Pioneer SA 7800 négative voltage on base and Emitter of power output transistors

[Lynyrd]

Hello.
Need help for reanimated one PIONEER SA 7800.
All 4 output transistors are dead.
I replaced all burn resistors,4 drivers q23....q26,and tested all parts on power amplifier board.Voltages +&-46v...ok+&-48v...ok
But now on output transistors I have negative voltage.
I know,Its NSA Amplifier and need very strict control,but I don't understand where is problem.

Q1. B -14.5v E. -12.9v
Q3. B. -18v. E. -13.3v
Q4. B. -9.4v E -3.7v
Q2. B. -4.3v. E. -3v
On 4 collector's +...-46v....ok

Thank you

Service manual
https://www.manualslib.com/manual/940262/Pioneer-Sa-7800.html

 

[Lynyrd]

I was replaced 4 trimpots ,4 resistors 0.22 ohm,2 transistors was dead Q27 and Q28,replaced with c1845.
All diodes.....resistors,caps was tested dosoldered..... nothing....

 

[Lynyrd]

No power transistors on the board

 

[xavirom]

Check power lines +/-48V and/or colectors Q5 and Q9

 

[Lynyrd]

Q5.....C....-4v.....
Q9 C.....-13.8v
On pin 1....+ 47.9v
On pin 2......-46.3v
Q6...C...-10v
Q10.....-18v

 

[Lynyrd]

 

[xavirom]

The circuit associated with Q5 may not be working, check D5, Q5, R17 and R23. Something associated with Q19 may also be failing, there seems to be no current conduction from the positive side of the power supply on both channels.

 

[Lynyrd]

R17...ok 223 Ohm
R23....ok.....2.2 kOhm
D5.......ok
Q5......ok.
All parts dessoldered and tested with part tester

 

[Lynyrd]

Q19....ok
D13...ok
All resistors...ok

 

[xavirom]

Do the new transistors come from reliable vendors? Perhaps it is convenient to disconnect the output transistors and the drivers to eliminate suspects, and with these out of the circuit, it would be necessary to do more voltage measurements, based on q13 and q17, B-E of Q5, Q19, there are indicated in the diagram, current values that with a multimeter and ohm's law would easily allow you to know where there is or is not current circulation and to be able to advance

 

[Lynyrd]

No power transits on board

 

[Lynyrd]

Without drivers same history

 

[Lynyrd]

Q19
B.....+44.8
C...-5.8
E.....45.4

Q13
B...-5.7
C.....-6.5
E.....-5.9

Q17
B......-7.1
C......-8.0
E....-8.4

 

[Lynyrd]

Q5
B.....+45v
C....-3.4
E...45.7

 

[Lynyrd]

Why is negative voltage and not positive?

 

[xavirom]

Why is negative voltage and not positive?

Because on the bright side, something is wrong.
Measure voltage in terminals of R17, R23, R37 and R45, divide the value measured by the value of the resistance and it will give the value of the current that circulates through them, then compare the measured data with the one that appears in the diagram, this will give you It will provide information on what may be failing.
Q13
B...-5.7
C.....-6.5
E.....-5.9

There is no conduction in this transistor, Vbe is 0.2V

 

[Lynyrd]

Ok,I will replace this

 

[Lynyrd]

No,I desoldered and VBE is ok

 

[Lynyrd]

I don’t know what to do anymore, I checked everything I could, all the parts were soldered and checked, everything that needed to be replaced, but ......

 

[Lynyrd]

I thought maybe I mixed up the transistors, no, they all stand correctly and in their places