Widely available 2SA970 replacement

I am working on a NAD 7400 that has low output in the left channel. I believe have tracked down the issue to a 2SA970 that is serving as a current source.

Reading through other threads (most of them very old) there are mentions of other (now) hard to find replacements. Is there any jellybean type transistors that would easily sub for this?

Most critical spec is going to be the collector to emitter voltage, there is almost 60v across this in the working channel.
 
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It might help to see what it actually does on the circuit. If it is purely a current source then it is most likely not very critical and something like a 2N5401 or similar should be OK. Although it may have only 60 volts across it when operating normally it may see more than that under signal conditions or switch on and the 2SA970 is a 120 volt device.

Low output and suspecting a current source seems a bit of an unusual fault.
 
It is Q807 in the schematic.

It is right after this area that the signal doesn't match up with the right channel. I was suspecting that transistor because there is less than 1v drop across zener D802. I swapped the diode with the same result. There's no voltage dropping across R809 and R813. Both are getting their bias from that zener. For whatever reason those two transistors aren't switching on.

I'm actually having a beast of a time trying to trace the signal path after Q801/802. I can find the signal at the drivers (Q813)and output transistors (cut off the screenshot). Just seems like there aren't anything but current sinks/sources attached to the collectors/emitters of the differential amp. Looks to me that the right transistor in Q801 is a feedback path, correct?

So where does the signal itself go?





Screenshot_20230529_132302_Acrobat for Samsung.jpg
 
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The circuit is a fully balanced configuration and Q801 forms a differential input stage. Same for Q802.

The Zener. If the voltage is low then first suspect is R804, the 4.7k. Check it out of circuit.

So where does the signal itself go?
Its not quite that straightforward as the circuit has to be considered as a whole. Depending on the polarity of the signal, either the top half or lower half of the circuit comes into play. The gain of the circuit is set by the ratio of R812 and R811. Voltage gain is (6200/220)+1 which is 29.2 numerically.

Check that resistor first.
 
I checked R804 in circuit and it was reading about 4.6k (5%) but haven't looked at it out of circuit. There was continuity between the zener and resistor. It is all on one trace on the PCB.

What I was trying to determine was what are the next components in the signal chain beyond Q801/802.

I'm a bit more at home working on tube amps and radio receivers (tube or solid state), sometimes I get a little lost trying to follow the the signal through these amps when these current sources/sinks get involved. I was trying to work backwards from the output, but I don't fully understand how the signal is getting to the base of Q813 from Q801.

Thanks for your help so far.
 
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I don't fully understand how the signal is getting to the base of Q813 from Q801.

Q813 is the vbe multiplier used for setting the bias current. You can link Q812 collector to Q811 collector as far as thinking about signal flow goes. In other words add a link across C809.

Is the DC offset correct? You should see near zero volts on the audio output line which will be the right hand end of R812.

What I was trying to determine was what are the next components in the signal chain beyond Q801/802.

It is Q807 and Q808. The collector current of these drive the two main VAS (voltage amplifier stages) of Q811 and Q812. Q809 and Q810 are current sources for the VAS stages. The output stage (which isn't in your diagram) will likely be an emitter follower stage that connects to Q811 and Q812 collectors.

You have to get the DC conditions correct. Forget AC gain and low signal levels. You'll never fix it that way. You have a major error with that Zener voltage.
 
Q813 is the vbe multiplier used for setting the bias current. You can link Q812 collector to Q811 collector as far as thinking about signal flow goes. In other words add a link across C809.

Is the DC offset correct? You should see near zero volts on the audio output line which will be the right hand end of R812.



It is Q807 and Q808. The collector current of these drive the two main VAS (voltage amplifier stages) of Q811 and Q812. Q809 and Q810 are current sources for the VAS stages. The output stage (which isn't in your diagram) will likely be an emitter follower stage that connects to Q811 and Q812 collectors.

You have to get the DC conditions correct. Forget AC gain and low signal levels. You'll never fix it that way. You have a major error with that Zener voltage.

Ok, there's about 50mV on the right side of R812. On the right channel it is less than 10mV.

There are a few spots on the PCB that looks like they got a little toasty at some point without any obvious component damage. What I did find is that I measured resistance across the Zener in circuit, I get about 800 ohms. Not good. Compared to the right channel which is measuring several megaohms. I had pulled the Zener earlier and tested it out of circuit so I know it is good.

So apparently the PCB has gone conductive. Guess now I have to see if it is even salvageable by bypassing the traces there.
 
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So apparently the PCB has gone conductive. Guess now I have to see if it is even salvageable by bypassing the traces there.

That sounds a bit extreme :) Boards often discolour with heat due to a chemical reaction within the PCB material. When its really bad (really really bad) the board can carbonise and yes, it can become conductive but that is more usually seen in high voltage high temperature gear. 800 ohms would be very very low for a board to be the cause.

Do you still read this 800 ohms with Q803 removed? That is the killer question.

When comparing against the other channel be sure to use identical meter ranges and meter lead polarity.
 
OK. Finally got some time to look at this some more.
Turns out that the Zener was in fact bad. I didn't have a 5.6V on hand and swapped in a 6v instead. Now all of the DC operating voltages are (mostly) correct. I'll have to order the proper 5.6v Zener before I put everything back together but at least I corrected that fault.

Still have low gain on the left channel.

I look at the base of Q811/812 (and the complementary ones on the right channel) and I don't see the signal from the prior stage, but just some 120Hz ripple.
Right channel still works fine so I'm still not quite following how the signal path works on this. It just seems to magically show up at the collectors of said transistors, which is throwing me for a loop.

In any case, the loss of gain is somewhere between Q801/802 and Q811/812.
 
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I look at the base of Q811/812 (and the complementary ones on the right channel) and I don't see the signal from the prior stage, but just some 120Hz ripple.
Right channel still works fine so I'm still not quite following how the signal path works on this. It just seems to magically show up at the collectors of said transistors, which is throwing me for a loop.

That is correct. Those transistors (Q811/812) provide massive voltage gain.

If you apply for example 100mv signal to the input of the amp the output of the amp would be around 3 volts (gain of 30 set by the feedback network of R811 and R812). The signal swing at the base of Q811/812 would be very small indeed, perhaps as low as a few tens of millivolts. All normal.

This is a similar configuration. First image shows input and output voltages. Gain set by R11 and R12 here (same values as you have).

Second image shows the signal at the base of Q8 in my diagram. Look at the amplitude. Any supply ripple would show here as well.

Gain 1.jpg


Gain 2.jpg