Trying to connect fans to the amp and got a huge spark!

rg12

Member
2010-08-10 3:52 am
I decided to add two computer fans to blow on the cooling fins of my
amp's transistors.

I decided to try to find a 12v source in the power stage of the amp.

So, i decided to get a multimeter and try to explore the amp.

I connected the amp and fired it up (no music playing) and i put
the amp on the side with the pcb's copper tracks facing me.

I connected one probe to the chassis ground and started touching
random points (which was a big mistake).
I found out that most of the amp's voltage is 37.3, so i kept on going
and one time i touched with the positive probe one of the ends of
somekind of bridge in the power stage of the amp (what i mean by
bridge is a stripped wire that is soldered like a resistor) and bam!
i see a blue spark with a loud noise.
Everything still works well (the multimeter, the amp and thank god, me!).
The positive probe's tip coating broke, it was a big zap!

What did i do wrong?

The spark was kind of what this stupid guy does with the cap and the pliers http://www.youtube.com/watch?v=UvSwIhGLpZE&feature=related
Could it have been something with the power caps?
 
Last edited:

benb

Member
2010-04-24 1:52 am
You surely shorted something with a lot of current across the probe tip, apparently one of the main filter capacitors. I doubt the probe on the chassis ground had anything to do with it.

From your description the "bridge" is a jumper wire that one end may not have been soldered well, and wasn't making a connection until you touched it.

As far as 12V for fans, you'd probably want to pull that off the main voltage and drop it down rather than some random point that happens to be at 12V. Post pics and a schematic, and let's see.

That video looks scary. Presumably that's rather high voltage. He's trusting the rubber insulation on the tool to keep him from being shocked. He's holding the cap in one hand and the tool in the other. If something goes wrong the current from the capacitor will go through his chest. Holding each of the charging wires in each hand isn't something I'd do either.
 

rg12

Member
2010-08-10 3:52 am
When you say jumper i imagine the computer's motherboard jumpers
which are removable (to open the circuit) and in that case it was
only a permanent connected wire and it was and still is soldered well,
if it wasn't then the amp wouldn't work right or wouldn't work at all.

Here is a link to the schematic:
http://www.milkbands.co.il/Pioneer-SA-7300-int-sch.pdf

And here is a link to a thread with many of my amp's pics:
http://www.diyaudio.com/forums/soli...-mod-rebuild-log-pioneer-sa-7300-26-pics.html

What do you mean from the main? the main is 240v from what i know.
 

benb

Member
2010-04-24 1:52 am
Yes, that's one common meaning of jumper. Another is a wire connecting two traces on a printed circuit board.

By the main voltage I mean the 37.3V, the main supply voltage to the amplifier from its power supply. On the schematic, this would be across C67 or C68.

I really like clorifille's idea, though I'd prefer an older 50/60Hz-transformer-based "wall wart" supply rather than using a modern SMPS, a potential source for noise, inside audio gear.
 

mace1337

Member
2005-08-21 11:21 pm
Instead of randomly prodding around, you should just measure the voltages that the power supply inside the amp gives. If your fan causes the supply to sag on one side of the dual supply you can create a dangerous DC offset that can fry your amp and/or your speakers!

If you ask me, it's much better to get an old LCD monitor power supply (The aforementioned SMPS) and use that, much safer.
 

rg12

Member
2010-08-10 3:52 am
Instead of randomly prodding around, you should just measure the voltages that the power supply inside the amp gives. If your fan causes the supply to sag on one side of the dual supply you can create a dangerous DC offset that can fry your amp and/or your speakers!

If you ask me, it's much better to get an old LCD monitor power supply (The aforementioned SMPS) and use that, much safer.

Sounds convincing, where can i get a good transformer elsewhere?
I don't have anything i can throw out that has a 12v transformer.


IMO AC Fans are more reliable than cheap PC 12 VDC fans.
A.C. Fans | AllElectronics.com

I already bought DC ones, but quality quiet ones and also the AC ones
are deeper in size and don't fit my needs.
 
Quote:
I decided to add two computer fans to blow on the cooling fins of my
amp's transistors.

Why? Are the outputs running hot? If so investigate why the output stage is running hot instead of trying to band aid the situation. In proper operating condition the amplifiers heat sinks are capable of taking care of the situation.
Have you changed the heat sinks or done some modification which would cause them to run hot?
 
After reading thru some of this thread it brings the question to my mind as to why one would spend the time and money to bring back an amplifier that surely isn't worth the time. Why not purchase an SA8500 or 9500? Even these don't sound as well as something as primative as an SAE 2200 which is around a 100 watts a channel or a GAS grandson which is around 75 watts a channel. Oh, I'm not a Pioneer hater as I still have a SA8500 sitting on one of my shelves but in all honesty its noting to brag about. Just my .02
 

rg12

Member
2010-08-10 3:52 am
After reading thru some of this thread it brings the question to my mind as to why one would spend the time and money to bring back an amplifier that surely isn't worth the time. Why not purchase an SA8500 or 9500? Even these don't sound as well as something as primative as an SAE 2200 which is around a 100 watts a channel or a GAS grandson which is around 75 watts a channel. Oh, I'm not a Pioneer hater as I still have a SA8500 sitting on one of my shelves but in all honesty its noting to brag about. Just my .02

I took this thing as a project, i never said that i can make a monster
out of it, but it's a simple and nice amp that is built like a tank, so i
decided to take it as a project and once i will buy a better amp,
i will have the knowledge i need to have for upgrading it easily.