The sawtooth is just the supply ripple making its way into the amp. That's nothing to do with the replacement transistors, just the overall current drawn and the PSRR (power supply rejection ratio) of the amp.
This was exactly the kind of problems we saw in the other thread. The modern devices differ slightly in fundamental characteristics such as Vbe vs Ic which would require changing other component values to work properly.
This was exactly the kind of problems we saw in the other thread. The modern devices differ slightly in fundamental characteristics such as Vbe vs Ic which would require changing other component values to work properly.
Do you hear a "hummmmmm" in that channel? With that much ripple at the output, you ought to be hearing it. Somehow, you've got power supply ripple getting into the *input* to that channel. If the driver transistor were causing any trouble, it would be a high frequency oscillation, at say 100 KHz or more.
Yes, I do hear a hum in that channel.
Maybe the transistors I used worked after all. I could adjust the bias, it stayed somewhat stable, but I was having the ripple on the output transistor emitter. Maybe there's something else bad on the board and the driver transistors were working after all...
Also I need to replace the trimmers with smaller value (or larger?) as the bias was rising pretty fast with small turns. I will measure and see if I need them smaller or higher value.
Maybe the transistors I used worked after all. I could adjust the bias, it stayed somewhat stable, but I was having the ripple on the output transistor emitter. Maybe there's something else bad on the board and the driver transistors were working after all...
Also I need to replace the trimmers with smaller value (or larger?) as the bias was rising pretty fast with small turns. I will measure and see if I need them smaller or higher value.
Good news is that I replaced the driver transistors on the other board and there I don't see the ripple.
Bad news is that I tested all capacitors with esr meter in circuit and they all mirror in capacitance/esr to the ones from the other channel, and are up to spec. I also tested all the resistors when I took the off the board after first blow. All in spec. Also tested diodes/transistors with diode test on dmm. doesn't make any sense. I will keep removing parts from the board until I don't see the ripple.
Bad news is that I tested all capacitors with esr meter in circuit and they all mirror in capacitance/esr to the ones from the other channel, and are up to spec. I also tested all the resistors when I took the off the board after first blow. All in spec. Also tested diodes/transistors with diode test on dmm. doesn't make any sense. I will keep removing parts from the board until I don't see the ripple.
Progress!
I replaced all silicon on that channel. Diodes included.
Now it seems stable. I used 2sd669ac/2sb649ac made by Hitachi.
I can set the bias, and it stays at 20mA with no problems on both channels. Also I don't have the ripple anymore. Don't know what caused that.
I used 220pF for the driver and I will install the board into the case so I can make some tests with signal input.
I replaced all silicon on that channel. Diodes included.
Now it seems stable. I used 2sd669ac/2sb649ac made by Hitachi.
I can set the bias, and it stays at 20mA with no problems on both channels. Also I don't have the ripple anymore. Don't know what caused that.
I used 220pF for the driver and I will install the board into the case so I can make some tests with signal input.
It's working great. Bias is stable, and I'm listening to it right now. On headphones at the moment.
I will test for oscillation with my Tek 465, I don't trust my DSO.
I used 100pF at first with 2sd669. nigelwright7557 suggested I start low and go up if it oscillates. 2sd669 is 140Mhz vs 2sc1384 at 200Mhz.
Would this capacitor limit my bandwidth if it is too large? (on 2sc1384 there was a 500pF one). Also the complementary transistor doesn't have any capacitor (stock didn't have as well).
Seems that 2sd669 is a good replacement for 2sc1384 in this amp. Will make measurements with load as well.
Headphones sound crystal clear!
I will test for oscillation with my Tek 465, I don't trust my DSO.
I used 100pF at first with 2sd669. nigelwright7557 suggested I start low and go up if it oscillates. 2sd669 is 140Mhz vs 2sc1384 at 200Mhz.
Would this capacitor limit my bandwidth if it is too large? (on 2sc1384 there was a 500pF one). Also the complementary transistor doesn't have any capacitor (stock didn't have as well).
Seems that 2sd669 is a good replacement for 2sc1384 in this amp. Will make measurements with load as well.
Headphones sound crystal clear!
Yes, that one. So then I should start with no cap and check for oscillation? Does the ft of the transistor have to do with oscillation? Because the original one had 500pF for ft of 200MHz. I installed 140MHz one so I presume lower should do, but I understand that ideal would be no cap. Shall I try with no cap?
I'd be surprised if you needed more than 100 pF. It's actually with slower, high capacitance transistors where you need these caps, and the "slower" the more you need. The C-B cap changes radically with voltage, and it's the CHANGE in the cap value you need to swamp out. A changing capacitance causes a negative resistance to appear (math is complicated, but thet's the gist of it).
This is why you look for low Cob drivers.
This is why you look for low Cob drivers.
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