FYI: Recapping gotcha

[drspiff]

When recapping and replacing the large power caps, be careful grasshopper...

Attached is an illustration of a typical power supply found in old receivers. Notice the 2 capacitors after the full wave bridge. Also notice that the negative output of the supply is more negative than ground.

At first blush, the illustration looks symmetrical about the ground point. This might lead you to put your bright shiny new caps in back to back. Avoid this rush to finish. Let's say you put your caps back to back and tie the negative terminal to ground for each cap. Everybody knows that ground is negative, right?

If you do this, one of your caps will be in backwards. If you do not know what happens when a large electrolytic is reversed, suffice it to say, good for you. It is not a pretty sight. Even if the cap does not blow up, it is still ruined. Yep, You now have to buy another cap to replace the "bulgis vulgaris"

I know, it is a sad thing when a good cap goes bad. I always blame it on bad companions. Let this be a lesson to you grasshopper...

 

[rulerboyz]

I think it's a very common problem when you are new to this sort of work that you dive in without taking enough mental pictures to know how to backstep and put things back the way they were originally. All of a sudden you realize you've cut a wire and are not sure where it went. Also, not everything is nicely labelled positive and negative.

 

[EchoWars]

http://audiokarma.org/forums/showthread.php?t=52274&highlight=ground

Post #21, specifically the first sentence.

 

[cycle_rcr]

Almost all modern amplifiers use a bipolar power supply. + is a potential above 0, - is the same potential below 0. Single ended supply disappeared when good PNP transistors that were satisfactory complement to NPN were developed. I believe middle 1970s. The huge benefit to the bipolar power supply on the main output is that you can bias the idling operating output voltage to 0V. This removes the need for output coupling capacitor.

Single ended supply gives quasi complementary NPN-NPN output.
Bipolar power supply gives complementary NPN-PNP output (or P-channel, N-channel for that matter.)

Glad your cap did not squirt. That is a stinky smell. I did it once in a lab. 50,000 UF, about 28V, Peeeeew.

 

[gonzothegreat]

Here's a little tip for those with a decent (i.e. 4mp or better) digital camera. Take a good closeup of whatever you're about to work on and print it out. That way when you've pulled all the outputs and caps, you can look back at the photo and see how everything looked originally.

That picture has saved me from numerous "dammit!" moments.

 

[markthefixer]

One cap, about the size of a thumb,

150 volts dc power supply.

Installed backwards by production people.

Plugged unit in, lights dimmed, heard m80 type bang, saw cloud of mylar fluttering down over another test bay...

priceless....