RF T1001BD toast

[triniphen]

hello, im new to the forum. ive got a RF amp that dead shorted the outputs due to a small dust screen dropping onto a high wattage resistor and grounding out. unit wont power up and when trying to do so, 2 of a group of 5 of 330uf/63v caps gets very warm and one actually buzzes at me. im guessing this is on the output side but im not sure. any advice? thx.

[Perry Babin]

Have you pulled the shorted transistors out of the circuit?

[triniphen]

not yet i wanted a bit of advice first.

[Perry Babin]

Rockford needs to fix this problem. this is not the first screen to fall into an amplifier. If you complain enough, they may cover it under warranty.

Normally, when an amp fails, troubleshooting is relatively simple because you know where the fault is (in the case of output transistor or power supply failure). When the screens fall in, they can short the rail supply to the protection or preamp circuits and troubleshooting can be very difficult.

[triniphen]

unfortunately, the amp was bought from an 'unauthorized' dealer so warranty is nonexistent. as for the failure, there are'nt any signs of anything overheating w.r.t any transistor on the board, no weird solder joints of flux bubbles, etc. i thought i would replace the caps and bring the amp up on a limited supply. i could use some advice on checking the transistors before so, or if bringing it up at this point would be a mistake.

[Perry Babin]

Check the output transistors for shorts. Set your meter to ohms. Measure the resistance between the legs of each individual transistor. If you measure anything near 0 ohms between any two legs, the transistor is likely defective.


The caps are likely OK unless swolen or leaking.

[triniphen]

finally got all the parts ... (4) IRFP3415 and some caps. the old caps may be ok but for a few bucks id rather avoid dielectric deterioration. as for the mosfets, all 4 of them seem to be soldered or stuck to a common then substrate which is then screwed to the hinksink/case. even though they're fried i dont want to break the substrate. any thoughts.

[Perry Babin]

You have to use a butane torch to remove them. I use the one that Radio Shack sells.

Remove the board from the sink and clean all of the old heatsink compound off of the back of the insulator. After ~1 minute of heating the back of the insulator, the transistor will pull off of the insulator. Just before it releases, you'll be able to see the residue of flux begin to bubble along the edges of the transistor. Do not use excessive force. When the solder is completely melted, the transistor will pull away easily. Do NOT let it slip when you are pulling it away from the insulator. If you do, it will slap back onto the insulator and throw solder in all directions. You must wear safety glasses when doing this.

Try to concentrate the heat directly behind the transistor. You don't want to cause anything else to come unsoldered. There is likely to be a tiny thermistor on the insulator. You will have to be careful not to damage it. In the attached photo, you can see a thermistor on the right of the transistor. On the left, you can see a 3 terminal connector. Make sure it remains soldered to the insulator. This is in the power supply section but you'll find the same thing in the audio section.

[triniphen]

thats what i wound up doing, torch style. a bit of heat and off they came. i examined the substrate carefully beforehand, no surface mount components. they went back on just as easily, and aligned well. popped them back in with the new caps and the amp came back like a champ. benched it at 12.3v using a 300mv signal (50, 60, and 80 hz) to get a clean sine. i was using a dummy load so i was conservative and only went up to about 33v.