Onkyo M-508 Primary Fuse F801 - diyAudio
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Old 21st August 2008, 03:21 PM   #1
jblmar is offline jblmar  United States
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Default Onkyo M-508 Primary Fuse F801

Hi all,
Working on an Onkyo M-508 for a fellow member. The primary fuse (F801) blows out on power up. The copy of the schematic is cut off just at the point of the circuit I need to check.
My guess is a short in the primary winding of the transformer. Another possibility is a defective component(s) futher down the line.
Anyone experience this problem in the past?
Thanks,

Ron
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Old 21st August 2008, 03:32 PM   #2
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Maybe the circuit is here in the service manual
http://www.audio-circuit.dk/Schematics/Onkyo_M508.pdf
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Old 21st August 2008, 04:47 PM   #3
jblmar is offline jblmar  United States
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Thanks for the schematic, ACD. Yep, F801 is direct to the primary of the transformer.
My guess is a short there.
Any one have this issue?

Ron
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Old 21st August 2008, 05:49 PM   #4
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Have you tried disconnect the primary wires of transformer T001 and T002 measured the resistance? I could be a melt down in T001 windings.
If not, then try disconnect the transformer secondary wires at D801 Rectifier Bridge and see if problem is before/after the D801.
At the same time, you have the possibillity to measure the diodes in the D801 Rectifier bridge (one of them could be shorten)...
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Old 21st August 2008, 06:53 PM   #5
jblmar is offline jblmar  United States
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Thanks for your reply.
I did check the bridge diodes. I didn't have time to disconnect the transformer(s) and measure their output or resistance.
The fuse goes up in a very bright flash.

Ron
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Old 22nd August 2008, 04:18 AM   #6
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Disconnect and measure the restiance of the transformer windings WITHOUT connecting power.......
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Old 22nd August 2008, 01:16 PM   #7
jblmar is offline jblmar  United States
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Yes, always measure resistance without power applied.
I'll check D801 & D802 tomorrow.

Ron
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Old 22nd August 2008, 01:49 PM   #8
bekim is offline bekim  Albania
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Default M-508

Hi,

I have M-508 amp serviced 3-4 years ago and, as I can see might be one channel is blown if fuse pops up just like that.
But, yes you have to check the resistance of primary winding of the trafo's.

I think best is to check any transistor is showing short with your dmm. Or, any electrolyte (main ones) is leaking/short..


bekim
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Old 22nd August 2008, 01:54 PM   #9
AndrewT is offline AndrewT  Scotland
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Hi,
power it up through a light bulb tester and measure the transformer input and output voltages. That will reveal a lot.
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Old 24th August 2008, 12:42 AM   #10
jblmar is offline jblmar  United States
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Removed both D801 & D802. F801 didn't blow out.
It's safe to assume that the secondary is o.k.
Could be either a B- or B+ short.


Ron
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