planet audio pa2250d problems after replacing p.supply and outputs

[ppia600]

I used all matched date coded parts and replaced the irf power supply fets and the fb31n20d output fets. The amplifer powers on and plays sound but loads the 12v power supply down and the output fets get pretty warm. There's also a low pitched humming noise that plays through the sub I'm using. I checked the gate resistors and they all measure consistently. There do appear to be two transistors on what appears to be the driver board next to the fan that have solder joints darker than the rest. Not sure how to test them on the board and it might be a nightmare for me trying to remove them without damaging the small board. Something on the driver board is probably the culprit though since the fan was squealing and not working well until I lubed it, probably caused something to overheat before the initial failure of the amp.

[Perry Babin]

Transistors Q124 and Q125 have likely failed. When you replace them, replace them with MPSW42s instead of MPSA42s. Also install them well off of the board so that they are closer to the fan blades (see attached).

The inductors are very likely shorted or are going to be shorted. After replacing the driver transistors, twist the inductors while watching the idle current. If there is any change in idle current, they are likely shorted and you'll need to insulate the windings.

[ppia600]

Awesome, thanks as always Perry. I tried messing with the inductors and nothing changed. I do plan on gluing them either way though, don't want this monster coming back lol. I will definately check out those transistors like you say and extend them out closer to the fan. Thanks for the advice on the p/n change, is that for a different case style or higher dissipation?

[Perry Babin]

The larger case has more surface area which can dissipate more heat.

When you power up the amp (after replacing the transistors) monitor the DC voltage on the output between the time remote voltage is applied and the time the red LED goes out. If there is any DC, the board may have become conductive between the pads for those two transistors.

Do the inductors have the fine strands of wire (multiple ~22g in a bundle) or larger strands (single ~14g)?

[ppia600]

They are the thick single strand type, like in your pic. I replaced the multiple fine strand type in a previous D amp (you also helped with) with some single thick strand wire but the cores are a little too thick to fit under the cover now lol. I couldn't find any for sale large enough. That amp works fine but I'm going to have to rewind the cores or modify the bottom panel to raise the amp for clearance.

I will check for DC after swapping the small transistors.

[Perry Babin]

I should have posted this earlier...

Don't try to remove the driver board.

To remove the old transistors, apply enough new solder to the leads of each transistor allow you to heat all 3 legs at once. Lay your iron across all 3 legs and when the solder is hot enough, pull the transistor. This is easier to do with the fan out of the board.

If you have a desoldering pump, heat the side of the board with the most copper (pad and trace, not the side with the pad only) and desolder from the other side of the board.

Clean the area with acetone and cotton swabs to remove all flux. With a bright light behind the board, look for shadows between the pads of the two driver transistors. If it's clear (looks like other transistors), the driver board is OK. If there are extremely dark shadows, the board may be conductive. The attached photo shows what I mean by a dark shadow (on the top/left transistor through-hole). This particular board has some slight discoloration between the pads but that's not a problem. The dark shadow on this board was a problem.

When you reinstall the new transistors, solder the top leg in with only about 1/16" (at the most) protruding through the board. Solder it in place and push the transistor down until it's at the angle you want. Reinstall the fan so you can make sure the transistors won't hit the blades. Then solder the remaining two legs of each transistor.

[ppia600]

Yeah, that is how I removed the original power supply and output fets. Works pretty well. The areas around the drivers are slightly darkened, hopefully not severely affected though. I'm waiting on some w92's now.

[Perry Babin]

W42s?

[ppia600]

Originally Posted by Perry Babin

W42s?

Yeah, oops that's what I meant

[ppia600]

I replaced the a42's with w42's and the amp still does the same thing. Plays through the woofer and doesn't go into protection but still makes a constant humming/buzzing sound and draws almost ten amps for a few watts output. The only area getting warm is the "output" transistor section. I couldn't find any other shorted or overheated components on the board. Also checked for any cracked solder connections but all appeared ok. I don't understand how the amp isn't in protection but is running hot on the output side and drawing a ton of current.

I tried measuring secondary output voltage on the transformers and it was about 25vac from center to outer on either side, no matter what volume the amp is at. The fan also starts out at a normal speed on power up and then drops down to about half speed, I've also already lubed it because it was working very slowly and squeaking after my first repair attempt.