitters for Q603 and Q605, and the collector for Q607. So I do not get the 0.5V drop across R627 (390ohm) or R639 (68ohm).
The Q603 collector -ve voltage is a mistake I assume.
The voltages on the schematic are otherwise ok up until Q605 and Q607 (Darlington pair). Q601 BEC - 1Vbase, 0.5Vemitter, 31Vcollector Q603 all read around 31V, but varies a bit by 0.3V. Not seeing a convincing diode drop Vbe...
Q605 B : 31.03V, E : 31.2V C : -26.12V
Q607 B : -26.12V E : -26.12V C : 31.23V
I'm not entirely sure where the voltage path is to the -26.12V (marked as -25.4V on the schematic). All 5 of the transistors after Q607are out. It can only be via R643, R645, R647, then R649 - I could put back Q611 and Q613 with Q609, but it would be good to understand how putting them in would decrease the current flow to the negative rail.
I replaced C625, which made no difference. The only thing that helped was lifting one leg of R647. Think there is only 3 resistors that join Q605 and Q607 to the -26V negative rail. I have replaced Q605 and Q607 with ones from Farnell. There was a thread where despite the Q607 seemingly being ok on a tester, there was some 'leak', which resulted in the similar negative voltages I am getting.
I'm trying to understand the circuit and the principles behind it. For instance the lack of 0.5 voltage drop from +30.7V supply rail to +30.1V on the Darlington pair ? Is the problem that I have not put all rest of the transistors in, but I can't see how they would feedback. Unless it is that 0V connection between the emitters of the output transistors ?! And I'm now seeing that I slightly misread the schematic, thinking C625 provides a path from Q605 collector and Q607 emitter to the minus rail.
What am I missing ?
Thanks for any help or suggestions.
Simon
Wanting some help with a repair of this old NAD3020 amplifier that has blown the left channel.
I have hopefully attached the relevant schematic. Soft clipping is off and Pre-out / Nor in connected.
When I got it the right channel played fine, but the left had no output and a -10V DC offset (from memory). The MJ2955 output transistor Q617 was blown, I had one spare, so put it in. Still no dice. Noticed R651 between the emitters of the driver transistors Q611 and Q613 was black but still measured 180ohms, but I replaced. Also R629 1W was slightly discoloured and it's neighbour R631 was black. R647 was also slighly burned. The PCB was blackened under R629 and R631. I replaced R631 and R647, but R629 was still reading good and was 1W. I guess it gives a clue as to what part of the circuit where the current is flowing more than it should. The other side looked fine. Someone had replaced R642 on the right side, but no signs of thermal stress.
I then went back and looked at the transistors in the signal path. I pulled them and used a component tester. Q601 was faulty, but but Q603 and Q605 were fine. I replaced Q601 with an ebay one, but still nothing, and the R651 I had replaced was getting hot again. I pulled the drivers Q611 and Q613, and also the biasing transistor Q609, which were also faulty with the component tester. I checked Q605 and Q607, which tested fine. I had ordered some BD237 and BD238 at the same time from ebay, so put them in. I had checked most of the resistors in circuit, and the voltages up until Q601 to Q603 were fine, but thereafter they were way off. Unfortunately, as I was testing and then not one time I forgot to put the DBT in circuit, and I blew everything beyond Q607 again. I attributed the fact it wasn't working to perhaps less than satisfactory parts from ebay, but they were cheap and quick (from a UK seller). I have since ordered replacement transistors, and have everything except the 2n3055 and MJ2955s.
I think there are still problems with the circuit, and I took advantage of the fact I had isolated a lot of the circuit by not having the transistors in. I had also noticed problems with the +ve rail voltage.
The first thing is I don't get the diode drop from D603, except the the cathode end of the diode - that screws up the voltages for the emitters for Q603 and Q605, and the collector for Q607. So I do not get the 0.5V drop across R627 (390ohm) or R639 (68ohm).
The Q603 collector -ve voltage is a mistake I assume.
The voltages on the schematic are otherwise ok up until Q605 and Q607 (Darlington pair). Q601 BEC - 1Vbase, 0.5Vemitter, 31Vcollector Q603 all read around 31V, but varies a bit by 0.3V. Not seeing a convincing diode drop Vbe...
Q605 B : 31.03V, E : 31.2V C : -26.12V
Q607 B : -26.12V E : -26.12V C : 31.23V
I'm not entirely sure where the voltage path is to the -26.12V (marked as -25.4V on the schematic). All 5 of the transistors after Q607are out. It can only be via R643, R645, R647, then R649 - I could put back Q611 and Q613 with Q609, but it would be good to understand how putting them in would decrease the current flow to the negative rail.
I replaced C625, which made no difference. The only thing that helped was lifting one leg of R647. Think there is only 3 resistors that join Q605 and Q607 to the -26V negative rail. I have replaced Q605 and Q607 with ones from Farnell. There was a thread where despite the Q607 seemingly being ok on a tester, there was some 'leak', which resulted in the similar negative voltages I am getting.
I'm trying to understand the circuit and the principles behind it. For instance the lack of 0.5 voltage drop from +30.7V supply rail to +30.1V on the Darlington pair ? Is the problem that I have not put all rest of the transistors in, but I can't see how they would feedback. Unless it is that 0V connection between the emitters of the output transistors ?! And I'm now seeing that I slightly misread the schematic, thinking C625 provides a path from Q605 collector and Q607 emitter to the minus rail.
What am I missing ?
Thanks for any help or suggestions.
Simon
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