NR-719 Stereo Receiver, Loss of Bias

[Mr_bibbles]

Hey Y'all,

Got kind of an interesting one (for me at least). Got a Nikko NR-719 (https://www.hifiengine.com/manual_li...o/nr-719.shtml) that passes signal through both channels, and sounds pretty good until you start applying more signal to the left channel, then it starts to distort. Right channel sounds good at all volumes, bias measures good, BUT

*The left channel shows 0mv Bias regaurdless of the trimpot.*

Trimpot measures good, I have the - and + rails, but something is getting rid of the bias voltage and I don't really understand how this transistor fed bias circuit works. I traced the voltage through a little bit, found that at the emitter of q705 I have about .5V as opposed to the 1.2V listed in the schematic. Can anyone give me a quick synopsis of the bias circuit, or perhaps a link to an article about it?

Thanks boys and ghouls

[Enzo]

Is there a postable schematic? I don't register for web sites.

[g1]

schematic attached

[Enzo]

Watch polarities. Do you mean -0.5v? instead of -1.2v?

The bias is the voltage across Q705 - in other words base to base on those pre-drivers. Schematic says 2.4v. You got that? If it is zerov or close, then Q705 maybe shorted.

[Mr_bibbles]

Hey Enzo! Of course here you go:

The service manual won't upload but if you need more information I can get it from there. Thanks!

[Mr_bibbles]

Watch polarities. Do you mean -0.5v? instead of -1.2v?

The bias is the voltage across Q705 - in other words base to base on those pre-drivers. Schematic says 2.4v. You got that? If it is zero or close, then Q705 maybe shorted.

Yup! Apologies I'll be more careful.

I actually have already replaced q705 (2sc945l) with a new substitute 2n3904, as I figured the issue was around there.

From base q705 to base q707, I read -34v.... Is this what you meant by "base to base?"

Some more measurements:

Q705:
B -.64 V
E .0ish V
C -.65

Q707
B -34.4
E -35V
C -.65V

[Enzo]

Do you realize that 2SC945 and 2N3904 have their legs in different order 2SC types on a TO92 are left to right looking at the flat side: E-C-B. 2N3904 and other 2N types are: E-B-C.

You can replace one style with the other, but you have to turn the transistor sideways so as to get the legs in the proper holes.

Q705 and Q707 are not the predrivers. Q705 is the bias transistor, and Q707 is the VAS stage (VOltage aamp)

Q709, 711 are the predrivers. Note the bases are at either end of Q705, and at 1.2v each they add up to 2.4v across Q705. Or should.

MAke sure that replacement part is installed the right way.

[Mr_bibbles]

Ahh jeeze, rookie mistake! This explanation has been incredibly helpful though, I now understand the basics here- Two driver transistors balanced oppositely tied to a bias transistor?

I do indeed have 1.2 and -1.2 at each driver, adding up to -2.4 from base to base. This seems to imply both drivers are intact, and that q705 is not outright shorted! I still have 0mv at the bias TP, which should read between 4mvdc - 14mvdc....

*BUT*

I can read the voltage changing through the base of q705 as the trimpot moves, which implies that everything up to that point is good? It is around .6VDC, very close to the other side.



Is it possible that One of the two finals has an issue? I will go through and measure everything post-bias transistor. I did notice that the Left channel final's voltage frops from .6V to about .3V when the volume is turned up with signal applied. Conversely the other side goes up from .5v to .6v under the same conditions.

[g1]

You see the Q705 base voltage changing via the trim pot, but how about the C to E voltage?

[Mr_bibbles]

You see the Q705 base voltage changing via the trim pot, but how about the C to E voltage?

Apologies for the delay, out of the shop for the weekend.

Looks like it does indeed change the voltage, at minimum trimpot setting I'm getting -2.57vdc, a slight turn changes it to -2.3vdc. I didn't risk turning it more than a 1/8th of a rotation for fear of overbiasing, but it does appear to change.

[g1]

The C to E voltage should correspond to the voltages at Q709 and Q711.
The 2.4V difference is approx. where they should be turning on. Less that 2.4V should be turning them toward 'off' or colder bias.
So I think you are interpreting the direction of the trim pot wrong. You are unable to get the bias hotter.
Or it's possible you are not getting an accurate reading of the mV across the emitter resistors R735 and R737. DC millivolts, one probe at each end of resistor.

[Mr_bibbles]

The C to E voltage should correspond to the voltages at Q709 and Q711.
The 2.4V difference is approx. where they should be turning on. Less that 2.4V should be turning them toward 'off' or colder bias.
So I think you are interpreting the direction of the trim pot wrong. You are unable to get the bias hotter.
Or it's possible you are not getting an accurate reading of the mV across the emitter resistors R735 and R737. DC millivolts, one probe at each end of resistor.

Thanks G1-

I think you may have got to the heart of it. The r737 shows 0v across it, and the other r735 has .5v across it. Does this mean that the 2sb616 has failed? r737 is not OL, measures OK in circuit.

Trying to learn from what y'all are saying....

So the pre - drivers are fed through their bases by the biasing transistor, and they in turn feed the base of the finals through their emitters. The finals then work together to sum the negative and positive rails from their emitters THROUGH emitter resistors and then to the output?

[g1]

Not completely sure how you mean the finals 'sum', so I'll say this. One turns on or the other. When the top turns on, the output swings positive, as it allows current from the positive rail via the transistor and it's emitter resistor. When signal swings negative, the top turns off and the bottom final conducts, causing the output to swing negative. The finals are just current amplifiers.

I'm not sure if you have a defective output transistor or what. .5V across .47 ohms is 1 amp, so Q713 is idling pretty hot. Is there roughly .6V and -.6V at bases of Q709 and Q711? Are those 220R emitter resistors both good?

[Mr_bibbles]

Not completely sure how you mean the finals 'sum', so I'll say this. One turns on or the other. When the top turns on, the output swings positive, as it allows current from the positive rail via the transistor and it's emitter resistor. When signal swings negative, the top turns off and the bottom final conducts, causing the output to swing negative. The finals are just current amplifiers.

I'm not sure if you have a defective output transistor or what. .5V across .47 ohms is 1 amp, so Q713 is idling pretty hot. Is there roughly .6V and -.6V at bases of Q709 and Q711? Are those 220R emitter resistors both good?

The 220's are both good! At the base of q709 I have 1.14VDC and at base q711 I have -1.4VDC, but those are both pretty close to the schematic printed values of 1.2 and -1.2... did you mean the voltage at q715 an q713??

Base 715 i have -.8VDC, and q713 I have .5VDC. A different tech had replaced the left SD586 with a D716, but hadn't replaced it's matched mate. The owner said it's been working fine for a while, so I don't think this is the issue but I've ordered a replacement pair already. Does this help illuminate what might be wrong?

[Mr_bibbles]

BINGO!

About 20 minutes after I posted the last reply, my new parts came in and I installed a matched pair of finals, new thermal grease / mica insulators. Biases up like a charm, set it at 7MVDC both sides (suggested is 4-14MV) and it's sounding good!

I really appreciate you guys taking the time to explain how the output section works. I've always had a loose understanding but seeing it in practice and diagnosing WHERE voltage issues are was educational.