BOSOZ Problem

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Hi All,

I need some troubleshooting help. I finished building the BOSOZ with separate PS and preamp circuit. I tested the PS and it's fine. After connecting the preamp fuses blow. I used 1 & 2 amp slow-blow fuses.

I built the circuit based on the article with a few exceptions:

-Used 2 power supply circuits- one for each channel.

-Used 4- 28.4V + 28.4V transformers at around 3-5 amps each. (I know overkill, but they were cheap.) Too much power on surge maybe??

-The FETs on the preamp are not bedded down on the circuit board. They are upright. The center pin is soldered to the connection where the case would be screwed down.

-The switch has a built-in fuse holder. It requires 2 fuses. The circuit only required one. I used 2 fuses.

The PS output voltage is 58+/-. The zenner array on the PS is set for the 60v setup.

No burn or burn odor emmitted when powered on.

Test equipment I have available: analog meter, 2 digital meters and a dual channel scope and Radio Shack transitor checker. And of course the light bulb socket thingy. I haven't learned how to use the scope yet.

When testing the PS, I used the light bulb trick, and it actual helped fix a ground problem. Can I use it again to help futher troubleshoot this issue?

Your suggestion are greatly appreciated.

Vince
 
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Hi Babowana,

No, there are no NTC thermistors on the front side, just what was indicated on the PS schematic. Should I try one for each side of AC in?

A couple of pics of the whole setup and connections would make it easier to figure for us

Oh, I don't know...it's kind of ugly. I don't want to embarrass myself in light of all Your great projects. :eek: But, for the sake of fixing it, I will post a few pics. :D

One other thing-
Also, when I had it plugged in to my amps and speakers, even though there was no power, if I turned up the pot on the preamp, there was some distorted sound coming from the speakers.

V~
 
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No go. There's a major problem. I have to recheck the wiring.
The NTC thermistors didn't help. :dead:

Best thing to test the PSUs is use a 15k resistor to ground, correct? I'm going to test everything and use only 1 PSU, instead of 2 to reduce failure points.

I'm really bummed.

Thanks,

Vince
 
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Still Blowing Fuses

I wired up the PS with only one PS board and tested it with 15k resistors. I get about 58 volts +- and no blown fuses.

I checked the caps on the audio board by using an analog multimeter. The needle jumps and slowly dropped on all caps.
I'm not sure if this test works for caps when installed on the PCB.
The resistors check out ok. The zeners all show about the same resistance. Their orientation looks ok.

On the audio board I swapped out all 4 IRF610 MosFETs just in case. This time I bolted the FETS and heatsinks to the board. I'm still blowing fuses.

I'm not sure what else to do. I may remove the switch with the built in double fuse holder. Maybe that's the wrong part for this type of circuit?

Should I start with a new PCB and all new parts?

Any suggestions would be nice.

Thanks,

Vince
 
Vince-

This shouldn't be too tough to figure out.



Best thing to test the PSUs is use a 15k resistor to ground, correct? I'm going to test everything and use only 1 PSU, instead of 2 to reduce failure points

Actually, your PS's are driving roughly 750 ohms to ground on the negative side. Make sure your PS's don't blow fuses with that load.

Are your boards from AudioXpress or did you etch them yourself?
I assume they are the same pattern as presented in Nelson's article?

The size of your trafos wasn't clear -- are they 90VA or 150VA? Remember, your thermistor won't help much when its hot--so if your inrush surge current is enough to blow a fuse, turning the PS off then on again should blow a fuse.

this should not be too tough to figure out....

JJ
 
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JJ,

I'm using the boards from Audio Xpress. They're not great. After replacing the IFP610s, the pads are almost gone.

The transfos are about 150 va.

I took out the thermistors, ran the PS w/ 15k resistance and a 1 amp slo-blo fuse. The PS is running ok. As soon as I add power to 1 or both channels, the fuse blows very violently and is very hot when removed. This happen with up to 3 amp fuse.

Actually, your PS's are driving roughly 750 ohms to ground on the negative side. Make sure your PS's don't blow fuses with that load.

I guess you mean test the PS with 750 ohms on the negative side only? If the fuse blows then, does it indicate a PS issue?

Thanks for your help,

Vince
 
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Hi JJ,

I put a 750 ohm resistor on the negative side of the PS and the fuse blew. What does this indicate?

I kept the positive side at 15k. Is that ok for this test?

I switched to the second PS, put in a 4 amp slo-blo, used a 750 ohm on the - and 15k resistor on the + side. The bridge rectifier on the + side burned. The fuse burned also.

Any ideas?

Thanks,

Vince
 
Vince-

If you don't have the transformers hooked up exactly as shown in Nelson's article, you will (in my experience) smoke the rectifiers. I tried to use a single transformer, and smoked four of them before I gave up on the bridges. Unfortunately, I am not a PS expert and I never worked out what the problem with my hookup was. I assume the trafo outputs were phased incorrectly, and it was basically making a short through the bridges. But that is just an untested guess.

You said the trafos were cheap, and you seemed uncertain about the VA rating. Did you get them from PE by chance?

Can you post a schematic of how you have the trafo's hooked up, and are you sure of the output's on the trafos?

JJ
 
I put a 750 ohm resistor on the negative side of the PS and the fuse blew. What does this indicate?

Either a short, or bad devices on the board somewhere or (my guess), is incorrect transformer hookup.

I kept the positive side at 15k. Is that ok for this test?
I kept the positive side at 15k. Is that ok for this test?
It should be able to drive 750 ohms, I would test with that.

JJ
 
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I bought the tranformers from Apex Jr. They are 28.4-0-28.4 Volts. There is no other way to wire the secondaries.

Instructions from Apex Jr:

"Wire the 2x28.4v toroids as follows-

120v input tie the 2 black and brown together along with tying the 2 white and blues together.

Secondary: Black is center tap and each side is red and yellow."

http://www.apexjr.com/miscellaneous.html#Toroids

I don't have the center tap connected. It doesn't go anywhere on the PS diagram anyway. On the PS PCB it just make a series connection for the transformer. Should I ground it to earth?
 
I used those transformers without any problems. This is how I connected the secondaries to the PSU board:

BLACK soldered in the hole marked "A"
RED soldered in the hole marked "B" (This just ties to the board, could use "C")
YELLOW soldered in the hole marked "D"

I used this setup in a stereo configuration, single ended without incident.

edit: It appears that you hooked up your transformer secondaries with the red and yellow in the "A" and "D" position. I think this may be your problem. Try configuring as I have suggested above. I think that may solve your problem.

Also remember, if you are running single ended, solder a link or resistor from the "- IN" to "GND."
 
BLACK soldered in the hole marked "A"
RED soldered in the hole marked "B" (This just ties to the board, could use "C")
YELLOW soldered in the hole marked "D"

I used this setup in a stereo configuration, single ended without incident

hmm, that is interesting, as it is definitely different than would be indicated by the way they have the leads marked at Apex Jr. If it worked for you Raoul, then it is definitely worth a try. Maybe the secondaries are not correctly identified?

Another way to tell if it is in the regulator section vs. the transformer and bridge section is to connect an unregulated power supply in where the bridge goes. (That is how I am running my boards)

I can post details if you are not sure how to do it.

JJ
 
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