I purchased this Onkyo M504 a few years ago and recently it just powered off, no sound and nothing to the meters. When I opened the amp the top fuse on the board attached as pic 1 noted a blown fuse, I searched this site and found several posts regarding a faulty switch might be the cause of the blown fuse, I replaced the switch, attached the wiring and original snubbers to the new switch, placed a new fuse in. When engaging the new switch, the VU meter lights came on, but within a few seconds the same fuse blew again. I am far from technician level, and it has been a long time since I built a couple of Heathkit amps, so my experience is old and limited for sure, but this amp seems pretty straight forward and I am in hopes it is something I might can repair.
From looking at the wiring, it appears the board that the fuses sit on has connections from the power switch, grey and brown wires which connect to posts at each fuse location, the AC power cable has conections to both sides and the accessory AC outlook on the rear of the amp does the same. Then from that circuit board yellow and white wires go from each fuse side to the two transformers.
I would presume with the wiring and the fuse location that the issue is prior to the transformer stage and if not the switch causing the fault, is it the AC power cord, or something else?
The solder joints look to be OK, and none are loose. There is no apparent or noticeable damage to the circuit board. Any help is greatly appreciated, I do love this amp, I have other options to run, but I do want this one back to working order. Thanks in advance
From looking at the wiring, it appears the board that the fuses sit on has connections from the power switch, grey and brown wires which connect to posts at each fuse location, the AC power cable has conections to both sides and the accessory AC outlook on the rear of the amp does the same. Then from that circuit board yellow and white wires go from each fuse side to the two transformers.
I would presume with the wiring and the fuse location that the issue is prior to the transformer stage and if not the switch causing the fault, is it the AC power cord, or something else?
The solder joints look to be OK, and none are loose. There is no apparent or noticeable damage to the circuit board. Any help is greatly appreciated, I do love this amp, I have other options to run, but I do want this one back to working order. Thanks in advance
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There is a fuse per channel - so obviously there is a problem with one channel.
Get service manual (e.g. from eletrotanya).
Use DBT - but one regulator of B2 uses a 15V zener D453 to provide reference voltage and I would bridge that with some 7k5 resistor like in the other rail while using DBT.
Check B1 and B2 voltages - if one of them is missing your outputs may not be blown but may sit on a rail.
Check output transistros for short.
Get service manual (e.g. from eletrotanya).
Use DBT - but one regulator of B2 uses a 15V zener D453 to provide reference voltage and I would bridge that with some 7k5 resistor like in the other rail while using DBT.
Check B1 and B2 voltages - if one of them is missing your outputs may not be blown but may sit on a rail.
Check output transistros for short.
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1. Lift wires 1 and 2 (secondary) from the board (disconnect the secondary from the bridge rectifier), replace the blown fuse and power up the amp. If the fuse does not blow, the transformer is okay.
2. Reconnect (solder) wires 1 and 2.
3. Remove the bridge rectifier from the PCB and test it. If it tests okay, place it back and then remove wires 3 and 4 (these are the power supply rails to the amplifier PCB). Turn the amplifier on. If all goes well and the fuse does not blow, you just proved/double-checked and confirmed the bridge rectifier is okay... which means there's something wrong with the AMP PCB... get the circuit diagram.... i.e. do not reconnect wires 3 and 4 (because the fuse will blow again...); instead, start checking the output transistors for short.... check the filter capacitors (and the bypass caps)...
2. Reconnect (solder) wires 1 and 2.
3. Remove the bridge rectifier from the PCB and test it. If it tests okay, place it back and then remove wires 3 and 4 (these are the power supply rails to the amplifier PCB). Turn the amplifier on. If all goes well and the fuse does not blow, you just proved/double-checked and confirmed the bridge rectifier is okay... which means there's something wrong with the AMP PCB... get the circuit diagram.... i.e. do not reconnect wires 3 and 4 (because the fuse will blow again...); instead, start checking the output transistors for short.... check the filter capacitors (and the bypass caps)...
This amp has a separate bridge rectifier, D706, for the protection, meters, and lamps and is known to go bad. There is also a fuse for this rectifier, F702. Make sure nobody has installed an oversized fuse in that location. F802 supplies the voltage to the power transformer that supplies D706.
Craig
Craig