SA-7500II output transistors voltages

R

robyrony

New Member

• Sep 3, 2015

• #1

Hello,
I have a Pioneer SA-7500II that I recapped a while ago, plus changed the output transistors with MJ15032 and 33. I measured the voltages from the output transistors and compared them with the service manual. I found that my amplifier has lower values at the base and emitter.
I attached a picture from the service manual and put my readings near it (with blue colour).
Can someone please tell me if this is normal, or my amplifier is not working the way it should.
Thank you.

 

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  7500II.jpg

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sregor

AK Subscriber

Subscriber

• Sep 3, 2015

• #2

I am (90+percent) fairly sure the red numbers are wrong. Looks like the put the voltages for the driver transistors (Q8 and Q10) on the outputs. The drivers both have + and - 1.2 volts on the base, so the emitters (which go to the bases of the outputs) should be abou + and - .6 volts, and the output emitters should be near zero... My 2 cents.

 

petehall347

the brandy coffee man

• Sep 3, 2015

• #3

it appears the manual is wrong ..
looks like you have a problem though with the 100mv DC offset calculated from your readings ..
measure DC mv where the 0.5 ohm emitter resistors join ..
also check the bias /idle current .. as i suspect this might be incorrect also . DC mv across the test points ..
TP1 to TP2 left channel ..
TP3 to TP4 right channel .

 

Last edited: Sep 3, 2015

R

robyrony

New Member

• Sep 3, 2015

• #4

Thank you, for your responses.
The voltages across the emitter-resistors are:
starting from the left, in mV: 27 - 27 - 22 - 22
and bias: left-52 mV, right-42 mV (jumper leads are cut on both channels).

 

Last edited: Sep 3, 2015

petehall347

the brandy coffee man

• Sep 3, 2015

• #5

what is the DC voltage at speaker terminals ?

 

R

robyrony

New Member

• Sep 3, 2015

• #6

left channel: -4 mV
right channel: +11 mV

 

petehall347

the brandy coffee man

• Sep 4, 2015

• #7

all looks fine going off the bias and offset voltages .

 

markthefixer

On Hiatus, hospitalized and NOT doing Clutches

Subscriber

• Sep 4, 2015

• #8

The DC offset and idle current voltages look good, no problematic transistors or capacitors in it.

It DOES have a protect relay, so the q20 and q21 transistors could eventually be upgraded, and a diode installed across the relay coil - but if it's working, don't bother.

And it does seem to be measuring out quite satisfactorily.

Just enjoy it, don't sweat it.

 

R

robyrony

New Member

• Sep 4, 2015

• #9

Thank you for your responses. I'm happy that I have a good working amplifier.
Have a nice day.