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Looking at the schematics of the SX-D5k and the SX-3900, the SX-D5k power amp runs at a higher voltage. And although the circuits are basically the same, a higher voltage will give you more power.

Ok, so all transistors and e-caps replaced. You also ran the amp with Q13 and Q15 disconnected and still noticed the static in the remaining operational amps. It will not hurt to replace D1 (or swap with D3). Still, I would start with giving the board a good visual inspection. The higher...

Check for a short between pin 11/12 and 13, or between pin 15/16 and 14?

On R33, I think you have voltages swapped? Q7 base should be around 2V, the other side of R33 should be 19V. 27.9V is not ok. Check R33 and R31?

I am playing with the thought that the right rear channel static influences the other amps through the +30V or -42V supplies, most likely the latter since the -42V power supply has some output resistance of 1.5 ohms. Since you have replaced all transistors and el-caps, I can only think of a bad...

You mentioned that you grounded input of the amps to see if it had its effect on the static. Was this the input of the power amplifiers or the control amp?

Ok. Voltages on VR1 are a bit off, which can either be the cause or the result of the DC offset. You replaced Q1, Q3, and Q7. What about Q5, Q9, and Q11? To rule out these, you could swap them with Q6, Q10, and Q12 on the same board, and see if the static moves to the front channel. The front...

Reading through the thread again, and I see I made a mistake. It was the A board that you had trouble with, right? And I asked you to have a look at the B board... Apologies. Can you repeat the steps from post #89 onwards, but now for the troublesome board?

Ok, is pin 7 properly connected to ground? My first suspicion is VR1. Can you measure voltages on all pins of VR1? Top end should be 0,07V and bottom -0,12V, per the service manual.

Ok, so you changed all electrolytics and transistors on the amp boards. Does adjusting VR1 (dc balance) change average dc on the output of the rear right amp?

The 2/4 channel switch does 2 things: It sets the B1 voltage for the front amps (in 2 channel mode a bit higher so you have more output power), and disconnects the speakers of the rear channel when set to 2 channel mode. You observe that the static is less in 2 channel mode, a state where the...

Ok, no suspicious voltages. Yes, it is painful to remove the board, I know. But we need to be sure. As a next experiment, let's remove the output transistors from the suspicious rear amplifier. You can do that by removing pins 33-38 from the B board. Do not connect a speaker to that amp, but...

Your power supplies are ok, there is something wrong with the B amplifier board. Can you measure voltages on all pins of the B board? Also worth doing a good visual inspection. Any bad joints, especially on components that tend to get hot, like R49-R52?

Right. The 2/4 channel switch basically sets the B1 voltage to 45V in the 2 channel position. Apparently this leads to a state where the protection kicks in. You tested with the inputs of all amps connected to ground, and still had static on all channels. Common denominator would still be the...

Ok, so the regulated power supplies are rock solid now (especially +B3 and -B3? And you see fluctuation on B1 and B2? On the back there is this 2/4 channel switch. Did you clean it already?

I agree with John, there are lots of parts that tend to be unreliable. When restoring my QX-949a I removed the boards completely to make working on them easier. Instead of unwrapping the wires from the pins, I desoldered te wire wrap pins from the boards. Getting the boards out completely makes...

Man, this is frustrating! Only a handful of components, and almost all replaced. What else can be wrong? In your first posts you mentioned the voltages fluctuating quite a bit. You don't see that anymore now? The only thing connecting the 30V and 13V supply is R9. As an experiment, can you lift...

Do you have some resistors lying around? Can you try changing R7 (36k) to 33k?

Checking dc and bias is a good start. The QX-949a makes extensive use of small blue Sanyo caps. Over time they start to behave less and less as capacitor, but more as a resistor. It is wise (or should I say necessary) to replace these. They are probably the source of your problem.

That could very well be the case. You have seen how the 13V supply affects the 30V supply, so a defective C15 can definitely cause audible rumbling noise. Film or ceramic capacitors usually don't break, but always good to have some common values at hand. In that case, order a couple with a...

I think you nailed it Keith, congratulations! Pioneer lists C15 as a ceramic type. Anything will do here I would say, something like 810-FA28X7R1H10200 from Mouser.

I am still thinking pin 6 as well. I know you replaced Q4, and that things got a little better. But E, B, Pin 6 connected: 11.1V, 11.3V is not ok. I would expect to see 0.6-0.7V difference as you see on the good supply. Do you have another KSC2073, or try the SD313 again for Q4? And just to be...

I would suspect there is something wrong with Q4. Are you sure that Q1 and Q4 are properly isolated from the heatsink? What are voltages on the legs of Q4?

Do you have access to a (regulated) power supply that can feed the tuner board separately? And measure current? Or more brutal: a power supply that is able to deliver some decent current. You should be able to feel (or see...) if something gets hot on the tuner board (one of the IC's or maybe a...

With Q3 removed, what happens if you also pull C10? If no change, then also pull C9 and see if that changes anything.