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Post by lee on Dec 13, 2023 20:54:08 GMT -6
Does anyone have any advice for locating a short on a DIY preamp? I have a mildly smoking 470µF cap, so that might be close to the area of the short, but I'm afraid to power the thing in order to read VDC because of it.
Either way, I have a good, working example to test and compare if that is helpful. Does it make sense to measure the components first? Or find continuity in far away areas of the PCB? I'm not quite sure where to begin.
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Post by audiospecific on Dec 13, 2023 21:34:08 GMT -6
Does anyone have any advice for locating a short on a DIY preamp? I have a mildly smoking 470µF cap, so that might be close to the area of the short, but I'm afraid to power the thing in order to read VDC because of it. Either way, I have a good, working example to test and compare if that is helpful. Does it make sense to measure the components first? Or find continuity in far away areas of the PCB? I'm not quite sure where to begin. Hello, I'm new here but an electronics tech that is a musician. What preamp circuit is it, and can you take a picture of the gutz.
470Uf is that your power supply? if so check the diodes by lifting one leg out of circuit and test them.
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Post by audiospecific on Dec 13, 2023 22:18:56 GMT -6
Also, expect to change the 470uF cap.
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Post by ragan on Dec 13, 2023 22:55:31 GMT -6
Does anyone have any advice for locating a short on a DIY preamp? I have a mildly smoking 470µF cap, so that might be close to the area of the short, but I'm afraid to power the thing in order to read VDC because of it. Either way, I have a good, working example to test and compare if that is helpful. Does it make sense to measure the components first? Or find continuity in far away areas of the PCB? I'm not quite sure where to begin. If the cap is smoking, it is very likely pulling the overcurrent. Is anything else in series with that cap? Have you measured resistance across the cap (and/or anything else in parallel with it) and verified that it's shorted, or are you going purely off the fact that it was smoking? |
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Post by lee on Dec 14, 2023 9:51:27 GMT -6
Does anyone have any advice for locating a short on a DIY preamp? I have a mildly smoking 470µF cap, so that might be close to the area of the short, but I'm afraid to power the thing in order to read VDC because of it. Either way, I have a good, working example to test and compare if that is helpful. Does it make sense to measure the components first? Or find continuity in far away areas of the PCB? I'm not quite sure where to begin. If the cap is smoking, it is very likely pulling the overcurrent. Is anything else in series with that cap? Have you measured resistance across the cap (and/or anything else in parallel with it) and verified that it's shorted, or are you going purely off the fact that it was smoking? I haven’t measured anything else yet, mostly out of not knowing where to start. The BYO kit thing is new to me. The preamp is an AML ez1081 (there’s a schematic on their page) and I’ll take a photo. I first knew something was wrong when it wouldn’t bias. Was supposed to be around 50-60mV but instead measured over 400. |
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Post by lee on Dec 14, 2023 15:16:43 GMT -6
I don't have a shot of the back. Will do when I get home tonight. I've circled where the smoke is coming from. Attachments:
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Post by lee on Dec 14, 2023 16:53:00 GMT -6
Does anyone have any advice for locating a short on a DIY preamp? I have a mildly smoking 470µF cap, so that might be close to the area of the short, but I'm afraid to power the thing in order to read VDC because of it. Either way, I have a good, working example to test and compare if that is helpful. Does it make sense to measure the components first? Or find continuity in far away areas of the PCB? I'm not quite sure where to begin. Hello, I'm new here but an electronics tech that is a musician. What preamp circuit is it, and can you take a picture of the gutz.
470Uf is that your power supply? if so check the diodes by lifting one leg out of circuit and test them.
Glad to have you here, and with that skill set, that is surely an asset. I'll do everything you mention as soon as I get a chance. I can't say if that would be my power supply (not experienced enough with EE to be able to say for sure) but I might guess so. Here is the build doc and schematic: AML 1081 Docs |
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Post by lee on Dec 14, 2023 17:01:23 GMT -6
 Here is the power section |
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Post by audiospecific on Dec 14, 2023 19:21:25 GMT -6
I had techs send me those boards when they either couldn't figure it out or they badly burned them. Those boards are a little flimsy and i had to repair a couple that techs burned up the lands on the gain rotary switch area. I recommend always to use a temperature controlled soldering iron. I have a rework station with a temperature controlled vacuum de-solder. Which 470uF cap was smoking? The one at the output? Or the one in the power supply?
Since it wouldn't adjust, did you check the diodes and transistors that is the output circuit? (Q6, Q10-12, D7, D8)?
BTW, you don't have to lift anything to check them because the short will be obvious.
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Post by lee on Dec 14, 2023 20:04:52 GMT -6
The smoking one (again, it was in the area, I'm not positive it was this one) appeared to be C24, which would make it the output; it's right next to R46-R49. I'll check those diodes and resistors tonight if I can.
To be clear, when you say "the short will be obvious", are you testing values in "diode" mode on the MM, and a short is indicated by values wildly off? Sorry, I actually need to be spoken to like a child so I can get it the first time and know it for the future!
Thank you again
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Post by audiospecific on Dec 14, 2023 20:10:09 GMT -6
I don't have a shot of the back. Will do when I get home tonight. I've circled where the smoke is coming from. I didn't notice this post, yes, check those transistors and diodes. you probably find one that shorted.
Also, you need to make sure the topside circuit gets connected. I have a note to myself that they are not plated thru holes.
What did you use to hook up to it to measure +20db? Because you need to terminate the input of the next device with a 600-1000 ohm resistor.
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Post by audiospecific on Dec 14, 2023 20:22:31 GMT -6
The smoking one (again, it was in the area, I'm not positive it was this one) appeared to be C24, which would make it the output; it's right next to R46-R49. I'll check those diodes and resistors tonight if I can. To be clear, when you say "the short will be obvious", are you testing values in "diode" mode on the MM, and a short is indicated by values wildly off? Sorry, I actually need to be spoken to like a child so I can get it the first time and know it for the future! Thank you again
If it reads less than 0.45V forward bias, Its shorted.
If everything checks out, check the capacitor, it should just charge up to infinity in ohms.
You should start with the bias adjustment from base to emitter On Q10 with about 100 ohms.
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Post by doubledog on Dec 14, 2023 20:24:10 GMT -6
I'd double-check Q9 and make sure that is oriented correctly (right next to C24). if not... smoke.
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Post by audiospecific on Dec 14, 2023 20:43:22 GMT -6
yes. I was going to go slow for him, but check Q9 and re-flow U1 on the topside to make sure its connected. Check the center pin of U1 to pin 5 of the card edge connector for 0 ohms. Measure between TP6 and TP7 for 1 ohm
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Post by audiospecific on Dec 14, 2023 20:52:44 GMT -6
Also check R50. If U1 was not soldered all the way into the circuit, that is what smoked.
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Post by lee on Dec 14, 2023 21:32:52 GMT -6
This is all really helpful info! Thanks everybody.
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Post by audiospecific on Dec 14, 2023 22:17:27 GMT -6
This is all really helpful info! Thanks everybody. Not a problem. I'm glad someone appreciates me and I don't have a bunch of internet wieners like I had on other sites, that would nick pick what I say and arbitrarily call me an uneducated idiot. Because that is far from the truth.
Oh, I put the computer screen up on my big monitor and blew up that picture of the topside. Yes there is a lot of soldering not connected on the top and yes, you did burn up that R50 resistor.
It shouldn't have gotten up into the amp, unless you installed a pnp transistor where a npn should have been, or have some leads turned around on one.
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Post by audiospecific on Dec 14, 2023 22:18:33 GMT -6
I'd double-check Q9 and make sure that is oriented correctly (right next to C24). if not... smoke. Does anyone have any advice for locating a short on a DIY preamp? I have a mildly smoking 470µF cap, so that might be close to the area of the short, but I'm afraid to power the thing in order to read VDC because of it. Either way, I have a good, working example to test and compare if that is helpful. Does it make sense to measure the components first? Or find continuity in far away areas of the PCB? I'm not quite sure where to begin. If the cap is smoking, it is very likely pulling the overcurrent. Is anything else in series with that cap? Have you measured resistance across the cap (and/or anything else in parallel with it) and verified that it's shorted, or are you going purely off the fact that it was smoking? Thank you for helping. |
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Post by ragan on Dec 14, 2023 22:57:23 GMT -6
This is all really helpful info! Thanks everybody. What is the resistance measurement across C20 compared to the good unit you have? |
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Post by lee on Dec 14, 2023 23:56:00 GMT -6
This is all really helpful info! Thanks everybody. What is the resistance measurement across C20 compared to the good unit you have? Ragan, the resistance number on both units start at around 250-400 kΩ and then start dropping. They both settle around 3.200 kΩ. So far, I haven't found a cap with a wildly different measurement, resistance or Farads. What's the deal with the resistance? Will it always go toward infinity (or drop), even if slowly? |
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Post by lee on Dec 15, 2023 0:04:16 GMT -6
This is all really helpful info! Thanks everybody. Not a problem. I'm glad someone appreciates me and I don't have a bunch of internet wieners like I had on other sites, that would nick pick what I say and arbitrarily call me an uneducated idiot. Because that is far from the truth.
Oh, I put the computer screen up on my big monitor and blew up that picture of the topside. Yes there is a lot of soldering not connected on the top and yes, you did burn up that R50 resistor.
It shouldn't have gotten up into the amp, unless you installed a pnp transistor where a npn should have been, or have some leads turned around on one.
As for the solder flowing through the holes, I'm going to have to improve my technique, sounds like. In the mean time, I'll go through and attack those empty pads from the topside. I can see that R50 now that you mention it. Cooked. What do you think the next move is? |
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Post by lee on Dec 15, 2023 0:22:35 GMT -6
Also check R50. If U1 was not soldered all the way into the circuit, that is what smoked. You know what may have happened, when I first took this back apart, I noticed that I had forgot to cut the excess off the legs on U1, and that one or more may have been touching the chassis, or came close to it. God, this all reads like a crime against electronics, but I have to learn somehow. |
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Post by audiomaintenance on Dec 15, 2023 0:40:03 GMT -6
You do not need to solder the top side of the PCB. They are plated through holes.
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Post by lee on Dec 15, 2023 0:47:34 GMT -6
Since it wouldn't adjust, did you check the diodes and transistors that is the output circuit? (Q6, Q10-12, D7, D8)?
BTW, you don't have to lift anything to check them because the short will be obvious.
Q6, Q10-12, D7, D8 all appear fine. I'm not sure what kind of voltage drop I'm supposed to be seeing, but the numbers were in the ballpark of each other. (Showing my ignorance yet again!) |
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Post by lee on Dec 15, 2023 0:48:26 GMT -6
You do not need to solder the top side of the PCB. They are plated through holes. OK Understood! These are the moments you rue the day you opened your designs up to the novice kit builder, haha. |
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