When the transistor shorted, it increased the load on the amp's supply and the 12v supply. I wouldn't be overly concerned about it at this point.
If the output was not hot when it failed, they may not be reliable. Again, I think you need to pop one open.
Did any of the originals fail or did one simply have broken legs?
If the output was not hot when it failed, they may not be reliable. Again, I think you need to pop one open.
Did any of the originals fail or did one simply have broken legs?
Would sound still work if the transistor had already shorted? I was still getting audio out of the subwoofer and each time a hard bass note hit the transformer/inductor simultaneously made the whine/hiss. Then it began to make the sound more and more for a few seconds until the amp died.
Orignals didn't fail. Simply a broken leg on one original TIP142T. At one point, I considered putting a piece of wire to reinstall it, but instead decided to just buy a new set of transistors.
Orignals didn't fail. Simply a broken leg on one original TIP142T. At one point, I considered putting a piece of wire to reinstall it, but instead decided to just buy a new set of transistors.
If you can't find any good tip142s, you may have to do that.
You could also try the TIP102/107. They're rated a bit lower but may work well enough. I think this amp has more than enough output transistors to be reliable with TIP102/107.
Someone should comment if they know that the TIP102/107 wouldn't work as a sub.
You could also try the TIP102/107. They're rated a bit lower but may work well enough. I think this amp has more than enough output transistors to be reliable with TIP102/107.
Someone should comment if they know that the TIP102/107 wouldn't work as a sub.
Ok will do. In the meantime I ran another test I reinstalled the new transistors and slowly raised volume. With very low volume I tested the B+ voltage and it was at 11.9v. At the first moment I heard the the transformers squeaking it was low volume still. I then raised it 2 notches at which point I noticed the voltage drop and fluctuate to as low as 10.5v. This was still a couple notches below the max volume I listened earlier. Could it be that the power supply or the wire from the PS to the amp can’t handle it and that caused an output transistor to short? When would the voltage on B+ be considered too low?
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Using my function generator I fed a 60hz signal to the amp and found the exact volume point where the transmormer squeaks. It’s at 3Vpp. Also I let it run a few seconds and measured the temp on the output transistors and they were climbing. I turned it off at 100F. Wasn’t certain if I could let them go higher. Datasheet states storage temp max at 150C. Is that how high it can heat up? Also the voltage on B+ only dropped to 11.5v at this volume.
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I decided to buy individual heatsinks for them. I tested it again and noticed that one transistor gets much hotter than the others. I let it run until it reached 150F. The others were all below 100F. I pushed the volume higher on this test than yesterday. Is this normal behavior? I may try the original transistors to compare.
It appears to be random but mostly one transistor heats up. One test two heated up. But it’s not tied to a location or transistor or even tied to original versus new. I swapped out the new for the originals and same random transistor heats up. At first I thought it could be bad soldering on the one heating up so I applied more solder to those but it would then continue to randomly pick one Q2-Q6 transistor. I can’t tell if any others are heating up since they are under the board on the heatsink. I stop the test when a transistor reaches 150F which takes about 2-3mins of playing a 50hz sinewave at 4Vpp and the load is 1.5ohms.
What I think is likely happening is that one starts to heat up and then, it's threshold decreases and it starts to pass more current. At that point, it starts to avalanche and overheats. When all are on the same heatsink, their temperatures are kept approximately the same so they tend to share current more evenly.
Maybe that’s what happened to the new transistor that shorted. I have noticed they cool down pretty quickly after the amp is shutdown. So maybe when I took the temps it was too late. Therefore, should I install the new ones on the pcb and heatsink and see how they perform? Are there any tests I can run with them hardwired to see if they are performing to spec prior to installing them?
I thought of another test. I applied a house fan onto the amp and heatsinks while playing music and the 50hz tone. It stayed cool and played for 10mins without an increase in temp. Q2-Q6 were all room temp. I then tried it with the new transistors and one got warmer than the others but not hot. It was 110F while the others were 85F. I even pushed the volume higher for one additional minute and the new transistors handled it