Power supply for main amp blows fuses after replacing filter caps.

[acedogblast]

Hello I have a Marantz sr5012 that I have been repairing. After I have replaced the 2 main filter caps the 2 fuses for the power amplifier blows. The original capacitors are ELNA LAO 10,000uF 71V and the replacements are Nichicon LKS 10,000uF 80V.
I assume a short circuit connection has occurred so I set my multimeter to measure ohms across the 2 fuse connections and both read 55 Kilo-ohms so I assume that is ok.

I am certain that I do not have any bridges on my soldering and after probing around I do find a temporary 0 ohm reading when probing across the positive and negative voltage rails that after a few seconds starts growing into OL. I assume this is just due to the meter charging up the filtering capacitors. My current plan now is to replace the fuses and see it they blow again if I have the amp board disconnected. The ratings for these fuses are T6.3AL 250V. I have checked all other fuses and they are intact.

[petehall347]

Re: Power supply for main amp blows fuses after replacing filter caps.

post a pic of how new caps are connected

[acedogblast]

Re: Power supply for main amp blows fuses after replacing filter caps.

Can't show the actual PCB right now but I do have a schematic.

The fuses that are blown are F4000 and F4001.
The replaced filter caps are C4052 and C4054.
Going to replace the fuses with slow blow fuses and will see what will happen. The original fuses look like quick blows to me since they are just a straight thin wire inside. I think the new filter capacitors have a slightly higher capacitance than the old ones which causes to pull more current on startup.

[petehall347]

Re: Power supply for main amp blows fuses after replacing filter caps.

need to check your work .

[acedogblast]

Re: Power supply for main amp blows fuses after replacing filter caps.

need to check your work .

I will have to take out the main PCB which is buried under 8 other PCBs which is why I can't show you the solder connections right now. If the replacement fuses blow without the power amplifier connected then I will have to take main PCB out then.

[lotas]

Re: Power supply for main amp blows fuses after replacing filter caps.

You did not reverse the polarity when installing new capacitors.

[acedogblast]

Re: Power supply for main amp blows fuses after replacing filter caps.

You did not reverse the polarity when installing new capacitors.

I am certain I have not. I have a picture of the original caps on the PCB where both negative stripes are pointing to the transformer. Looking into the PCB right now I can see 2 negative stripes pointing to the transformer and the stripped marking on the PCB that indicates negative terminal.

[petehall347]

Re: Power supply for main amp blows fuses after replacing filter caps.

when caps have been confirmed to be correct way then check the bridge rectifier .. new caps may have stressed it . failing that look for shorts .

[mmartell]

Re: Power supply for main amp blows fuses after replacing filter caps.

I am certain I have not. I have a picture of the original caps on the PCB where both negative stripes are pointing to the transformer. Looking into the PCB right now I can see 2 negative stripes pointing to the transformer and the stripped marking on the PCB that indicates negative terminal.

I'm not quite certain what you are saying here. You can clearly see on the schematic that ONE of your caps should be pointing in the OTHER direction.

Unless I'm missing something...

[R_J]

Re: Power supply for main amp blows fuses after replacing filter caps.

The caps seem to be in the correct way based on the sr5011 similar manual. Disconnecting the plug going to the amp circuit should tell you where the short is located, but if it IS in the amp circuit you will need to disharge the main caps BEFORE reconnecting the CN4212 plug or the amp may be damaged. So check the voltage ACROSS C4052/C4054 to make sure it is near 0 volts before reconnecting the plug.

It may be that the leads from the new caps are shorting against the chassis after reinstalling the board?

[mmartell]

Re: Power supply for main amp blows fuses after replacing filter caps.

Oh I see what he's saying now. I thought he meant the grounds of both caps were on the same line/trace to the transformer.

And yes good point about the leads possibly touching the case - did that myself once lol.

[acedogblast]

Re: Power supply for main amp blows fuses after replacing filter caps.

Got new fuses and they did not blow if I have the amp board disconnected. So something must be wrong with the amp board.

There is a short on the power amp board between -B and GND. When I remove the heatsink the shot is gone. I noticed some burnt thermal paste around one of the output transistors. Should the transistors be electrically isolated from the heatsink?

[petehall347]

Re: Power supply for main amp blows fuses after replacing filter caps.

yes they have to be insulated .

[acedogblast]

Re: Power supply for main amp blows fuses after replacing filter caps.

yes they have to be insulated .

That is rather difficult and horrible design. I check the transistors and they have an electrical connection on the middle pin (collector) to the back flat surface. When I took apart the heat sink during the repair I removed the old white colored thermal compound and between the heat sink and the transistor is a small thin plastic sheet. Is that suppose to electrically isolate the transistor to the heat sink?

When I reassembled the heatsink I used Arctic MX-6 thermal compound which on their website claims to be non-conductive. So you are saying that when I reassemble the heatsink I need to make sure there is no electrical connection on all 3 pins of the transistor to the heatsink?

[R_J]

Re: Power supply for main amp blows fuses after replacing filter caps.

You need to make sure the collector (rear tab) is insulated from the heat sink, that is why they use a mica or rubber insulator. these must be there.

[acedogblast]

Re: Power supply for main amp blows fuses after replacing filter caps.

You need to make sure the collector (rear tab) is insulated from the heat sink, that is why they use a mica or rubber insulator. these must be there.

Ok so that is the cause of the short. Sorry, this is my first time repairing an amplifier. I can see some burnt thermal compound around one of the screw holes on the heat sink. I will try to reattach the heatsink again with care to make sure the mica insulators are aligned to prevent a short. Thanks everyone for the tips.

[petehall347]

Re: Power supply for main amp blows fuses after replacing filter caps.

there should be insulators on the screws as well as the mica.

[acedogblast]

Re: Power supply for main amp blows fuses after replacing filter caps.

there should be insulators on the screws as well as the mica.

The amp board didn't have screw insulators. After refitting the heatsink on a few times there still is a short. I think I might have damaged one of the insulators when I removed the heatsink the first time so here goes another 2 day wait for new insulators to arrive.

[lotas]

Re: Power supply for main amp blows fuses after replacing filter caps.

The amp board didn't have screw insulators. After refitting the heatsink on a few times there still is a short. I think I might have damaged one of the insulators when I removed the heatsink the first time so here goes another 2 day wait for new insulators to arrive.

Rather, you burned the transistors, which collided with the heatsink without insulation.

[R_J]

Re: Power supply for main amp blows fuses after replacing filter caps.

There is no need for these screw insulators, the transistors are TO3P case style, You will need to check all the output transistors to make sure none are damaged (shorted) They may be hard to check because they are darlington transistors with internal 70Ω resistors, but just make sure there are no E~C shorts, if all the same types check the same they are likely ok.

What was wrong with the original capacitors that you needed to replace them?

I can see some burnt thermal compound around one of the screw holes on the heat sink

This transistor may have been damaged, so you will need to inspect it to make sure there is no burnt carbon track on the back of the transistor and no rough metal if it did indeed arc. This is if the transistor itself did not short internally