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Onkyo M504 Protection Mode Problem

WillRobinson

WillRobinson

Active Member
Hello all!

I recently picked up an Onkyo M504 and P304 pair from a local estate.

Both were in very nice shape, they were stored in a closed cabinet where they collected very little dust. I opened them up, blew out what little dust there was, did a quick visual inspection and saw nothing out of order, checked the DC offset - all good. So I hooked them up and they sounded great, ran for about an hour, no issues.

The the next day decided to use the amp in my Bi-amp setup. hooked it up and noticed there was no output. After a few munutes double checking the cables I heard a relay click in the M504, and the Servo Operation indicator was no longer lit.So I turned the amp off, waited 10 minutes, turned back on, Protection relay clicks and Servo Operation indicator lights. Hmm.
So I check the DC offset again, the left channel was good but the right showed DC climbing steadily, climbing up to around 4V then the protection relay clicks.

So off the the bench it went, here are clues I found:
Base and Emitters of all 4 Right channel output transistors read the same climbing dc voltage, collectors read the correct voltage.
D420 and D418 on the main board are not lit.

Any ideas of where to look next? This is a more complex unit than I have ever opened up.
 
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WillRobinson said:
D420 and D418 on the main board are not lit.
Click to expand...
Not familiar with this unit sorry - what does this mean - are these LEDs?

Go to hifiengine.com - register & download any service manual that is there. Provided that the DC offset is user adjustable - setup the DC offset of the unit very carefully using the procedure described there. You will need a small screwdriver & a DMM
 
slimecity said:
Not familiar with this unit sorry - what does this mean - are these LEDs?

Go to hifiengine.com - register & download any service manual that is there. Provided that the DC offset is user adjustable - setup the DC offset of the unit very carefully using the procedure described there. You will need a small screwdriver & a DMM
Click to expand...

Yes D418,420 are LEDs on the main board.
D417,419 are the their counter parts for the left channel, which are both lit.

I don't believe the DC offset is adjustable, the service manual has no information on it.
 
Update!
I pulled and tested all right channel output transistors, all test good. Then did the same for all the small transistors and a few diodes on the main board, they also tested fine. After reinstalling all the small transistors I powered the amp up and it was working perfectly fine, D420 & D418 were lit, the problem went away.
So I chalked it up to most likely a bad solder joint on one of those transistors.

I ran it on a dummy load for 30 minutes to be sure the problem wouldn't return before hooking it up to some speakers. It didn't.

Great! Back to the audio room!
After playing for about an hour everything sounded great, It was sitting idle for a few minutes and I heard a POP. Uh oh, back to the bench...

The pop was F802, the fuse in-between the power switch and the right channel's power transformer. I'm guessing something is shorted on the right side. Really hoping it's not one of the main filter caps, they look like a PITA to replace.

The quest continues...
 
I have an M504 on my bench with a similar problem. There is a short somewhere in the left channel that blows F803: I cannot find any faulty components and I am totally con-fused (pardon the pun).I checked the output transformer and that looks OK with continuity and no shorts to Ground. I disconnected the main filter caps (it is a PITA) but they all checked out OK. I removed the rectifiers and they are OK as are the caps on the same board. I disconnected the 3 power supply wires to the main board (that disconnects the main board?) and the fuse is OK! Hmph, running out of components to take out of the circuit. Those wound-up wire connections are a pain - maybe someone has a technique for unwinding them so that they can be rewound later?

I am starting the amp up with the power switch on and I have a Variac and a 60W lamp in series so I know the problem is still there if the lamp glows brightly and the meter lamps do not light up... I cut the power if that happens to save blowing lots of fuses. If I remove fuse 803 the right channel work fine - this is a UK amp so there is no F801.

The short is definitely on the main board but where is it? I noticed that if I put a meter across the power supply to the main board on the right channel it is OL but on the left channel it reads 350MOhm... maybe that is telling me something?
 
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hello.... i have my unit... and the same problem... i think the power supply get unbalanced..... the power supply get +69v & -76 volts.... so the diference in the power supply vs. the ground make the drivers to go to the negative output... please advice.. if this is correct.... Why? what happen to the power supply that it get unbalanced? please advice... my kind regards.
 
Based on the age and the amount of related data, I’d put a wager on it being the power caps and now likely other parts in the chain.
 
Trouble shoot the regulated +/- 68 vdc power supply. Could be a bad zener or a bad capacitor.
 
in the RIGHT CHANNEL.... the +69 v & -76 v. with the LEFT channel... without main fuse ( F801 DAMAGE AND NOT REPLACED YET) there seems to be two monoblocks completely separete on the power...

it seems to me ... that a bad capacitor could make diferent fault... like less voltage and very big ripple.... but never to elevate the voltage 7 volts... over the regular value.....
 
fbatarse said:
in the RIGHT CHANNEL.... the +69 v & -76 v. with the LEFT channel... without main fuse ( F801 DAMAGE AND NOT REPLACED YET) there seems to be two monoblocks completely separete on the power...

it seems to me ... that a bad capacitor could make diferent fault... like less voltage and very big ripple.... but never to elevate the voltage 7 volts... over the regular value.....
Click to expand...
Trouble shoot the regulated +/- 68 volt power supply for each bad channel.. Yes, it is a dual mono design..
 
the voltage i read is at the big capacitors of the power supply.... the negative is bigger than the positive voltage.... but it doesnt seems to me ... that the capacitors are the problem.... something else.. could be....?
 
Transformer secondary AC voltage problem or a Rectifier (bridged rectifier) problem or and issue with the big capacitors.-- Gots to be one of those.Unless something downstream is loading down the rail.
 
WillRobinson said:
Really hoping it's not one of the main filter caps, they look like a PITA to replace.
Click to expand...

I think either blhagstrom or maybe leesonic has a workaround for this.

I shudder every time my 504 powers up. Totally virgin, untampered, and overdue for a rebuild.

Best of luck to all with these most photogenic of amplifiers.
 
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