Pioneer SX-550 "POP" on startup and hiss at high volume

[hippo008]

Hello all,

I just finished a repair of my SX550 I got for $25. After replacing some caps in the power supply, new LED lights, cleaning the pots and setting the bias it sounds pretty good! I am very happy with its turnout. I am having two persistent problems though, on startup there is a loud POP as the current hits my speakers. As I understand there is a muting circuit thats active for about 7~ seconds while the unit turns on to prevent this pop. I am an electronics beginner so I need some help figuring out where to probe to find the issue. I am also having an issue with a hiss at higher volumes. I have been told this is often a transistor problem in these units and is (usually) easy to fix.

I *think* the issue lies within the FM Muting board but im not sure, I dont know where to begin.

 

[Watthour]

Welcome Aboard!

One item affecting the power-on surge is the setting of the volume control. It is usually best to turn off and on with the volume at minimum.

As you indicated, there is a "soft" anti-surge circuit in the SX-550 (largely a function of C19/C20) which can minimize this phenomenon, but it does not include a speaker relay and thus cannot completely eliminate it.

There is a muting function on the RF board, involving the HA1137 and Q6, but they are only intended to mute the interstation noise experienced when tuning a FM stereo broadcast, and have no bearing on the power-up function.

Regarding the hiss, this can occur in the PHONO mode due to the gain of the EQ amplifier, and can also occur in all modes if the early amplification stages of the audio amp section have noisy devices. While the devices typically used in the SX-550 AF section are not on the list of notoriously noisy / "shoot-on-sight" transistors, they can occasionally break down and create noise.

However, there ARE some of these devices on the RF board (AWE-072) and those should probably be replaced as a matter of course, and especially since the unit is already producing hiss. Q14, Q15, Q16, and Q17 are 2SA725 and 2SC1312 types, known to be problematic. Those can be replaced with KSA992F and KSC1845F devices if you can find them. Since the voltages in that circuit are well below 30V, it would also be acceptable (and potentially preferable) to replace those with BC560C and BC550C devices, since they can have an even lower noise factor and better gain.

Of course, exercise caution and refer to the device pinout/connection data sheets since the replacement devices may have a different orientation from the originals.