Pioneer SA-8800 with 60hz hum on the power amp section

[EvanF]

Right channel, with shorting plug in the main in (more noisy pic)!! Now left channel has no noise after power cycle!

 

[EvanF]

Right channel, with shorting plug in the main in (more noisy pic)!! Now left channel has no noise after power cycle!

Rught channel noise now:


All scope pics with no load or shorting plugs or jumpers unless specified.

 

[mbz]

Based on your recent input, focus is on louder right channel
which has a 120Hz.
Post #20, first pic looks like a common 120Hz ripple but inverted
No idea on second pic.
Post #21 (shorting plugs) Pic 1 looks like a phase shift effect
Pic 2???
Post #22 again 120Hz ripple but inverted, ie, normally the charge
up time is much quicker than the decay time. Possibly indicates
a problem on the negative half of the amp, ie, ripple from neg rail.

Amp uses separate unregulated power supplies for L&R channels.
Expect to see ripple on voltage rails (pins 21&18).

Candidates from neg half are
- Q42 check waveform at Q42 emitter (R122)
- C56 check waveform at Q42 base (R116)
- D14 check waveform at cathode

 

[mbz]

Any update on this?
The waveform looks a bit like ripple on the negative voltage rail,
check at pin 18, right power amp or collector of Q7 or Q8. Suspect
that this is leaking into the audio path via a tired part. If you
get no hits on the parts mentioned then you can check at the base
of each transistor, signal will be lower but gets amplified.

 

[EvanF]

Based on your recent input, focus is on louder right channel
which has a 120Hz.
Post #20, first pic looks like a common 120Hz ripple but inverted
No idea on second pic.
Post #21 (shorting plugs) Pic 1 looks like a phase shift effect
Pic 2???
Post #22 again 120Hz ripple but inverted, ie, normally the charge
up time is much quicker than the decay time. Possibly indicates
a problem on the negative half of the amp, ie, ripple from neg rail.

Amp uses separate unregulated power supplies for L&R channels.
Expect to see ripple on voltage rails (pins 21&18).

Candidates from neg half are
- Q42 check waveform at Q42 emitter (R122)
- C56 check waveform at Q42 base (R116)
- D14 check waveform at cathode

Q42 base with freeze spray (ultrasonic crap when warm, no change in buzz with spray)

 

[EvanF]

Q42 base, freze spray, no change in buzz

 

[EvanF]

Any update on this?
The waveform looks a bit like ripple on the negative voltage rail,
check at pin 18, right power amp or collector of Q7 or Q8. Suspect
that this is leaking into the audio path via a tired part. If you
get no hits on the parts mentioned then you can check at the base
of each transistor, signal will be lower but gets amplified.

D14

 

[mbz]

Ripple at Q42b points to upstream problem.
D14 looks real bad to me.

 

[mbz]

D14 trace taken on cathod/band side? Would expect 80mVp-p ripple on anode side.

 

[EvanF]

Ripple at Q42b points to upstream problem.
D14 looks real bad to me.

D14 trace taken on cathod/band side? Would expect 80mVp-p ripple on anode side.

Cathode

 

[mbz]

Replace D14 if trace from cathode side, something like 1N4004, a 1N4003 would probably do, need 200V, 1A.

 

[mbz]

Check waveform on other side(anode) of diode, expect the same. You could try an in circuit diode test (powered off).
Outside chance problem is due to a c-e short of Q16 or Q14.
Check waveform at Q14b, Q16b and Q16e

 

[EvanF]

Check waveform on other side(anode) of diode, expect the same. You could try an in circuit diode test (powered off).
Outside chance problem is due to a c-e short of Q16 or Q14.
Check waveform at Q14b, Q16b and Q16e

14b

 

[EvanF]

Check waveform on other side(anode) of diode, expect the same. You could try an in circuit diode test (powered off).
Outside chance problem is due to a c-e short of Q16 or Q14.
Check waveform at Q14b, Q16b and Q16e

16b

 

[EvanF]

16b

 

[EvanF]

Check waveform on other side(anode) of diode, expect the same. You could try an in circuit diode test (powered off).
Outside chance problem is due to a c-e short of Q16 or Q14.
Check waveform at Q14b, Q16b and Q16e

16e

 

[mbz]

14b not ok, D14 probably ok

 

[EvanF]

16e

16e

 

[mbz]

14b, 16b, 16e voltage scale difficult to read, 50mV/div? Way to much.

 

[EvanF]

14b, 16b, 16e voltage scale difficult to read, 50mV/div? Way to much.

.5V/div