Pioneer sx 1250 awh-021 wrong board

oh shit yes just found it but strange i have two pioneer sx 1250 here and both boards do not have this D6 its a resistor instead
 
It's there.
The SM images are mirrored from the bottom of the boards.
Takes some getting used to.
Look at the components and get oriented.
 
There must have been a design change after the documentation was completed. Here is an Eq board that was done by @mattsd and it looks the same as yours:

SX-1250+equilizer+amp+board+after.JPG
 
ok great i was being told look at the board it must be there ...i looked and looked and looked started to feel crazy.... i never seen one sx 1250 with that diode. But i had a underpopulated board i was going to rebuild and i got stuck at that Diode.
 
A resistor is cheaper than a zener diode. The circuit sets the constant current for the input differential amp by providing a constant voltage. With a fat cap already there (C32) the resistor voltage divider works just as well. YMMV. :dunno:
 
Just for the record, that circuit and board layout are counter-intuitive. While one is tempted to think they are a mirror image layout, they aren't.

The diode D5 is the reference for the 65v to 49v regulator.
the -49v regulator is a mirror tracking regulator, which tries to keep the base of Q14 at -0.6v, with the R38(47k) / R39(47k) + R40 (120 ohms) voltage divider
connected to the +49v dc and -49v dc rails.

R37 is the limiting resistor paired with the C32 / D6 24v zener making QUIET and clean 24v dc for the phono / eq amp's input stage.
THEN the bean counters changed D6 to a 5100 ohm resistor, although with the low draw of 120k resistors, the voltage divider scheme gets ~ 25.5v and MAY be quieter than the zener diode.
 
Good analysis Mark and Meryln,
Hard to say if the bean counters inspired/demanded the design change or if it was the designer. Making a dual +/- tracking reg eliminates one zener at least.
I was thinking that I'd do the same for the 1980 PS design, make it a tracker but left it as is, no bean counters breathing down my back, could have saved 2 zeners with that mod :)
 
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