Pioneer SA-900 hum

[METZLER955]

New member herein, I also have a SA 900. Problem with intermittent right channel, when stereo switch is selected. Have you ever encountered this issue? I cleaned switch with de-oxit a few months ago..maybe helped some. Any pointers would be appreciated. When it's working...absolutely fantastic.

 

[modernlover]

New member herein, I also have a SA 900. Problem with intermittent right channel, when stereo switch is selected. Have you ever encountered this issue? I cleaned switch with de-oxit a few months ago..maybe helped some. Any pointers would be appreciated. When it's working...absolutely fantastic.

i had issues with the mode selector knob when I first got it. Just shooting deoxit into the switch didn’t help. I had to take the switch apart to really clean the contacts.

 

[modernlover]

After a few more weeks of poking around, there is now no hum when the pre and main are separated, and the muting switch reduces the level of the hum. Now I just gotta figure out what the culprit is.

 

[modernlover]

Dropping in again. Decided to recheck everything on the power supply, and it all tested good.

Then I decided to replace the new main filter capacitors I had installed with some newer, beefier ones. Just in case one was bad. They both checked out fine, but I still swapped them out to test. And it did make a difference. I suspect that when I replaced the old capacitors, I didn’t secure the connection to the terminals well enough.

Messing with the ground pad on the power supply board can move the hum, right now it shows up if the muting switch is thrown or if the pre/main are decoupled.

I also tested using some alligator leads to attach the grounds of the filters to another point on the chassis, and that seemed to make an improvement as well.

So I’m still trying to track down exactly where and what I can do to finally get this thing done. Without having to attempt making a significant modification to the power supply board like Mark suggested. Just don’t feel comfortable doing that.

 

[markthefixer]

just don't locate a connection to ground on the green or blue paths i drew on the boards in post #16..

Those are the locations of the AC current flow that due to ANY ground resistance whatsoever (amps times ohms), will impress itself (millivolts or even microvolts) upon the quiet dc ground.

 

[modernlover]

So after last weekend’s alligator adventure, I decided to test to see if something that was connected to the capacitor grounds were the issue.

First thing I did was desolder the two wire attached to the larger of the two. One for the power light and the other that went to the other capacitor ground. I clipped these elsewhere on the chassis. This didn’t affect anything so I tied them back in to where they were.

Then I moved to the other one. Three wires attached to this one. The ground coming off the larger cap, the wire that goes to the the ground point and the ground wire to the headphone jack. This time I only took out the headphone ground wire and clipped that to another point. And lo and behold the annoying hum is practically gone. It’s still there, just very low and I have to really concentrate to hear it through my headphones.

Not sure exactly why. When I replaced the caps the first time I followed the same wiring as it had originally. And every picture I’ve ever seen has the headphone ground tied into the smaller filter capacitor ground.

What’s likely is the issue is still present, but it is beyond me what/where it truly is. I think it might be time to call it done. Unless I can find someone around where I am to see if they are willing to touch it.

 

[HPMguy]

Did you attach the ground wires in EXACTLY the same positions? It matters, because if the headphone ground wire is on the wrong side of the cap ground wire you will create what is known as a ground loop. In this case, if the cap ground wire is between the headphone ground and the actual chassis ground point, you will likely hear ripple from the cap just exactly as if the cap were not grounded. Electricity is strange stuff and the order in which wires are soldered to a "star ground" matter a lot. Even the very tiny difference in resistance between the grounding points can result in voltages going where you don't want them.

A high resistance solder joint at the ground points can do the same thing. So can a bad ground somewhere else. And so can a bad solder joint on a shunt to ground cap somewhere or a cap backwards so that the unwanted hum "signal" isn't being grounded and removed.

 

[modernlover]

Yes, I have done a lot of reading about ground loops and ripple. I’ve found as many images as possible that I can compare to. I have reworked, rewired and rebuilt the ground point that the filters tie into. I’ve double checked polarity and reflowed/re-soldered everywhere. I have gone over everything so many times.

I removed the tabs off the old caps, mechanically secured them to the snap in tabs, and soldered in.

This is basically what I did initially when I first replaced the filters and is when the hum first showed up.

Removing the headphone ground and moving it somewhere else is the most significant change.

 

[HPMguy]

Check ALL of the ground points in the device, and especially look for cracks in boards. Wiggle any ground wires to see if that changes the hum, and if the main ground is a screw joint, a little DeOxit there won't hurt any. Sure sounds like a ground loop, and most Pioneers of that vintage are a rat's nest of wire, all single strand a prone to breaking when flexed, even inside the insulation. Kenwoods are only better because they changed to ribbon cables sooner which is less prone to internal breakage.

And check ANY pad you have removed components from, I ran around in circles for weeks with a Kenwood KR-845g I'd recapped where the FM would go out and make terrible racket intermittently, and finally found a broken trace on a capacitor I'd replaced. It cracked just past the solder, and unltil the pad and lead came loose from the board completely, I couldn't find it.

You can use a non-conductive probe of some sort to gently flex boards and poke any components you have replaced, if the hum changes when you do so, look for the cracked trace or bad connection.

An oscilloscope would be handy here, so you could determine the initial source -- 120Hz is mail filter cap ripple, and if it's not being removed by the voltage regulator transistors or filter caps, you could trace it through the amp, or start with the outputs and go backwards. 60Hz is a bad rectifier, etc.

Another way to look for the issue is to verify ground path from component lead to next component lead or pin. Use a very gentle touch, pressure on a pad with a cracked trace can restore the connection while testing and go open when you remove the probe.