MA6600 blows fuse

[tutor_turtle]

I have a customer I repaired this amplifier for twice. Both times it went into protection and the heatsink got very hot.

In 2016, I replaced the left channel amplifier board as an assembly. At that time the whole assembly was only $220
16 months ago, the same symptoms appeared on the right channel which was also replaced as an assembly. Price: $280

Now the customer tells me the fuse blows instantly upon power up.
Accompanied by a snapping noise and burnt electronic smell.

I brought it up on a Variac, at about 70 volts, I can hear a relay kick in, I assume this is the speaker proaction relay.
no movement on the current meter.
At about 85 volts I can hear another relay clicking, and the current meter jumps to the clicking.

I disconnected both power boards, these are not the source of the excessive current draw.
I can't find any burned components.

The main power supply for the amplifier output board rails looks deceptively simple. A bridge rectifier and two caps.
The bridge rectifier measures good on a DVM on diode mode.
Could the big caps have gone bad?

There is a second power board called the back up power supply board.
The diodes and caps are very small, can't be more than 24 volts at 2-3 amps.
I wouldn't think this could take out a 6.3A slow-blow fuse.

Has any tech out there encountered this problem to help me "cut the cards"?

 

[Bob the Tech]

The only relays in that unit that aren't small signal relays are the two on the backup power supply. When it's connected to mains the backup supply generates an always on low voltage for the microcontroller which then controls everything else. The only thing that should trigger the unit to turn on in this state is the standby pushbutton. Bringing the unit up slowly may be confusing the microcontroller and causing unpredictable behavior. You might be hearing the relay in one leg of the mains kick in before the relay in the other leg just based on the two different relay's responses to the same coil voltage.

Anyway, once the micro triggers the unit to turn on it throws those relays and the rest of the supplies come up - B+/- and the ones on the main board. It's definitely possible the B+/- rectifier or caps have failed but I'd bet it's one of the low voltage regulators or associated caps is shot/shorted. Can definitely still blow the fuse.

There's no speaker protection relay in the MA6600.

You should also never have to replace a whole amp board like that, these boards can be diagnosed to the part pretty readily.

 

[tutor_turtle]

The only relays in that unit that aren't small signal relays are the two on the backup power supply. When it's connected to mains the backup supply generates an always on low voltage for the microcontroller which then controls everything else. The only thing that should trigger the unit to turn on in this state is the standby pushbutton. Bringing the unit up slowly may be confusing the microcontroller and causing unpredictable behavior. You might be hearing the relay in one leg of the mains kick in before the relay in the other leg just based on the two different relay's responses to the same coil voltage.

Anyway, once the micro triggers the unit to turn on it throws those relays and the rest of the supplies come up - B+/- and the ones on the main board. It's definitely possible the B+/- rectifier or caps have failed but I'd bet it's one of the low voltage regulators or associated caps is shot/shorted. Can definitely still blow the fuse.

There's no speaker protection relay in the MA6600.

You should also never have to replace a whole amp board like that, these boards can be diagnosed to the part pretty readily.

Thank you Bob,
Typically I troubleshoot to the component level.
The customer wanted his equipment back ASAP and was given the option of the quick, but more expensive fix, and opted to do so.

I counted five low voltage supplies on the main board. +/-15, 12, two 5's , 3.3

I'm wondering, can I isolate the low voltage supply and only bring up the power rails, given the soft start power button needs low voltage to function.
That way I eliminate one failure path.

 

[tutor_turtle]

The only relays in that unit that aren't small signal relays are the two on the backup power supply. When it's connected to mains the backup supply generates an always on low voltage for the microcontroller which then controls everything else. The only thing that should trigger the unit to turn on in this state is the standby pushbutton. Bringing the unit up slowly may be confusing the microcontroller and causing unpredictable behavior. You might be hearing the relay in one leg of the mains kick in before the relay in the other leg just based on the two different relay's responses to the same coil voltage.

Anyway, once the micro triggers the unit to turn on it throws those relays and the rest of the supplies come up - B+/- and the ones on the main board. It's definitely possible the B+/- rectifier or caps have failed but I'd bet it's one of the low voltage regulators or associated caps is shot/shorted. Can definitely still blow the fuse.

There's no speaker protection relay in the MA6600.

You should also never have to replace a whole amp board like that, these boards can be diagnosed to the part pretty readily.

Bob,

The trace from K1-pin 4 to RT1 pin 2 is 2/3 burned through.
I checked the diodes, regulators and power supply capacitors on a DVM.
Nothing obvious is awry.
I'm out of my comfort zone on this one.

My business is vintage hifi. 1980's or older. The older, the better I like it.
My specialty is tube amplifiers.
I did the last two repairs as a favor and because it was a simple board swap.

I am going to recommend this to a shop that is comfortable at this level. I'm not.
As Clint Eastwood said: "A man's gotta know his limitations"

Normally I would recommend Audio Classics, but the last client I sent their way said that their prices have gone bananas.

Where is your shop located and what is your shop rate?
This gentlemen lives in Hudson Mass.

If he agrees, I'll send him your way.

Best regards, Steven.

 

[Bob the Tech]

Hi, I'll send you a DM.