SX-750 Hum from relay

[Davron94]

Hi All, I've got a SX-750 unit that has a low hum on all source inputs when first turned on. The hum starts automatically if the source is set to AM or FM, but will not start on phono or Aux until playback of audio starts. If I lightly tap the relay it stops, but will come back again after a few seconds. All other functions and playback through sources is good. Although it does need a good contact cleaning on all the pots and switches. Do the relay contacts need cleaning or should it just be replaced?

 

[mrk229]

Relays don't cause hum. If they are crudded up what typically happens is the audio cuts out or is very distorted.

You probably have some bad solder joints in the power supply section which is on the same board as the relay.

Pioneer power supply's typically have several components that run very hot and over time the heat/cool cycle compromises the solder joints.

 

[Davron94]

Relays don't cause hum. If they are crudded up what typically happens is the audio cuts out or is very distorted.

You probably have some bad solder joints in the power supply section which is on the same board as the relay.

Pioneer power supply's typically have several components that run very hot and over time the heat/cool cycle compromises the solder joints.

OK, there does not seem to be any cutting out of the audio though. I would think that a bad solder joint would cause intermittent issues, not a hum.

I did clean the contacts of the relay with contact cleaner and a thin card. Now the hum is permanent and does not go away when tapping on the relay.

 

[satellite65]

OK, there does not seem to be any cutting out of the audio though. I would think that a bad solder joint would cause intermittent issues, not a hum.

I did clean the contacts of the relay with contact cleaner and a thin card. Now the hum is permanent and does not go away when tapping on the relay.

A bad solder joint or cracked trace at a filter cap can cause hum.

 

[mrk229]

OK, there does not seem to be any cutting out of the audio though. I would think that a bad solder joint would cause intermittent issues, not a hum.

I did clean the contacts of the relay with contact cleaner and a thin card. Now the hum is permanent and does not go away when tapping on the relay.

As I said, the PC board that the relay is on a combination of the power supply and the protection circuit.

A bad solder joint in the power supply, especially at a capacitor could cause the supply voltage(s) to have AC on them causing hum.

Above the red line is the power supply, below it is the protection circuit.

Check the solder joints on the red squared items, they run very hot.

And as @satellite65 mentioned check the rest of the power supply area for funky solder joints or maybe cracked/lifted/broken traces at the capacitors.

 

[EastPoint]

The two 330 uf 50v caps on the SX-750 power supply board are under-specced and will go bad and cause hum. Replacing them with 470 uf 63v usually cures the issue. A full recap/rebuild of that board (and the rest of the 750) is ideal, but those two caps are probably causing your hum.

 

[Davron94]

Awesome feedback everyone! I'll go through the power supply board and see where there may be solder issues. Hopefully I'll find a smoking gun and then just rebuild the PSU.

 

[Davron94]

Found the issue! Blown C17 cap. looks to be from overheating. several solder points were corroded on the board also. Will be restoring the whole board anyway!

 

[Bruno Primas]

C17 and C18 are ALWAYS an issue on these. They are 50v and there are 50 volts or more running through them. Replace with 330uf 63v and 105°C caps.

 

[Blast]

C17 and C18 are ALWAYS an issue on these. They are 50v and there are 50 volts or more running through them. Replace with 330uf 63v and 105°C caps.

Indeed. I wish I had thought about that when I replaced all the caps in my 750 a couple years ago!

Davron94, I'm glad to hear that the C17 cap replacement solved the problem!

Additionally, I had to replace my relay afterwards due to it sticking. The original IDEC brand MS4U-24VDC is no longer available so I subbed an Omron MY4-02DC24 (available at Mouser, and others).

I had to cut off all the NC pins off the Omron (like the factory did to the original Idec) but it's the perfect sub.