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Originally posted by J M Fahey
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Thanks Juan! That makes sense as I've had my amplifier working before, plugged it in turned it on and it's working fine and then turn it off and on again and it blows. Is this the same for me MJ2955's? is it worth replace both the NPNs and the PNPs -
IŽd replace all 6 with the VERY robust and "overkill" MJ15022/24 as NPN or MJ15023/25 as PNP and sleep like a baby.
That said,
1) still recheck bias,
2) after that test amp LOUD for some time, heatsinks will get hot but still "touchable by hand".Juan Manuel FaheyComment
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Ordered a couple, will report back when I've got them inOriginally posted by J M Fahey View PostComment
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Go the new transistors in the read this on my tester and DMM (they all test about the same for each)
mj15023g - HFE 96.9 - 0.545V drop
mj15022g - HFE 58.4 - 0.516V drop
The datasheet says the Max HFE is 60 but I think my tester reads an extra 30 or so HFE on PNP transistors for some reason as the chancces of getting 3 bad transistors that read the same is extreamly low. I brought them from RS Online, there the ON semiconductor transistors.Comment
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hfe also varies by current through the part, so your meter does not approximate the real world.Education is what you're left with after you have forgotten what you have learned.Comment
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The Vbe on these PNP's are normally a bit higher than their NPN mates. And the beta (hfe) values are typical Even though they are compliments to each other, when you look at their hfe curves, their Ft curves and others between the NPN's and PNP's, you mind says to you they don't look like they're compliments at all, but, they are. Put them into circuit, have circuit values that compensate for the differences, and they all usually work together in harmony and you got musical current flowing out of the speakers in spite our apprehension of what we don't really understand.Originally posted by wirelessliquid View Post
So, pop 'em in and see what we have.Logic is an organized way of going wrong with confidenceComment
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Q15 E -18.39
Q15 B -15.60
Q15 C -17.12
Q6 E -16.84
Q6 B -17.12
Q6 C -0.1V
Q16 Emitter -29.89V (I think this transistor is in backwards)
Q16 Base -18.82V
Q16 Collector -18.22V
Q7 Emitter -15.54 (Definatly Negative)
Q7 Base -16.12 (Definatly Negative)
Q7 Collector 29.44
Q8 E 16.12
Q8 B 16.70
Q8 C 29.44
Q10 E 16.70
Q10 B 17.12
Q10 C 29.44
Q9 E 16.70
Q9 B 17.12
Q9 C 29.44
Q18 E -17.12
Q18 B -17.62
Q18 C -29.89
Q19 E -17.62
Q19 B -18.21
Q19 C -29.89
Q17 E -17.12
Q17 B -17.62
Q17 C -29.89
Am I right in saying I've put Q16 in upsidown with reversed E C, would this explain why turning R42 has almost no change on -18VComment
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Yes, Q16 is backwards. We've all done that before. But still, the base of Q17 should be the same voltage as Q16 emitter (collector in this case, with it backwards).Originally posted by wirelessliquid View Post
I'm still working from the gif Sunn Alpha Slave Power Board schematic that I downloaded from this thread early on. So, I hope there's not major differences in the circuitry between them.
One thing that is puzzling me is the voltages on Q6 & Q15 current limiter xtrs. Their emitters are common to the output buss, and the buss is, again, sitting at a negative voltage. Q15's B & E are way greater than 0.7V...that part is suspect. Q6 as well, though it's B-E difference is also much greater than 0.7V.
Then, looking at the voltages of Q7 emitter and Q8 base. My schematic shows that is a direct connection, yet there's a huge difference in potential. Same with Q8 emitter voltage and Q9/Q10 base voltages. Typo? Also Q17 emitter and Q18/Q19 base voltages. Those should be the same, as it's a common connection.
Are there open traces on our board? From the insertion/removal of parts multiple times, I've had the trace break from the solder pad in those cases, which causes problems like this. Go back and take a look at the voltage & current chart I made from the schematic I have. You should be very similar to those potentials when everything is installed correctly, and all the traces are intact. There shouldn't be any DC level shift on the output buss as you still have. At least the new outputs and drivers are still good.
Do you have the schematic for the board you're using? Sunn Beta Bass Amp. I should have gone searching for that. I just followed the link in your initial post to the service manual for the Sunn Beta Bass Amp, looked at the power amp schematic, multiple generation losses from copies, but, the schematic does at least look identical, not seeing any added base resistors in the output stage (for RF purposes).Logic is an organized way of going wrong with confidenceComment
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This schem may be cleaner.Originally posted by Enzo
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I'll get Q16 changed round the other way first and I'll take these reading again as I assume they will change somewhat with the transistor in the right way. Hopefully this should allow me to get some change in voltage when moving R42. I'll double check the new readings against the pin out for each transistor make sure I'm not reading any backwards or got any in backwards.
I checked Q15 with my tester after and it showed it to still be functioning.
I think these massive differences must be caused by either broken tracks, I'll have to snoop through these tonight and see If i can spot any. I feel like I'm getting really close to getting this fixed now
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Thanks, g1! yes, cleaner, and actually still the same circuit, so there weren't any surprises.Originally posted by g1 View PostLogic is an organized way of going wrong with confidenceComment
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Getting close to that win now, I think Q15 is also in backwards, whats the verdict?
Nothing got hot just warmed up after 15 minuets like an amp would, IC1 warmed up slowly
I managed to pull the bases Q16 & Q7 down to around +/-1.8V
Tested on +/-30V power supply around 150mA draw
Q1 E 14.21V
Q1 B 14.73V
Q1 C 27.01V
Q2 E -0.737V
Q2 B -0.060V
Q2 C 6.79V
Q3 E -0.717V
Q3 B -0.035V
Q3 C 6.72V
Q4 E 8.46V
Q4 B 9.09V
Q4 C 28.20V
Q5 E 28.84V
Q5 B 28.20V
Q5 C 1.778V
Q6 E 0.002V
Q6 B 0.193V
Q6 C 1.451V
Q7 E 1.185V
Q7 B 1.772V
Q7 C 29.25V
Q8 E 0.600
Q8 B 1.178V
Q8 C 29.25V
Q9 E 0.019V
Q9 B 0.597V
Q9 C 29.25V
Q10 E 0.021V
Q10 B 0.595V
Q10 C 29.25V
Q11 E -14.13V
Q11 B -14.71V
Q11 C -28.91V
Q12 E 5.207V
Q12 B 4.589V
Q12 C -28.73V
Q13 E 1.753V
Q13 B 1.1V
Q13 C -1.75V
Q14 E -29.35V
Q14 B -28.73V
Q14 C -1.753V
Q15 E -1.436V
Q15 B -0.198V
Q15 C 0V
Q16 E -1.14V
Q16 B -1.749V
Q16 C -29.76V
Q17 E -0.585V
Q17 B -1.140V
Q17 C -29.76V
Q18 E -0.018V
Q18 B -0.583V
Q18 C -29.76V
Q19 E -0.001V
Q19 B -0.583V
Q19 C -29.76VComment
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CR2 looks suspicious. The voltage on Q6 collector should be similar in nature to Q15 (collector @ 0V), so it seems like there's leakage thru CR2. Your output buss now looks normal, based on the emitter voltages of Q6 being 0.002V. Q15 may be one part where there's damage to the circuit trace. Q15's emitter should be the same as Q6 emitter, both sitting on the output buss.Originally posted by wirelessliquid View PostGetting close to that win now, I think Q15 is also in backwards, whats the verdict?
Nothing got hot just warmed up after 15 minuets like an amp would, IC1 warmed up slowly
I managed to pull the bases Q16 & Q7 down to around +/-1.8V
Tested on +/-30V power supply around 150mA draw
Q1 E 14.21V
Q1 B 14.73V
Q1 C 27.01V
Q2 E -0.737V
Q2 B -0.060V
Q2 C 6.79V
Q3 E -0.717V
Q3 B -0.035V
Q3 C 6.72V
Q4 E 8.46V
Q4 B 9.09V
Q4 C 28.20V
Q5 E 28.84V
Q5 B 28.20V
Q5 C 1.778V
Q6 E 0.002V
Q6 B 0.193V
Q6 C 1.451V
Q7 E 1.185V
Q7 B 1.772V
Q7 C 29.25V
Q8 E 0.600
Q8 B 1.178V
Q8 C 29.25V
Q9 E 0.019V
Q9 B 0.597V
Q9 C 29.25V
Q10 E 0.021V
Q10 B 0.595V
Q10 C 29.25V
Q11 E -14.13V
Q11 B -14.71V
Q11 C -28.91V
Q12 E 5.207V
Q12 B 4.589V
Q12 C -28.73V
Q13 E 1.753V
Q13 B 1.1V
Q13 C -1.75V
Q14 E -29.35V
Q14 B -28.73V
Q14 C -1.753V
Q15 E -1.436V
Q15 B -0.198V
Q15 C 0V
Q16 E -1.14V
Q16 B -1.749V
Q16 C -29.76V
Q17 E -0.585V
Q17 B -1.140V
Q17 C -29.76V
Q18 E -0.018V
Q18 B -0.583V
Q18 C -29.76V
Q19 E -0.001V
Q19 B -0.583V
Q19 C -29.76V
Apart from that, you've got an amplifier again! Looking at the emitter voltages of the two pairs of output xtrs, Q9, Q10 and Q18 are partially conducting, while Q19 hasn't yet started. Advancing the bias control R42 a bit should get current flowing thru Q19 as well. At present, you're running between 18-20mV across the 0.33 ohm resistors, about 61mA. See what it takes to get Q19 to conduct (assuming it's not a mis-reading). You're at that threshold of where the bias adjustment will show a sudden increase in your supply current on your lab supplies, so you're happening! Great work!!Logic is an organized way of going wrong with confidenceComment
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I don't think it would be CR2 that's the culprit but thereno harm in checking. The emitter on Q6 is attached the the output buss @ 0V which is a direct connection to the emitter on Q15 which is at -1.4V. the collector on Q15 is 0V which is opposite to the collector of Q6 @ 1.4V. I think Q15 also has its CE reversed. the collector of Q15 should be 0.4V to 0.7V lower than the base of Q16.
Time to put the Ossiloscope into use to get the perfect bias, I do need to clean the output on my func gen. Little adjustment and I should get the 1mV over 0.33Ohm resistor.
I'll be checking the pre amp board properly next to make sure nothings getting hot and see if I can get some sound out of my amp
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