ppi x3 power supply transistors... what are they???

I've got this ppi x3 crossover and the four power supply transistors are fried. They are labeled "Q1,Q2,Q3, and Q3". They were all C9031h and I replaced them with PN2222 type that seemed to be close enough according to specs. I tried powering it on and the new ones fried as well. It belongs to a friend who connected the + and - backwards and fried the protection diode and the four transistors. Now I'm starting to wonder if someone else had already swapped them. I'm also wondering why they are installed in parallel pairs instead of just using two higher current tranistors. Has anyone else encountered this problem on one of thes ppi xovers?
 
ppia600 said:
I've got this ppi x3 crossover and the four power supply transistors are fried. They are labeled "Q1,Q2,Q3, and Q3". They were all C9031h and I replaced them with PN2222 type that seemed to be close enough according to specs. I tried powering it on and the new ones fried as well. It belongs to a friend who connected the + and - backwards and fried the protection diode and the four transistors. Now I'm starting to wonder if someone else had already swapped them. I'm also wondering why they are installed in parallel pairs instead of just using two higher current tranistors. Has anyone else encountered this problem on one of thes ppi xovers?



Could be, or maybe the toroid is shorted. I have seen these little switchers cook the windings on the toroids many times when they get RP'ed at the power plug/ If it is the toroid you will have to rewind it.

It could be the simple 8 pin control chip if it uses one like the big crossover they make does. Also I have found the 9013 to difficult to replace with anything other than that exact part number, and that was a popular part back then. A pic is worth a bunch of relply
 
I assume it is more likely that the small driver - transistors are fried . This results in an "always connecting" state for the power mosfets. Since the transformer is very low in DC resistance the mosfets burn again.

You should be able to follow the curcuit board lines from each gate of the mosfets. Each gate is connected to a small resistor, usually something between 22 and 150 Ohms. Following the resistors you find the resistors are connected to small signal transistors. The transistors are usually operated by the timing control chip (TL494 or an SGxxxx).

If the transistors are burned also replace the resistors following towards the mosfets´ gates, they are likely to be burned as well.
 
Re: Re: ppi x3 power supply transistors... what are they???

1moreamp said:




Could be, or maybe the toroid is shorted. I have seen these little switchers cook the windings on the toroids many times when they get RP'ed at the power plug/ If it is the toroid you will have to rewind it.

It could be the simple 8 pin control chip if it uses one like the big crossover they make does. Also I have found the 9013 to difficult to replace with anything other than that exact part number, and that was a popular part back then. A pic is worth a bunch of relply

Oops, I forgot to add the pic:

[IMGDEAD]https://i66.photobucket.com/albums/h246/2slofouru/P2120010.jpg[/IMGDEAD]

You can see they are both sitting in a little grease, there was only the sil pad when I got it. I would be amazed that those wimpy transistors could short that transformer but I'll check it. I guess the driver chip could be clamping them on so they overheat. I probably have several of those in broken amps I could swap one in. Otherwise I'll have to order one as I don't feel like paying an arm and a leg at a local supply store.
 
Does it blow the transistors with the audio board disconnected?

Does it blow the transistors if you disconnect the secondary windings of the transformer (center tap can remain soldered in)?

Are both of the 1 ohm resistors in tolerance?

With the transistors out of the circuit, check the DC voltage on pns 9 and 10 of the 494. It should be ~5-6v.

If your multimeter has a frequency counter, measure the frequency on pin 9 or 10. It should be relatively high (~80-100k).

If you don't read 5-6v DC on pins 9 and 10, post the voltage on each of the pins.

494
Pin 1:
Pin 2:
Pin 3:
Pin 4:
Pin 5:
Pin 6:
Pin 7:
Pin 8:
Pin 9:
Pin 10:
Pin 11:
Pin 12:
Pin 13:
Pin 14:
Pin 15:
Pin 16:
 
On many of these PS units that use any drivers on the output of the TL494. They often seem to use PNP and NPN types in a driver configuration So the 9013 devices may be the wrong part, as the circuit appears to need a NPN and a PNP device to work correctly and drive the toroid correctly. Typical transistors would be a MPSA-06 and MPSA-56 in a totem pole driver circuit

Now I would need to see the bottom of the board so I can draw out that 4 transistor circuit, but I would say that it appear to be a totem pole driver circuit that requires both P and N devices to work properly. PPI might have used a different scheme but only the board circuit can tell me that info

The only other thing I see that could be bad is the TL494. Based on your post that the thing was hooked up backwards to power <RP'ed>
In that case the chip is surely damaged as they usually have no RP protection, and the output driver section of the TL494 usually get fried by RP...LMK what you find :)
 
Thanks a LOT guys, I got it finished last night finally. Npns worked, like the originals. I used some 3904's from radio shack, they even had matched date codes, lol. I used plenty of grease between the heatsink, AND replaced the timer. Apparently something was wrong with the timer as well. I checked every resistor in the circuit and they all were fine. This thing must have good output voltage, the total rail voltage is 24v, not quite 30 like some but pretty good. My friend is going to be so happy today when he picks it up. He has two ppi a600's to run his subs and a ppi four channel for his mids. He's been waiting on this crossover to install everything. Man am I glad its finally working. :)