MTX Thunder 1501D

[spooney]

A buddy gave me his 1501d to work on and says that it will not power on.I got it home checked out the fets and haven't found anything to be bad.I decided to try and power it up through a 3 amp fuse.The amplifiers power light came on for a moment and then the fuse blew.Is it possible that my fuse is too small?Does this amp pull more amperage than that upon turn on?

[Perry Babin]

You may need to use a 15 amp fuse to power up this amp. The transistors must be clamped down tightly to prevent damage in case there is another problem.

There is an LED in the center of the rail caps. Do not handle the board until the LED has gone completely dark. The rail capacitors store a lot of energy and can be dangerous. If it takes more than 20 seconds for the LED to go dark, the discharge circuit may be defective. In that case, you will have to repair the discharge circuit or discharge the capacitors each time you remove power from the amp.

[spooney]

i just noticed near the preamp section that there is a blown open trace going from smd resistor R115 and D199.Also D 198 is blackened.Would this have anything to do with turn on problems?

[Perry Babin]

It's not likely that those are causing any problems at this point. They are connected between chassiis ground and shield ground. The diode with the striped end connected to the shield burns when the chassis ground for the amp is lost and the amp tries to ground through the RCA shields. In the later model amps, they left that diode out of the circuit to prevent this from happening.

If the diodes are shorted, you'll have to remove them to determine which pads are connected to the shield.

If you get it working, you need to solder the shield on the RCA jack to the RCA jack housing/frame. They get loose and destroy speakers when the shield ground is lost (due to the bad connection between the shield and the frame).

[spooney]

I just finally got around to powering this thing up with a larger fuse.Using a 10 amp fuse i powered it up and it did not blow but the amp will not turn on and stay on.It turns on for a second then turns off then turns on again and makes a humming noise and turns off.After it repeats that a couple times the led in the center of the rail caps turns on followed by another led thats next to all the rail caps that blinks while the amp makes a sound almost like a warning buzzer of some sort.Never seen anything like this.It takes quite a while for the led in the center of the rail caps to go completely dark as well.

[Perry Babin]

Is the voltage on your DC power supply dropping significantly when the amp tries to turn on?

LED238 will illuminate as the amplifier comes out of muting and the drive circuit is activated. If it senses a problem like over-current, the amp will go into protect and the LED will go off.

Is there any DC across the positive an negative speaker terminals before LED238 comes on?

[spooney]

I just jumped back into this thing for the first time in a long time.I recently got a power supply so I decided to try to power this thing up again.Upon applying remote power this thing maxed out my 25 amp supply and sent it into protect for a split second.It killed a 15 amp fuse instantly and a 20 amp fuse just barely survived.After the inital inrush of current the amp settles back down to around one amp until the it comes out of muting and then it sits right around five amps with no load or signal attached .The amplifier does remain powered up through the whole process but the transformer(or something near it) is making a horrible humming noise.I was looking closer today at the power input caps ( 35 volt 8200 mfd) and I noticed that plastic sleeve on them has burnt through to the metal on teo of the three.There are also some small square yellow caps near the power input caps that looked blackened as well.Is it possible that these caps are shorted and or out of tolerance and causing the spike in amperage at power up?

[spooney]

heres a couple pics of the bad caps: http://www.putfile.com/apeboy187/images/169401

[Perry Babin]

The damage to the caps appears to be superficial. The yellow caps are blackened from the failed power supply FETs.

These amps draw a lot of current on startup. 5 amps is more idle current than I'd expect.

Do the inductors near the speaker terminals get hot quickly?

Do any of the caps near the speaker terminals get hot?

Do any of the heatsink mounted components produce enough heat to make the heatsink get warm or hot if you leave it powered up for 10-15 minutes?

[spooney]

So the power supply fets are bad?I didn't get any shorts on them.What other signs besides those bad yellow caps would I be able to find.I will power the amp back up and see if it gets hot.