Onkyo M-508 Primary Fuse F801

[jblmar]

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

 

[Jan Dupont]

Maybe the circuit is here in the service manual
http://www.audio-circuit.dk/Schematics/Onkyo_M508.pdf

 

[jblmar]

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

 

[Jan Dupont]

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)...

 

[jblmar]

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

 

[Jan Dupont]

Disconnect and measure the restiance of the transformer windings WITHOUT connecting power.......

 

[jblmar]

Yes, always measure resistance without power applied.
I'll check D801 & D802 tomorrow.

Ron

 

[bekim]

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

 

[AndrewT]

Hi,
power it up through a light bulb tester and measure the transformer input and output voltages. That will reveal a lot.

 

[jblmar]

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

 

[Jan Dupont]

Follow the light bulp advice from AndrewT.
Use it my self each time, and have a small collections of bulps to suit each test object (25, 40, 60 and 100W bulps)

 

[jblmar]

Found the problem. Left channel is blow out. Output and driver transistors are all shorted.
The question now is what caused the short?

Ron

 

[bekim]

Might be many things.
I would suggest to be careful when you're repairing, might be any small trannies to be blown out in right channel. As well, first check LED's in amp board, they look like standard diodes.

When I bought my M-508 was dead left channel; it seems that, was factory fault, many of wires were missing, so practicaly my amp is relatively new!

You can check all output trannies and if one pair has survived try only one pair (pnp & npn) to put in there, not all six, in case there is major damage. There is dual diode - change it with two 1N4148 in series; three electrolytes 10uF/16v per channel - change in both channels and, see if any lines in pcb are burnt..

Make a list of what is burned and start from the small part to the bigger ones

Wish you luck on this, if I had chance to buy more of these amps I would do it without hesitation, the sound just warm and nice.

bekim

 

[jblmar]

Yes, I will check the circuit back to the previous stage.
Thanks for the LED heads up.

What ever the original fault, it took out a great amount of components.

Ron