SP9 schematic questions...

[HVfanatic]

I downloaded SP9/2 schematics, but my understanding needs some discussion:

I just claim what I understood, so please support / correct me!

Phono:

Q1: First stage
V1: Second stage
Q2, Q3: Cascoded fet stage (third stage)
Q5,R13, R14: Mosfet source follower.

My questions:

Q13 seems to be for protection - why???
Q4, Q6 are n-channel jfets. Drain and source are connected - why???
Annother point is, that there cannot flow any current into the gate. So it seems they are just there "for fun". Can Anybody please explain???

Line:

Q7: First stage
V2: Second stage
Q10, R27: Mosfet source follower.

Q8, Q9 are n-channel jfets. Drain and source are connected - why???
Annother point is, that there cannot flow any current into the gate. So it seems they are just there "for fun". Can Anybody please explain???

Audio Research SP-9 MKII Service Manual free download,schematics,datasheets,eeprom bins,pcb,repair info for test equipment and electronics

Regs, Dirk

 

[HVfanatic]

Just appended the schematics in question...

 

[analog_sa]

The additional components safeguard the mosfets during tube warmup. Why use fets instead of diodes i am not too sure.

 

[SY]

Low leakage and capacitance during normal operation (when they're reverse biased).

 

[HVfanatic]

So it's like protection diodes - what is the advantage??? Protection diodes do not contribute noise, if they are not in "regulating" area...

Is this correct???

The bipolar I mentioned is alsofor protection?

Regs, Dirk

 

[HVfanatic]

Another point:

The schematic shows some devices denoted as:
FET, WHITE ORANGE GREEN
FET, GRAY BLUE
FET, ORANGE GREEN
FET, RED BROWN
FET, WHITE ORANGE
FET, ORANGE GREEN BLUE
FET, YELLOW
FET, YELLOW RED

What are these transistors? What can I use for replacement, if broken??? Is there a substitute list around?

Regs, Dirk

 

[analog_sa]

What can I use for replacement, if broken??? Is there a substitute list around?

The idea seems to be to shop for replacements from ARC or use the authorised repair facilities. Finding suitable replacements should not be difficult. Are you considering building a clone? The SP9 is not ARC's finest hour IMHO...

 

[HVfanatic]

Hi,

I got a broken one in my hands. I want to repair it, but I have no glue which mosfets are used. SP8 is superior, but unluckily mine is SP9...

Could you recommend me the substitutes you would use? would you exchange parts on both channels???

I was told, that the power supply is broken... I'll have to check next days. My first time on high voltage stuff...

 

[bear]

Low leakage and capacitance during normal operation (when they're reverse biased).

That functions as a diode??

The gate is very high Z.

The usual connection makes them run at IDSS, but iirc is with the gate strapped to the source? or is it just D and S?

Can't put any current through the gate, can you??

So those "diodes" are going to try to pull the source of the mosfet UP towards B+ when the tube is not conducting, then the tube conducts and pulls the gate voltage back down?? Hmmm...

_-_-bear