Pioneer SX 737 popping fuses during recap

DixieMan1994

Active Member
Hey guys

Well, I am having some interesting issues now. Started a recap of my SX 737 and started on the protection/power supply board. Got a few caps in a and had one with a bad connection, popping the main fuse. So I of course got the capacitor bridged, installed the new fuse in the screw in holder, and gave her a whirl. Nothing. So I start checking things out and I notice the inline soldered fuse leading to the screw in fuse is blown. I change it out, and success, the unit fires up and works like a charm. :banana: Then I start again working on my recap, I replaced one of the 50v 330uf caps (the one closest to the back of the unit of the three), POW! there goes the screw in fuse again. I checked the solder joints, those came out fine, then I noticed that while I had the board out of its connectors I had the back wire post on the right side (from my angle from working from the back) very close to the chassis. Ok, so I readjust the board accordingly, pop in a new fuse, and success again. :D Well then I power the unit off, then power it on again immediately. Zap! There goes the inline fuse. :yikes::screwy: I cannot see anything visually to cause that, and I have no idea what is going on, or what I possibly did. What do you all think? The whole popping fuses thing is confusing as hell now :saywhat:
 
Assuming you put the caps in with the correct polarity, I would disconnect the rest of the circuit from the power supply, so there's no load on the power supply.

I would then power up the unit to see if it still blows a fuse.

If not then the issue is further downstream from the power supply.

If it still blows the fuse then I would re-examine the re-capping work done so far for polarity, short circuits, blown transistor (if in the power supply circuitry) and resistors.

You can even get more involved and disconnect the AC input from the rectifier bridge and feed the relevant DC power into the system (post rectifier) using an external power supply. Set the current limit on your supply to match the fuse. Then see what happens. If there's a problem the current limit on the power supply will kick in and the voltage will then drop to zero.... at least no more blown fuses and you now have a good test bed to use to fix the problem.

There has been the odd time where I have cranked up the current limit on an external power supply just to see what smokes up on the cct board. This is very rare and I only do it when I am confounded to the nth degree in what the root cause is. I then replace the smoked component and carefully check the associated components around it. Some I replace and others I just measure and leave in place (if metered ok).
 
Assuming you put the caps in with the correct polarity, I would disconnect the rest of the circuit from the power supply, so there's no load on the power supply.

I would then power up the unit to see if it still blows a fuse.

If not then the issue is further downstream from the power supply.

If it still blows the fuse then I would re-examine the re-capping work done so far for polarity, short circuits, blown transistor (if in the power supply circuitry) and resistors.

You can even get more involved and disconnect the AC input from the rectifier bridge and feed the relevant DC power into the system (post rectifier) using an external power supply. Set the current limit on your supply to match the fuse. Then see what happens. If there's a problem the current limit on the power supply will kick in and the voltage will then drop to zero.... at least no more blown fuses and you now have a good test bed to use to fix the problem.

There has been the odd time where I have cranked up the current limit on an external power supply just to see what smokes up on the cct board. This is very rare and I only do it when I am confounded to the nth degree in what the root cause is. I then replace the smoked component and carefully check the associated components around it. Some I replace and others I just measure and leave in place (if metered ok).

What would be the best way to disconnect the power supply?
 
Hey guys

Well, I am having some interesting issues now. Started a recap of my SX 737 and started on the protection/power supply board. Got a few caps in a and had one with a bad connection, popping the main fuse. So I of course got the capacitor bridged, installed the new fuse in the screw in holder, and gave her a whirl. Nothing. So I start checking things out and I notice the inline soldered fuse leading to the screw in fuse is blown. I change it out, and success, the unit fires up and works like a charm. :banana: Then I start again working on my recap, I replaced one of the 50v 330uf caps (the one closest to the back of the unit of the three), POW! there goes the screw in fuse again. I checked the solder joints, those came out fine, then I noticed that while I had the board out of its connectors I had the back wire post on the right side (from my angle from working from the back) very close to the chassis. Ok, so I readjust the board accordingly, pop in a new fuse, and success again. :D Well then I power the unit off, then power it on again immediately. Zap! There goes the inline fuse. :yikes::screwy: I cannot see anything visually to cause that, and I have no idea what is going on, or what I possibly did. What do you all think? The whole popping fuses thing is confusing as hell now :saywhat:

So you ran the unit before the recap was complete?
 
I did, I do that to make sure if something happens, that I know what the last thing I did was that could have caused it.
 
Cap may be backwards, defective, or solder connections bridged somewhere. It didnt blow fuses before, so I would back out all replaced caps.
After you get it back to original, I would suggest you use a dim bulb tester, that can save you some grief and fuses.
There are writeups of dim bulb testers on AK. Basically they use a incandescant buld in series with the hot leg of power feeding you receiver. Upon powerup, the bulb usually flashes bright for a second, and then dims down to little or nothing. If there is a short, of the type to blow fuses, the bulb comes on bright and stays bright.
 
Cap may be backwards, defective, or solder connections bridged somewhere. It didnt blow fuses before, so I would back out all replaced caps.
After you get it back to original, I would suggest you use a dim bulb tester, that can save you some grief and fuses.
There are writeups of dim bulb testers on AK. Basically they use a incandescant buld in series with the hot leg of power feeding you receiver. Upon powerup, the bulb usually flashes bright for a second, and then dims down to little or nothing. If there is a short, of the type to blow fuses, the bulb comes on bright and stays bright.

Hey Tom. Hey check my thread "blown 3A fuse" I have been going back and forth with Mr. LarryDeroin on the issues I have been having. Everything is up to date there. :)
 
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