3.6k very hot!

The 3.6k cement 5watt resistor in my MC50 is very, very hot! Is this normal? Thanks.

Chris

Searing75 said:

The 3.6k cement 5watt resistor in my MC50 is very, very hot! Is this normal? Thanks.

Chris

Click to expand...

I know nothing about that unit, but it might be right. The 5 watter on my Marantz 2230 PS board runs very hot, but it's not dissipating 5 watts. It's simple to calculate how much power your resistor is dissipating.

If you have the space, you could substitute a 10 watter. Either way, you can elevate the resistor off the board (if it's on a board) to let it breathe and get it away from nearby components.

It was expected to get warm....if I remember right the voltage drop across it is 38 volts or so.

Is the amp misbehaving? Have you had it bench tested?

c_dk said:

It was expected to get warm....if I remember right the voltage drop across it is 38 volts or so.

Is the amp misbehaving? Have you had it bench tested?

Click to expand...

With 38 volts dropping it should be barely warm - 0.5W dissipation. Measure a voltage drop on it. Then calculate power like P=U^2/R
It should be 3W or less.

There you go. My 5 watter gets hot, and it's dissipating less than 1 watt. I think the only concern would be heating nearby components.

The amp is running great! I can never leave well enough alone though.

I will attempt to learn how to measure how much wattage it is dissipating. :scratch2:

Thanks.

Chris

Searing75 said:

The amp is running great! I can never leave well enough alone though.

I will attempt to learn how to measure how much wattage it is dissipating. :scratch2:

Thanks.

Chris

Click to expand...

Those resistors run hot.........best to move them away from the chassis.....

Nothing unusual about that.....

Now put the bottom on it and play some guitar.....:thmbsp:

If you can raise it up enough, maybe you can stick a heat sink underneath it?

Without a bench test we are only speculating what hot verses warm is. It is possible the pre driver is failing drawing excessive current.

Searing75 said:

The 3.6k cement 5watt resistor in my MC50 is very, very hot! Is this normal? Thanks.

Chris

Click to expand...

How many DC volt do you have between ground and each end of this 3,6K resitor ? :scratch2:

On very early MC250 the 3.6K was a 10W mounted between 2 terminal strips, it got slightly warm. In later designs including the 2100, 2105, and 2300 it was mounted on the circuit board where it eventually will cook the board to a toasty brown, just make sure it is mounted off of the board and let it get hot, never have seen one fail.

Thanks all! Mounted it high off the board. It's hot, but the amp has run for many hours of playing guitar through it, and no issues. I may try a 10 watt, if it fits, to see if i can bring the temp down for peace of mind. Thanks again.

Chris

Searing75 said:

The amp is running great! I can never leave well enough alone though.

I will attempt to learn how to measure how much wattage it is dissipating. :scratch2:

Thanks.

Chris

Click to expand...

Use a meter to measure the DC voltage across the 3.6K resistor. Then use the following formulas to calculate the power dissipated by the resistor.

Voltage/Resistance = Current (e.g. 80/3600 = .022 amperes rounded off)

Power = Voltage x Current (e.g., 80 x .022 = 1.76 Watts), or

Power = Current x Current x Resistance (e.g., .022 x .022 x 3600 = 1.74 Watts)

So with this example, the dissipation is roughly 1.75 Watts. The differences above are simply due to small rounding off errors.

A 10 Watt resistor will not produce less heat (dissipation is dissipation). However since it is physically larger, the same power dissipation is spread over a larger area and thus the heat is less concentrated in a particular area of the circuit board.

Now, if your voltages are out of specification (e.g., too high...check the service manual), that is a different matter.