Treadmill blows fuses

[newtothis]

Hi, I recently bought a broken treadmill on a wimp - it didn't power on one day - and thought i could fix it [emoji4] - I couldn't but maybe you can help ?

When power was added my house fuse 10A blow - after changing  the house fuse (to a 13A) the internal fuse (10A) keeps blowing immidiately on the PCB even with everything disconnected.

What i checked myself.
Im pretty sure there is a short somewhere as there is only 2ohm restance  between the 2 AC inlets on the board - and I Think it must be a component as the board seems good I pretty much tested all components.

There is a varistor on the board which is desoldered and checked (>megaohm resistance)

a NTC thermistor in series with the AC fase is 2 ohm at ambient temp this is the source of the resistance between the 2 inlets the NTC is oone of the first components after the fuse.

There is 2 bridge rectifiers one for the motors and one for the control.
They seem to be in order with diode check.

Ac fase to positive = beeps
Ac fase to negative = no beep
AC2 to pos. = beep
Ac2 to neg = no beep
Pos to neg = no beep
Neg to pos = beep

Relay seems to be NO and not fused.
Capacitor is not a short
Resistor is the expected range.

I would gues that the short should be at or before the bridge rectifiers but i do't know what to check anymore

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[james_s]

Usually there is a triac or SCR that controls the motor, it could be mosfets, I would check those next. Actually just check every semiconductor you can find. If you really get stuck you could try connecting it to a current limited DC power supply set to a couple hundred mA and poke around measuring voltage, the resistance of the traces can be enough to tell where the short is.

[Assafl]

There is typically a safety SCR that shorts the motor to brake it (when the safety latch is pulled the belt brakes immediately). These fail shorted so it may be what you are seeing.

[newtothis]

Thank you for the replies - I will test the triacs/SCR as well.

I would like to clarify that the fuse blows when the board is completely disconnected from the motors, control panels, lcd and only the AC connected. etc.

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[newtothis]

Ok so I finally got around to buy a new solder tip (the old one was so bad it hurt) and poked around and finally found and repaired! the issue - the smaller of the bridge rectifier had a full short - desoldered the bridgerectifier and added a new rectifier with 4x1N0007 soldered on the backside. The treadmill works as a charm [emoji4]
 I'm a little uneasy in repairing AC, but i made sure there is no solder bridge etc. and covered everything with hot glue so i hope this insulate [emoji52]

Finding the issue was quite a journey although I actually think it was quite obvious in hindsight [emoji4] anyways you wanted to give an update and thanks for the help

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[Shock]

Good work.  Avoid generic hot glue unless you need to. It's not used for insulation in electronics more to prevent mechanical stress. If you do use it (and especially on power supplies) it's better to get high temp glue designed for the purpose.