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....
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....
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
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.
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.
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.
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.
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.
Thanks for the replies guys.
They are on the same side, but that is because the sub channel uses two parallel devices. The other channels use single D718/B688.
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.
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.
They are on the same side, but that is because the sub channel uses two parallel devices. The other channels use single D718/B688.
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.
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.
OK, well I guess the problem WAS caused from a solder bridge all along. It was so tiny I didn't notice it at first. I only saw it when I got in real close to desolder the TIP35.
This tiny spec was definitely not from me, so I don't know how sub channel ever worked at all with it being there. The bridge connected the base of one of the TIP35 to the positive rail of the 4-channel amp section power supply.
The sub channel appears to be working fine now. There is only 14mv offset on the output. I just need to reconnect the preamp board and run a signal through it now.
Thanks again everyone; especially Perry.
This tiny spec was definitely not from me, so I don't know how sub channel ever worked at all with it being there. The bridge connected the base of one of the TIP35 to the positive rail of the 4-channel amp section power supply.
The sub channel appears to be working fine now. There is only 14mv offset on the output. I just need to reconnect the preamp board and run a signal through it now.
Thanks again everyone; especially Perry.
An externally hosted image should be here but it was not working when we last tested it.
ppia600 said:
I don't think so. It didn't look like it was ever taken apart before. The seller said he was the original owner and bought it new ~2 years ago. He said it started going into protect recently so he put it up for sale.
The solder bridge was also making the positive rail of the sub channel supply high at 53v somehow. I had measured this before I found the bridge. Now both rails measure 47v.
Next time I try to repair an amp I will be sure to look twice for even the smallest solder bridges.
I Am An Idiot said:
I would believe that if it weren't for the super clear photo he provided. If you look closely it appears there is a huge glob of solder melted onto the top of the original solder on the board, it looks as some novice at soldering was trying to add solder to make the connection better somehow and the curve on the top looks to be where they were trying to smooth it down a little. If it was indeed mounted in a vertical fashion as the photo appears, the solder wouldn't have expanded toward the direction of the camera lens and then trickled down just enough to touch the other leg. You can actually see flux in between the original solder mass and the second glob applied to the top, mainly on the left in the picture. Just because the seller told you he was the original owner or that it wasn't repaired before doesn't mean its true.
Think I saw that amp on there but did not know much about them. The guts look similar to some others I have seen.
I have found a lot of solder bridges in cheap amps that have hand soldering. I swear some of them have a pound of solder on the board. Does not seem like it would have worked with it there when new so something put it there.
I have found a lot of solder bridges in cheap amps that have hand soldering. I swear some of them have a pound of solder on the board. Does not seem like it would have worked with it there when new so something put it there.
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