What are they used for?

[Richardjhy]

Below is from Yamaha RX-V757. What are D254 and R265 used for? They are on L, R channels. Center and 4 surround channels are the same as L, R channels except for a resistor and zenner between bases of output transtors.

 

[Doodski]

Here is a more inclusive schematic snipping so that the entire stage can be seen. I think the Zener diode is to compensate for any temperature related variation in the base-emitter voltage and provide bias. I am as always interested to see what one of our esteemed heavy duty experts has to say about this. R265 is to set the current limiting for the Zener diode operation.

 

[Doodski]

BumP!

 

[egellings]

Below is from Yamaha RX-V757. What are D254 and R265 used for? They are on L, R channels. Center and 4 surround channels are the same as L, R channels except for a resistor and zenner between bases of output transtors.
View attachment 399868

Level shifter.

 

[AnalogSteph]

I'd say neither of the above is correct. Under normal conditions, these parts don't do anything at all. Has to be for some sort of edge case, like getting base charges out of the opposite output transistor at high amplitude / high frequency. If both conduct simultaneously because one is slow to turn off, this can lead to substantial heating and thus degraded SOA.

 

[LTig]

I'd say neither of the above is correct. Under normal conditions, these parts don't do anything at all. Has to be for some sort of edge case, like getting base charges out of the opposite output transistor at high amplitude / high frequency. If both conduct simultaneously because one is slow to turn off, this can lead to substantial heating and thus degraded SOA.

I think you are on the right track. The bias current is set by Q5, and D254 with R265 prevent it getting too high.

 

[AnalogSteph]

Now bias isn't even adjustable, so unless Q5 happens to not be installed on the heatsink (which would be stupid)...

 

[Richardjhy]

Thanks all and more questions.

1. This receiver uses 3W 0.22 ohm as emitter resistor for L,R and C channels (See photo and R46 in the 1st snipshot) but uses 5W 0.22 ohm (R47 in the 2nd snipshot) for all 4 surround channels. Why L, R, C channels use resistors with lower wattage rate?

2. Yamaha uses 7 pairs of A1696/C4468 as output devices for all 7 channels and claims 100wpc/8ohm, but A1696/C4468 only rated 100W max. Can I mod the receiver by combining 3 pairs of A1696/C4468 for L,R channels to create a robust 100wpc 2.1 receiver?

3. Can I replace driver transistors A1770/C4614 (1W) with A1306/C3298 (20W)?

 

[Richardjhy]

Now bias isn't even adjustable, so unless Q5 happens to not be installed on the heatsink (which would be stupid)...

Many Yamaha receivers/amplifiers designed with bias current not adjustable. Their service manual just mentions to cut open a certain resister if bias is out of range.

 

[LTig]

Thanks all and more questions.

1. This receiver uses 3W 0.22 ohm as emitter resistor for L,R and C channels (See photo and R46 in the 1st snipshot) but uses 5W 0.22 ohm (R47 in the 2nd snipshot) for all 4 surround channels. Why L, R, C channels use resistors with lower wattage rate?
View attachment 400303
View attachment 400304

View attachment 400305

The biasing voltage of the surround channels is 1.2 V instead of 1.0 V, because the zener diode is missing. So bias current is higher.

 

[Richardjhy]

The biasing voltage of the surround channels is 1.2 V instead of 1.0 V, because the zener diode is missing. So bias current is higher.

C channel doesn't have the zenner diode as well but it's emitter resistor is the same as L, R channels.

 

[LTig]

C channel doesn't have the zenner diode as well but it's emitter resistor is the same as L, R channels.

Without seeing the schematic of the center channel and the voltages in red I cannot answer your question.

 

[Doodski]

Without seeing the schematic of the center channel and the voltages in red I cannot answer your question.

Here is the C channel.