SA-W2500 Sub blowing fuse - diyAudio
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Old 11th February 2013, 09:09 PM   #1
2dbit is offline 2dbit  United States
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Default SA-W2500 Sub blowing fuse

So, I have an SA-2500 that likes to pop the 2.8A fuse on the board. I replaced the fuse and with the speaker disconnected, and the volume at minimum, when connected to a current limiting bulb, the bulb is full bright.

I have included a schematic of the sub, along with highlighter indicating which components have been pulled from the board and tested.

Yellow = Test good
Pink = Fail.

The only component that I've found failed is Q704. Using the diode check on my multimeter, I found the transistor to check good as a diode between B and C and B and E. However, I have continuity both from C to E and E to C...

The power transistors (Q708 and Q709) have not been pulled, but check good in circuit....

I can't believe that only the one transistor could be causing this fuse issue...could be wrong though.

Any thoughts??? (schematic attached)


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Old 11th February 2013, 09:43 PM   #2
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Pretty much any one component that upsets the DC conditions could blow a fuse.

An amplifier is a loop so any wrong voltages can get passed around the whole loop.

This is why often amps are a pig to fix.

If the fuse is blowing then I would check the output transistors first.
If they are ok it must be something further back causing the problem.
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Old 12th February 2013, 01:03 AM   #3
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I will likely pull the power transistors for some out of circuit testing, but I find it odd that nothing ahead or behind this failed transistor is toast. I haven't checked everything yet, so perhaps I just need to investigate further.
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Old 12th February 2013, 02:26 AM   #4
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Hi,
Check the resistance at the load side of the fuse to ground. It will tell you if you have a short since you said that the output transistor check OKAY.
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Old 12th February 2013, 03:24 AM   #5
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Am I alone not being able to read this schematic?

Please post another one, with better resolution, and adding the part where the wires on the top right are going to.

Transistors should not have continuity between C.E or C-B.
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Old 12th February 2013, 04:29 AM   #6
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Here's a zoomed in look at the schematic...Click the image to open in full size.
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Old 12th February 2013, 05:44 AM   #7
2dbit is offline 2dbit  United States
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You asked where the wires went in the upper right of the original posted schematic? It's the Input control board...here's a snapshot of where they head to: Click the image to open in full size.
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Old 12th February 2013, 06:19 AM   #8
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Quote:
Originally Posted by tauro0221 View Post
Hi,
Check the resistance at the load side of the fuse to ground. It will tell you if you have a short since you said that the output transistor check OKAY.
I went a step ahead of your suggestion, the fuse is inline with the main transformer. I bypassed the power-on relay and powered the main transformer with the bulb current limiter in place. No illumination on the bulb, and all proper power outs from the transformer.

I then checked resistance between every combination of pins where the transformer connects to the amp board. I found no shorts, all readings were 1M or higher. Then again, this is with the failed transistor removed from the circuit. I'm tempted to power the main amp board with the failed transistor removed, but not knowing the reproductions of missing that transistor keeps me from doing it.
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Old 12th February 2013, 02:41 PM   #9
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Q 704 is the driver, and failure in this one most certenly will give imbalance in the amp, resulting in any fuse to blow.
The 2,8A(?) fuse is prior to the power transformer? Wich is located to the right of connector "C" in the schematics?
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Last edited by TANDBERGEREN; 12th February 2013 at 02:45 PM.
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Old 12th February 2013, 05:08 PM   #10
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Correct. I think I need to look closer at Q709. Seems odd to have a driver fail, and the power transistor survive
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