|
|
Post by danno27 on Jun 18, 2015 12:19:00 GMT -5
The amp I have is working except one channel. I bought this on eBay knowing this, figured it could be fixed since it is all capacitors and transistors and are all replaceable… I’m majoring in electronics technology, going back in the fall.. And I thought it may be something I could fix, but I need to ask somebody who knows what their doing so I don’t go flying across the room. Channel 2 has two blown fuses. I could replace the fuses but I figure, they had to blow for some reason. Some bad resistors maybe? That cap in the middle looks like it has some brown glue on it, I thought it was discolored. None of them are bulging. I can’t see any part of the board that is burnt.   |
|
|
|
Post by mastertech on Jun 18, 2015 12:37:35 GMT -5
Hi Dan and welcome. I just checked and now have the service manual for your unit. I have to run out for a couple hours so when i get back i will look it over and see what i think and post back here. Later.  |
|
|
|
Post by gjaky on Jun 18, 2015 15:22:00 GMT -5
Grab your DMM put it in Diode tester mode. Check if Q11 through Q18 transistors are shorted? If you are unsure how to do this, we can help.
|
|
|
|
Post by mastertech on Jun 19, 2015 14:42:08 GMT -5
Well i would think that all the voltages should be ok since the other channels work fine. If there are not individual line supplies per channel. The manual is not real clear on this.
I wonder if maybe there is something wrong with the relay for that channel and maybe it is not latching on for that channel speaker output.
|
|
|
|
Post by gjaky on Jun 20, 2015 5:25:58 GMT -5
Well i would think that all the voltages should be ok since the other channels work fine. If there are not individual line supplies per channel. The manual is not real clear on this. I wonder if maybe there is something wrong with the relay for that channel and maybe it is not latching on for that channel speaker output. But why would that blow the fuses? I am guessing that one or more output transistors are shorted. |
|
|
|
Post by mastertech on Jun 20, 2015 8:52:02 GMT -5
Well i would think that all the voltages should be ok since the other channels work fine. If there are not individual line supplies per channel. The manual is not real clear on this. I wonder if maybe there is something wrong with the relay for that channel and maybe it is not latching on for that channel speaker output. But why would that blow the fuses? I am guessing that one or more output transistors are shorted. I am not even seeing those fuses in the schematics I have. Am I missing them or is this schematic just crap? |
|
|
|
Post by mastertech on Jun 20, 2015 9:05:02 GMT -5
I guess if I were to make some assumptions (I don't like assuming with electronics), since those fuses are mounted next to JP1A and "if" they were fusing those 2 lines then they would be in the 48v AC lines which are then converted to the +/-63v lines that power the amp. So then, searching the amp section would be the place to start. If all assumptions were correct.
|
|
|
|
Post by gjaky on Jun 20, 2015 9:40:33 GMT -5
But why would that blow the fuses? I am guessing that one or more output transistors are shorted. I am not even seeing those fuses in the schematics I have. Am I missing them or is this schematic just crap? Yeah I don't see them either in mine, but I also think they are fusing the AC lines (5A seems to be enough for that), maybe the rectifier bridge itself can be checked. But we don't have any follow up... |
|
|
|
Post by danno27 on Jun 20, 2015 10:38:22 GMT -5
Grab your DMM put it in Diode tester mode. Check if Q11 through Q18 transistors are shorted? If you are unsure how to do this, we can help. Interesting... Yeah, don't know how to do this, didn't learn this yet. I'll have electronic circuits this up coming semester. I hope we get into more stuff like this. I have a handheld multimeter. I do this with the unit powered on? Or do i need a bench supply to supply to just the module itself? Bear with me. I'm sorry it took me so long to reply, I was at my sister's place and I was traveling all day yesterday and didn't have time to go on my computer. |
|
|
|
Post by danno27 on Jun 20, 2015 11:04:35 GMT -5
Well i would think that all the voltages should be ok since the other channels work fine. If there are not individual line supplies per channel. The manual is not real clear on this. I wonder if maybe there is something wrong with the relay for that channel and maybe it is not latching on for that channel speaker output. Interesting you should mention the relay, I've read on another forum while googleing possible solutions, and a guy replaced the relay and it fixed the nonworking channel. His had two non-working channels if I remember right. |
|
|
|
Post by mastertech on Jun 20, 2015 12:00:10 GMT -5
I agree with gjaky though that if the fuses are blown then there is a good possibility there is another problem and it would not be the relay.
Testing these components would be with power "OFF" and unit "UNPLUGGED".
Set DMM to diode setting and touch the 2 probes together and your meter should then read 0.001 roughly. This would be a "short" reading.
Then, on the amp board with the blown fuses look for any 3 leg components. Then with your probes test each component moving your probes to different legs in different directions and see if any of them read close to a "short" reading.
You can just remove those blown fuses for now and do not need to put anything in there yet. You can also pull that amp section out to get at the components easier if it comes out separately.
|
|
|
|
Post by danno27 on Jun 20, 2015 13:35:52 GMT -5
OK, I measured readings on outer pins for every transistor. Output Q12, Q11, Q14, Q18, Q17, Q16 gave me a beep on my meter.
Readings:
Q11=002
Q12=022
Q13=613
Q14=023
Q15=615
Q16=020
Q17=020
Q18=000
I don't know what these numbers mean though, I'm guessing they're all bad? Except 13 and 15?
Could a surge have damaged all of these transistors?
|
|
|
|
Post by gjaky on Jun 20, 2015 14:01:24 GMT -5
There are more measurements (as mastertech instructed as well) to do on Q13 and Q15 to decide if they are good. Not neccesarily all the rest six transistors are bad... Q13 and Q15 are the so called drivers, if they are good the chances are high the problem is only narrowed to the six transistors we mentioned. Three-threee of those are connected parallelly so if one behaves as a short circuit you measure the others as a short too. In a later step you'd have to desolder these transistors, to check which one is good and which one is bad, without anything other connected to them. Lesson #1  Check the image below, there are two kinds of bipolar transistors, NPN and PNP, these are used in your apmlifier as well. As you already saw the transistor have three legs, while it is not correct but for error finding we can assume a transistor behaves like two diodes connected in (anti)series. Measuring a diode on your DMM (in diode mode) should give a reading around 600, significantly lower or larger value may indicate a fault.  To fully discover the transistor's functionality you have to measure all three legs in all iterations: transistor pin1-2 with DMM's red-black lead, then pin 1-2 with DMM's black-red lead (note the sequence); pin1-3 with re-black, then pin 1-3 with black-red and so on. give us all these iterations for Q13 and Q15! |
|
|
|
Post by danno27 on Jun 22, 2015 10:13:36 GMT -5
Okay, I desoldered and tested these transistors over the weekend.
Q13:
1-2:R-B=1
1-2:B-R=599
1-3:R-B=1
1-3:B-R=605
Q15:
1-2:R-B=627
1-2:B-R=1
1-3:R-B=630
1-3:B-R=1
These I noticed were all the same, so I have them tapped on a piece of paper with measurements below each one...
Hope I measured them the right way with the silicone and writing on each chip facing me.
Q11-Q12, Q14, Q16-Q18:
1-2:R-B=1
1-2:B-R=561
1-3:R-B=1
1-3:B-R=565
1-2:R-B=000
1-2:B-R=000
1-3:R-B=000
1-3:B-R=000
1-2:R-B=001
1-2:B-R=001
1-3:R-B=001
1-3:B-R=001
1-2:R-B=1
1-2:B-R=548
1-3:R-B=1
1-3:B-R=554
1-2:R-B=584
1-2:B-R=1
1-3:R-B=586
1-3:B-R=1
1-2:R-B=587
1-2:B-R=1
1-3:R-B=588
1-3:B-R=1
|
|
|
|
Post by mastertech on Jun 22, 2015 10:15:04 GMT -5
What do you think when you see these readings?
|
|
