Pioneer SA 9900 strange problem after recap/rework

[pakko]

Hey everybody,

I need your help with my Pioneer SA-9900. I reworked the whole amp in the last days.
Replaced Caps and Transistor - like a lot of you here in this forum.
So, everything works fine except for one thing I don´t have a explanation for.
If the Amp runs with a source hooked up (let´s say input Tuner) and the Volume is set to zero - I still can hear the source! It´s not possible to reduce it totally. With no source hooked up you still hear a hiss/sough on both channels.
I located the problem to the control amp (AWG-031) - there the input (comes from Switch and Input boards) the at pins 10 and 13 is absolute quiet. But I do get a signal at pins 2,3,4,5 and 7 and on the other side as well on 16,18,19,20,21 (pins 6 and 17 are quiet)
What I did on this board - I changed all the caps, changed the 4 x 2SC1451 transistors and today I changed the 4 x 2SK34 as well.

By the way I replaced all 2SC1451 with 2SC1953

I say it again - everything else works fine - the amp section measures fine with Bias and DC Offset etc. - If I separate the amp, it´s clear the problem comes from the pre-amp section.

Hopefully someone has an idea how to go on.....thank you

 

[SaturationPt]

I don't know if it helps, but if my memory is correct this amp has a dual-attenuation where it lowers the input and output of the preamp together when you turn down the volume.

 

[rcs16]

If the Amp runs with a source hooked up (let´s say input Tuner) and the Volume is set to zero - I still can hear the source! It´s not possible to reduce it totally. With no source hooked up you still hear a hiss/sough on both channels.

Did it do it before you touched it?

 

[pakko]

Did it do it before you touched it?

Yes, it had the same problem

 

[pakko]

I don't know if it helps, but if my memory is correct this amp has a dual-attenuation where it lowers the input and output of the preamp together when you turn down the volume.

Thank you. In my case the position of the attenuator switch has no effect to the situation.

 

[bberkom]

I have had a few amps and receivers that don't fully block the signal when the volume pot is turned all the way down. It could just be age or mistacking in the pot. Is it the same level in both channels?

 

[pakko]

I have had a few amps and receivers that don't fully block the signal when the volume pot is turned all the way down. It could just be age or mistacking in the pot. Is it the same level in both channels?

Yes, it´s the same on both channels. The Volume Pots sits right before the Control amp and there is no signal at pin 10 and 13 when Volume is down. So I think the Pot is OK and works like it should.
When you switch the Balance pot to one or the other side the signal goes way down.

 

[pakko]

Is it possible than it has something to do with the muting gate?

 

[jobrewer1983]

Is it possible than it has something to do with the muting gate?

Have you tried to adjust your offset/bias? Doubt this will help, but you may want to ensure your died in.

Good Luck with it!

 

[restorer-john]

Breakthrough from other sources on source selectors was and is very common on old amps, due to the type of switching they used and the proximity of open contacts to one another. Capacitive coupling and the high impedance of the FET differential front end of the flat amp doesn't help either.

The hiss you hear is likely normal. All amplifiers have a residual hiss even with a shorted input that can be heard when your ear is near a tweeter.

The transistors with the blue paint marks on top are matched pairs- did you match your replacements?

 

[SaturationPt]

The balance pot lowers the level in both channels when turned to either side? Sounding like a signal-ground problem.

 

[pakko]

OK here we get closer - yes, the Balance lowers the signal to both sides. How can I go on with this signal - ground problem?

 

[kramden]

Using anything other than low leakage electrolytics or film caps for coupling will add a great deal of HISSSSSSS. You should also take a close look at the 220k ohm resistors at the output of this board. Discolored. I would also look at the 48 volt rails for cleanliness.

 

[pakko]

Here´s a pic before the changes

 

[pakko]

I´ll checkt the colored Resistor. Elna Silmic II are not a good replace for the tants?

 

[kramden]

The Pioneer aficionados like Zebulon and Mark The Fixer will hopefully offer their perspectives as they have on countless occasions. My own opinion is that if you want your Pioneer to sound more like a Nak, you would use the silmic II's. Heavy on the low frequencies adding a bit of squireliness to vocals but audible HISS at low listening levels. People DO love them though. But back to your problem o/p...no hijacks please. In any case, I don't think the use of any particular capacitor would help to alleviate the problem of being able to hear audio when the volume control is all the way down.

 

[LesE]

The usual recommendation for the replacement of caps from 0.1 uf to 1.0 uf is stacked film. Otherwise for CSZA, CSSA, and CEANL types, Nichicon UKL are recommended.

Les.

 

[ConradH]

+1 on LesE recommendation. Silmic is good, but not very low leakage like a tantalum. Use film or UKL series, but no, probably not related to signal leakage.

How are you looking at pins 10 and 13? It takes remarkably little signal to be audible. I'd look at the ground side of the pots and the ground side of the input and outputs of all boards, usually the shielded cables, but the maximum sensitivity setting of most scopes is barely enough. If both channels do it, and it's subtle, it might be normal, IOW, inherent in the layout and design.

 

[Fhamre]

I have had so called leakage on a unit,before. The solution was to replace the volume pot.That said others told me not to worry about it or just use the speaker switch to eliminate the issue.
Some suggested to try and repair the pot.