Mackie 1400i hot light on

[Original Burnedfingers]

I got a Mackie for free that has a hot light on and high fan. Never worked on one before. The amp won't pass signal in this condition. No blown fuses no shorted outputs.

Any ideas?

[ingenieus]

Check the bias stage. It might have a fault causing it to go into Class A mode.

[riotpack]

From the service manual.

1) "The hot LED lights to indicate that the amplifier has engaged protection due to heat sink or power transformer core temp"

2) "Extreme heat sink or transformer core temperature will activate OVER TEMP. U5A activates over temp if the LM35DZ detects temperature above 80 deg C.
Feedback resistor R137 rescales U5A voltage reference so that the heatsink must cool below 50 deg C before overtemp is deactivated. Overtemp can also be activated by the power transformer thermal breaker (via Q89). In either case, both channels are muted via D69 and D66."

Look for a fault around these components.

[Original Burnedfingers]

The hot light is on immediately when the amp is turned on. No heat build up on the heat sinks per my infrared thermometer.

[Zero Cool]

I have worked on many of these. look for open resistors. they wont show any signs of failure but when you measure them, they will be open! also look for cracked solder joints. front left corner of the board if memory serves.

[Original Burnedfingers]

Are you talking the main board? or the small board on the other side of the heat sink that the fan is plugged into?

[riotpack]

I realise the hot light is on immediately when it is not hot. My post above indicates which components may be triggering the fault. For instance check for dry joints around the LM35, check the base voltage on Q89 to see if the power transformer overtemp sensor is faulty etc etc. It is either coming from the power transformer sensor, the heatsink sensor (lm35 on main HS, or dry joint around there) or the opamp which triggers the overtemp. Test R205 to see if it is to spec and test Q91 also. Post some photos of the board (even the small ne) if you like.

[Original Burnedfingers]

Taking it apart right now. Will take some pictures of the board to post.

[Original Burnedfingers]

Found R205 on the small board on the other side of the amplifier board. R205 was open so I replaced it. I checked the transistors on the board and they tested good checked remaining resistors and they were good. Plugged it in and noticed the transistors Q89,90, and 91 area were hot. Amp is still in overtemp state. Have not found the LM35DZ yet and will check R137

[Zero Cool]

yup small board! and I always find a problem on the main board too but i cant think of it at the moment. keep going you are on the right track!