Power Switch - SX-550

[Doctor B.]

I found an SX-550 in poor cosmetic shape for $8.00 at a garage sale today. I plan to either give it to one of my kids or to put it in my office once it's cleaned up, gone over, and has new contact paper or veneer.

At first look, the only significant problem inside is that the power switch doesn't turn the unit off. It operates to select the speakers. Just the on/off function is affected.

I'm looking for suggestions of what to look for.

I have read Markthefixer's posts that include pictures of the inside of the switch.

edit: show thread

From the pics, it looks like it may be possible for someone to put in the cam that opens the switch backwards leaving it permanently on. Before I tear the switch apart to check for that what else should I check?

Does the large capacitor across the switch ever short? I could take a leg loose and test if that opens the circuit.

Where else might I look for a short that keeps this thing from turning off?

For a little 20W receiver, this thing puts out well. I have it hooked up to a pair of Baby Advent IIs and it sounds quite nice.

Anyone have a service manual that you are willing to scan and share?

 

[markthefixer]

The service manual is on the database on the tri-comp section, which at the moment isn't accessible.

Sounds like someone deliberately shorted the switch somewhere because the switch wasn't working right, probably burned contacts.....]

Look around the sockets on the back of the unit, the switched/unswitched power plugs for extra wires / connections ...

 

[Doctor B.]

Look around the sockets on the back of the unit, the switched/unswitched power plugs for extra wires / connections ...

Sure enough, there it is! The unswitched outlet is on the left and the switched outlet is on the right. The red wire on the left goes to the power switch and one of the white wires on the right comes back from it. It looks like somebody used a paper clip to bypass the power switch.

So, my plan is to remove the power switch, open it up, burnish the contacts, and then snip the paper clip.

Markthefixer, you're The Man. Thanks.

 

[markthefixer]

Sure enough, there it is! The unswitched outlet is on the left and the switched outlet is on the right. The red wire on the left goes to the power switch and one of the white wires on the right comes back from it. It looks like somebody used a paper clip to bypass the power switch.

So, my plan is to remove the power switch, open it up, burnish the contacts, and then snip the paper clip.

Markthefixer, you're The Man. Thanks.

Ironically, IF I was going to bypass a power switch, that is the way and place I would do it at...... but I have NEVER said it before now....

That is also where I measure the power switch on resistance from the outside, without opening the case, the unswitched and switched outlets (WITHOUT it being plugged in !!).

 

[Doctor B.]

It was a learning experience.

I was able to open the power switch, remove the contacts and sand them lightly with a dremel. Now the power switch works as it should and there is no static or popping sounds as happens when there is poor contact. I clipped the paperclip bypass and the receiver is playing nicely.

Here's what I learned in the process:

Trying to take a switch apart while it's in the unit is no short cut. Working in close quarters is difficult. The power switch casing is so close to the fuse behind it that it's difficult to get the casing open far enough to get the contacts out. I broke the switch casing. It's back together now with superglue. If those contacts ever need cleaning again I'll bet the brittle old plastic of the switch casing will break into bits trying to open it.

It would have been easier in the long run for me to desolder the speaker switch from the board and take the whole assembly out of the unit in order to open the power switch casing. There would have been less chance of breakage and putting those little nuts on the back of the switch on a bench would have been a lot easier than in the unit.

Working on this equipment is a challenge for me. It's like solving a puzzle. In the process I learn something new or I recall something I learned in tech school 36 years ago.

Best of all, it's a collaborative process as AKers offer help and suggestions and as I read of others' experiences.

Thanks to markthefixer for helping and to everybody who writes about your experience.

 

[DaWoofer]

I have the same model with the same problem. It's my computer audio amp and has been on for 3 years still running strong.

 

[Doctor B.]

Now that I've been there and back, I could walk you through the process if you want to dig in there and fix it.

 

[Switchblade]

Any reason you can't just replace the power switch?

 

[markthefixer]

Any reason you can't just replace the power switch?

Very specialized switch, another unit has to die to provide the transplant part.