mtx 4250 problem

[firemedicsafd]

I have an MTX thunder 4250d and it will not work. It turned on but no sound out of the speakers. When i opened it i found these things looking like they were broken. What are they? Where do i get new ones? Any special instructions on how to install replacement parts?

[Perry Babin]

It's difficult to tell from the photo but they appear to be the resistors that are in series with the output transistors. When they fail, it's generally due to shorted output transistors. There may be other damage.

Mouser.com should have them but you need to do a bit of troubleshooting before you place an order.

You need to check the output transistors (those adjacent to the resistors).

When checking the transistors with your meter set to ohms, you should not read anything near 0 ohms when the probes are touched to the terminals of any individual transistor in any configuration. If any are shorted (~0 ohms between any 2 legs of any individual transistor), they will likely be shorted from leg 2 to leg 3 but you should check 1-2, 1-3 and 2-3.

If you can power up the amp, measure the DC voltage on the legs of the output transistors. Place the black meter probe on chassis ground. Place the red meter probe on the point where you need to measure the voltage.

Have the transistors clamped to the heatsink when you have power applied.

[firemedicsafd]

Where could I score a schematic for this amp? Any special multiple meter I need to buy in order to check this amp out? Thank you!

[Perry Babin]

You're unlikely to find a schematic for this amp. If you do, please send me a copy of it.

Virtually any digital multimeter will work but if you can borrow a Fluke (brand) meter, that would likely be better than buying a cheap multimeter (especially if you don't plan on using the meter for anything else).

If you buy a meter, buy a digital meter that has a diode-check function.

[firemedicsafd]

i know how stupid this sounds....but what do the output transistors look like?

[Perry Babin]

The output transistors are the 3-legged devices clamped to the heatsink. They are next to the burned resistors.

[firemedicsafd]

The transistors both show near no resistance across all the legs in any order. Now what do I do?

[Perry Babin]

Desolder/remove them and make sure that there are no solder bridges between the pads of the transistors (could be inadvertently bridged when desoldering).

Then power up the amp (everything clamped to the heatsink and a 10 amp fuse in the B+ line). If it powers up, measure the DC voltage on all of the pads where the output transistors were soldered to the board. What is the highest/lowest voltage you read on any of the pads?

Also do this on both ends of each of the burned resistors.

Place the black meter probe on chassis ground when measuring the voltage.