Pioneer SX-9000 unsuccessful rebuild

[patra710]

I just finish a recap and transistor replacement on this Pioneer SX-9000. It does sound great but it wouldn't reach rated output before clipping. It started clipping at about 17 volts AC, reading across my dummy loads. Which is about 36 WPC. This stereo is rated at 50 WPC, which should be 20 volts AC. With the volume all the way up it does get 22 volts = 60 WPC. So it does have the power.
There are 4 pots on the main amp board that the service manual doesn't say anything about.

 

[patra710]

Sorry didn't finish my post, click on the wrong button.

I don't know how to adjust these pots. The two pots in the rear I can see when the board is back in the stereo but the two pots in the front I can't get to.
Any ideas of what I may try?
Thanks

 

[patra710]

I check the pre out and it look good with no clipping and reading 548 mV out, the manual says 500 mV so that side is good. I did replace all of the transistors on the main amp board. But I didn't replace the output transistors.

As you can see, someone else was inside this stereo before I was.
I did use the same value for the coupling caps for the output. This is a cap coupled output which I really don't understand just how it works.

 

[ivandezande]

Uh why are there SK's in there? That's a problem.

The back pots are bias, front are DC offset. All need to be replaced, they corrode and...well..bad things can happen.

 

[dly66]

It is good idea to replace the old pots in these vintage receivers the old carbon types can go open and bias goes to an extreme and an output transistor may go pop. Here are two threads that may help with setting the bias and DC voltage. On each channel I believe the collector of one transistor will be at the main power supply voltage. Generally you adjust the dc voltage of the other to about half that. Then you set the bias current usually as a voltage across a pair of resistors. I have done this with procedure with my 2000A. You can fine tune the dc setting then by driving it just into clipping and setting it so the clipping at the top and bottom is symmetrical.

Hopefully the following two discussion can help on the SX-9000. I have one on the shelf as a future project so I keep them in mind.

http://audiokarma.org/forums/index....-center-point-voltage-bias-adjustment.650259/

http://audiokarma.org/forums/index....-offset-procedure-needed.519417/#post-6762188

For the basic procedure of adjusting bias you can look at this document Conrad Hoffman has on adjusting a Sansui 2000A. Principles should be similar they are both cap coupled amps. The voltage and current (bias voltage) numbers will be different. The Pioneer should be more powerful and the way it is wired may be a bit different. Look for the section titled "Biasing the amplifier" for the bias basics and later in the next section where he talks about the diode he mentions the DC setting.

 

[dly66]

Mine has blown outputs on one channel and I was looking at four On MJ21194G transistors for the replacements ordered from Mouser or Digikey. Good to have all four the same for consistent sound. Have used them and their complementary PNP MJ21193 on several amps with blown outputs and have been happy with the results.

 

[john stumpf]

i just did a sx9000 and diy is spot on. that bias and center point info isnt in the service manual. i think its a good idea to change the pots but the originals appear to be rather robust. you could adjust them if you clean and lube with fader lube and ohm out to make damned sure there are no dead area's. i think there biggest fault is they are unsealed and dirt and tobacco smoke screw things up

 

[dly66]

And with age the metal surfaces tarnish. It is pretty common for people to install modern multi turn trimmers to allow for more precise adjustment than you can get with single turn.

 

[zebulon1]

I haven't found replacement pots for those style trimmers. On the coupled amps, the originals are fine. Mostly I'll use donor pots for replacements. Of course, remove them and check for condition with your DMM before using. They can become intermittent when a blow-out occurs.

 

[dly66]

I sometimes clean and lube them leaving them be. I did that on a recent Scott SS receiver that should be a few years older than a SX-9000.

 

[patra710]

Thanks for the information. I will order new output transistors and try to clean the pots. If I remember right, the top half of the wave on the scope was clipping and the bottom half wasn't. So getting those pots adjusted right just could be the answer.
I have been working on this SX-9000 for a very long time. I kept having one channel that would work then not work. It sat on the shelf for a long time. It turn out to be a dirty tape monitor switch.