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Quad 303: why are you blowing the main fuse?

Raccoon1400

Raccoon1400

Super Member
Quad 303 blows the main fuse for no apparent reason. I didn't test anything out of circuit, but I looked for shorts with my meter everywhere I could think of. I even compared numbers with another quad 303 I had on the bench. All outputs appear good, rectifier appears good.

I disconnected the driver boards, and the power supply board from the main 65V supply, and it still blows fuses.

Not sure what is left, any ideas? Troubleshooting tips?
 
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Raccoon1400 said:
Quad 303 blows the main fuse for no apparent reason. I didn't test anything out of circuit, but I looked for shorts with my meter everywhere I could think of. I even compared numbers with another quad 303 I had on the bench. All outputs appear good, rectifier appears good.

I disconnected the driver boards, and the power supply board from the main 65V supply, and it still blows fuses.

Not sure what is left, any ideas? Troubleshooting tips?
Click to expand...

have you been tinkering with the amp recently ?
 
primary should measure something like 30 ohms ..maybe less ..
try disconnecting the secondary side and see if it still blows the mains fuse .
 
As Pete says, but just to be clear, when you remove the power supply board, you are removing all DC converting components such as the bridge rectifier and the power supply filter capacitors correct?

If so, it points to a shorted power transformer.
 
cademan said:
As Pete says, but just to be clear, when you remove the power supply board, you are removing all DC converting components such as the bridge rectifier and the power supply filter capacitors correct?

If so, it points to a shorted power transformer.
Click to expand...

The main caps and the rectifier are still in the circuit when I remove the board, but I think they are all that remains. And they appear good. At least the ohm meter doesn't find any shorts.

EDIT: Just cut the wires at the input to the rectifier. Didn't blow the fuse. But rectifier tests good on diode check too.
 
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Disconnect the bridge rectifier. Disconnected, it should only read one way like a diode between the - and the + terminals. If it does, check for a shorted capacitor. If the capacitors(s) show a steady reading, it (they) are shorted.
 
cademan said:
Disconnect the bridge rectifier. Disconnected, it should only read one way like a diode between the - and the + terminals. If it does, check for a shorted capacitor. If the capacitors(s) show a steady reading, it (they) are shorted.
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See Edit in previous post.

If capacitors show a steady reading while measuring what?
 
If a capacitor is shorted, it will read steady where the meter won't move. If it is good, it will show it is charging up and the meter will read something like zero then continue on up into infinity.

Your short is either the bridge rectifier or a shorted capacitor!
 
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Don't test the bridge rectifier at the two ~ ~ points, but at the + and - points. The two ~ ~ are the AC input and it will read both directions if it is good or bad anyway!
 
Do you have four individual diodes? Check all four out of circuit or at least lift one leg of each.
 
cademan said:
If a capacitor is shorted, it will read steady where the meter won't move. If it is good, it will show it is holding a charge and the meter will read something like zero then continue on up into infinity.

Your short is either the bridge rectifier or a shorted capacitor!
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Measuring resistance? They all measure infinity.
Using the 200W DBT, I could measure some voltages. The main supply voltage is 70V. Would likely be higher without the DBT.
So I adjusted it on the power supply, down as far as I could. I then connected direct to wall. I set it to 67V, and no fuse blowing, with everything hooked up.

So, here's my explanation. The supply voltage was too high, causing the caps to draw more current during powerup. When I disconnected the power supply board, it couldn't regulate the supply, so it was still too high.
 
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maybe the main filter caps are old and going short . and pulling too much power at power up .. see if they get warm when running off the dim bulb .
 
petehall347 said:
maybe the main filter caps are old and going short . and pulling too much power at power up .. see if they get warm when running off the dim bulb .
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See previous post. Yes, pulling too much power at powerup, but due to rail voltage being set too high. So no short!
 
i thought rail was 65v ..and its measures 70v unloaded .. i would say its about right if that's the case..
edit .. just seen above post .. my guess its more likely the caps formed enough on the dbt so they didn't act like a short when powered from mains .
important bit does the amp work ok ?
 
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petehall347 said:
i thought rail was 65v ..and its measures 70v unloaded .. i would say its about right if that's the case..
edit .. just seen above post .. my guess its more likely the caps formed enough on the dbt so they didn't act like a short when powered from mains .
important bit does the amp work ok ?
Click to expand...

It measured 70V with the DBT. Likely it wanted to go higher when connected to line. I have a big bose 1801 here, the supply voltage was almost 10V lower on the 200W DBT.

EDIT: If I hook it to the DBT now, supply is only a couple volts lower.
 
as i understood it all being powered was the power supply so i wouldn't expect too much drop on a 200 watt bulb .
i would actually expect it to be a little high .

anyway does the rest of it power up ok now ?
 
petehall347 said:
as i understood it all being powered was the power supply so i wouldn't expect too much drop on a 200 watt bulb .
i would actually expect it to be a little high .

anyway does the rest of it power up ok now ?
Click to expand...

From what I can tell. Can't do sound check yet because of that fancy input plug. I may add RCA jacks. I did a nice clean install on the last quad I did.
 
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