Fusion FP-505: TIP36C Problem

[theAnonymous1]

I have a Fusion FP-505 5-channel amp I purchased "as-is" from ePay. When I powered the amp on to test it, it would start to draw huge amounts of current and then the protection would kick on.

I traced the problem down to the sub channel that had two shorted TIP36C. After removing the TIP36C the amp would power on normally and the other four channels work fine.

I ordered some TIP36C and hoped I could just drop them in with no problems. No such luck. When the amp is powered on the TIP36C start to heat up immediately. I don't measure any shorts like with the originals, so I have no clue what is causing them to heat.

Here is a pic of the board and a closeup of the TIP36C....

[Perry Babin]

Have you checked the driver transistors for the sub channel?

Are there bad connections on the bias transistor?

If the TIP36s are heating up, the TIP35s should also be heating up unless they're shorted. Are the 35s heating up also?

[theAnonymous1]

I'm not sure how to check the drive transistors beyond checking if they are shorted; which they are not. They are KTB1367/KTD2059.

The soldering on the bias transistor looks and measures fine (assuming the bias transistor is the small one between the TIP36C).

The TIP35C are not shorted and don't heat up at all.

I should probably note that I'm not 100% sure the parts I have are even TIP36C. They came from ePay, so they could be anything. They do measure as PNP transistors with an hFE 132 though.

[Perry Babin]

For the 36s to heat up quickly, they would have to have significant current flowing through them. Generally, the only way high current can pass through them (with no load connected to the amp) is for it to pass through the other output transistors for that channel (the 35s). If the 36s are heating up but the 35s are not, the 35s are shorted, are being driven on 100% (no voltage drop from collector to emitter) or the current is flowing through another path (wire/solder bridge is common).

If you have a current limiting resistor for the B+ line, power the amp up through it and measure the DC voltage across the emitter resistors for the TIP35s and the TIP36s. Do you have the same voltage across both sets of emitter resistors?

The voltage will be very low (probably less than 0.05v) so you need to get as accurate readings as possible.

Did you check the 35s with all possible lead configurations? Transistors don't always short collector to emitter.

[pinyoro]

To me it would seem as if you fitted 2 Tip 36C on the same channel, is this correct? (Going by the picture)

Originally posted by theAnonymous1
I have a Fusion FP-505 5-channel amp I purchased "as-is" from ePay. When I powered the amp on to test it, it would start to draw huge amounts of current and then the protection would kick on.

I traced the problem down to the sub channel that had two shorted TIP36C. After removing the TIP36C the amp would power on normally and the other four channels work fine.

I ordered some TIP36C and hoped I could just drop them in with no problems. No such luck. When the amp is powered on the TIP36C start to heat up immediately. I don't measure any shorts like with the originals, so I have no clue what is causing them to heat.

Here is a pic of the board and a closeup of the TIP36C....

[ppia600]

/\ What he said

[Perry Babin]

If I'm not mistaken, there are 4 transistors in the sub channel. The two TIP35s (or equivalent) are on the other side.

[ppia600]

Duh, I was counting the pairs incorrectly. Four pairs of two and one channel with four. Yeah the sub channel does need more oomph. If its not the drivers for the 36's maybe something in the preamp section then? So that would mean the two transistors directly below the ones he replaced (on the opposite side of the board) would their complements then, right? It also looks like each pair of tip's on the sub channel use one larger driver transistor compared to the smalle ones on the other four channels.

[luka]

Hi

I don't know if preamp can draw or cause to draw a lot of current, some short on board is still possible. You can check and post voltages from every pin on preamp IC too

[theAnonymous1]

Thanks for the replies guys.

To me it would seem as if you fitted 2 Tip 36C on the same channel, is this correct?

They are on the same side, but that is because the sub channel uses two parallel devices. The other channels use single D718/B688.

If the 36s are heating up but the 35s are not, the 35s are shorted, are being driven on 100% (no voltage drop from collector to emitter) or the current is flowing through another path (wire/solder bridge is common).

I measured the C-E voltage of both the TIP35 and TIP36. For the TIP36 I measured 7.77v and for the TIP35 only 0.003v. When the amp is powered off there is a residual voltage of ~5.5v across C-E of the TIP35.

If you have a current limiting resistor for the B+ line, power the amp up through it and measure the DC voltage across the emitter resistors for the TIP35s and the TIP36s. Do you have the same voltage across both sets of emitter resistors?

I tried a 20R and 5R current limiting resistor on the B+ line. With the 20R all of the emitter resistors measured 0.001v across them, and with the 5R they measured 0.018v.

I'm not measuring any shorts on the TIP35, but I will try removing them and see what happens.