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Thread: Workingman's 15 blew a fuse when turned on. (help me troubleshoot?)

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    Question Workingman's 15 blew a fuse when turned on. (help me troubleshoot?)

    I bought this from a kid in town who needed cash along with a very sad looking squire bass. When I plugged it in and turned it on it blew the 8A fuse between the plug and the first internal hot wire. it was a hard soldered fuse and i'll be replacing that tomorrow but i'm not sure where I should start trouble shooting... or even how to trouble shoot some of the components on an amp. Also, before the fuse blew there was a good deal of humming.

    I'm "ok" with a soldering iron but not a pro. I only paid $125 for the guitar and amp so i'm willing to risk bricking it so long as I'm taking that risk with as much information as possible. Thanks in advance for your help.

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    First things first disconnect the speaker and do not connect the speaker until the amp has been repaired. Do you own or have access to a multimeter to measure resistance and voltages? Something is drawing excess amount of current and is blowing the fuse. You said you heard a humming and that is probably DC voltage hitting your speaker before it blew the fuse, hopefully the speaker is not blown too. When you reinstall the fuse do not simply turn on the amp or you might just blow another fuse and, well, your right back where you started. Instead research and build a light bulb limiter (a.k.a. Dim bulb tester and Poor Man's Variac) which will allow you to power the amp on with out blowing fuses. The light bulb limiter will light the bulb up bright if the amp is pulling excessive current which will save your new fuse from blowing. When you first power it on via light bulb limiter do not have a speaker connected. Here is a schematic for that amp and hopefully it matches up to yours. Do your research and approach the amp repair using troubleshooting techniques offered through correspondence on this site. Do not just pull parts and replace without justification.

    Light Bulb Limiter

    The problem is probably due to shorted output transistors or a shorted bridge rectifier, but it could be other issues beyond that too.
    Attached Files Attached Files

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    Thank you very much for the reply.

    I do have a multi-meter. I'm going out of town today for a funeral but should be back tonight. My intention is take the meter to my office (where the guitar is) when I get home and start by testing the transistors and the bridge rectifier. I am pretty sure I've identified those but I may take a picture mspaint a circle around what I'm testing and post it here. I'll also build a LB Limiter to take with me. That amp has a switch that can eliminate the speaker and output via headphone jack... any reason I couldn't just use that mode while chasing the short? Also, instead of replacing the soldered in fuse with another is there anything wrong with replacing it with soldering in either an 8A breaker or replacable fuse holder? There seems to be plenty of room and this would also eliminate the need to heat shrink the fuse. (it was in clear HS when I removed it)

    Also, am I correct in my understanding that the schematic you shared does not show the AC power in but actually starts somewhere after the transformer? The only fuse I see in that diagram is on the board itself.

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    Welcome to the place.

    Quote Originally Posted by Stratapastor View Post
    Also, instead of replacing the soldered in fuse with another is there anything wrong with replacing it with soldering in either an 8A breaker or replacable fuse holder? There seems to be plenty of room and this would also eliminate the need to heat shrink the fuse. (it was in clear HS when I removed it)
    The internal fuse is there as a safety device to prevent the user from replacing the main fuse with one of the wrong value, which I will guess was done on your amp.

    On the back panel there should be a fuse holder with a removable cap that should have a 4 amp slow blow fuse in it. If this has been replaced with a 20 amp fuse or piece of foil, the internal 8 amp fuse will blow instead. When operated correctly, the internal fuse should never need to be replaced, therefore I recommend that you replace it with the original solder in type fuse.

    The fuse on the pc board is the speaker fuse. It is there to help protect the speaker from damage when there is a problem with the power amp.

    If you build the limiter, you can replace the outside fuse with the correct value one and temporarily bypass the blown 8 amp internal. That way you can order a new one when you order the other replacement parts that you will need. You will probably not find a pigtail fuse in any local stores.

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    Quote Originally Posted by 52 Bill View Post
    Welcome to the place.

    The internal fuse is there as a safety device to prevent the user from replacing the main fuse with one of the wrong value, which I will guess was done on your amp.

    On the back panel there should be a fuse holder with a removable cap that should have a 4 amp slow blow fuse in it. If this has been replaced with a 20 amp fuse or piece of foil, the internal 8 amp fuse will blow instead. When operated correctly, the internal fuse should never need to be replaced, therefore I recommend that you replace it with the original solder in type fuse.

    The fuse on the pc board is the speaker fuse. It is there to help protect the speaker from damage when there is a problem with the power amp.

    If you build the limiter, you can replace the outside fuse with the correct value one and temporarily bypass the blown 8 amp internal. That way you can order a new one when you order the other replacement parts that you will need. You will probably not find a pigtail fuse in any local stores.

    The slow blow was in place but the 8A still blew first. That said, I got antsy tonight and put an 8 amp in that I already had and it blew the slow blow... so hopefully tomorrow I'll be able to actually trouble shoot it with my fancy new slow blow fuse and light/power source. I noticed tonight that the cord that came with it does not have a ground prong... i'm wondering if that might be part of the issue.

    In positive news. I was able to replace the pot on the bass that came with the amp and it seems to play pretty well. With a little bit of set up it will be a solid guitar.

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    You should certainly replace the plug or power cord for your own safety, but not having a ground wouldn't make the amp blow fuses.

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    Quote Originally Posted by DrGonz78 View Post
    First things first disconnect the speaker and do not connect the speaker until the amp has been repaired. Do you own or have access to a multimeter to measure resistance and voltages? Something is drawing excess amount of current and is blowing the fuse. You said you heard a humming and that is probably DC voltage hitting your speaker before it blew the fuse, hopefully the speaker is not blown too. When you reinstall the fuse do not simply turn on the amp or you might just blow another fuse and, well, your right back where you started. Instead research and build a light bulb limiter (a.k.a. Dim bulb tester and Poor Man's Variac) which will allow you to power the amp on with out blowing fuses. The light bulb limiter will light the bulb up bright if the amp is pulling excessive current which will save your new fuse from blowing. When you first power it on via light bulb limiter do not have a speaker connected. Here is a schematic for that amp and hopefully it matches up to yours. Do your research and approach the amp repair using troubleshooting techniques offered through correspondence on this site. Do not just pull parts and replace without justification.

    Light Bulb Limiter

    The problem is probably due to shorted output transistors or a shorted bridge rectifier, but it could be other issues beyond that too.

    today I got the multi meter out and poked around... Assuming I was testing them correctly. all of the transistors seemed ok. The bridge rectifier just after the transformer is good. I was about to give up but then decided to test every diode and I found that D10 is measuring 83 both directions using diode test function on the multi meter. Any chance that this could be causing the issue i'm experiencing? If i'm understanding correctly that should only have resistance in one direction... but i'm learning as I go.

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    Supporting Member The Dude's Avatar
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    Edited: D10 is paralleled with a 100 ohm resistor R41. You are measuring the resistor also when you measure in circuit. It's not likely the problem.

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    What sorts of measurements are you getting on Q5? Also, please post your actual measurements on Q3-4 and Q9-10 for second opinions. You can measure the transistors in Diode check mode and even resistance mode if you don't have a diode checking function.

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    Quote Originally Posted by DrGonz78 View Post
    What sorts of measurements are you getting on Q5? Also, please post your actual measurements on Q3-4 and Q9-10 for second opinions. You can measure the transistors in Diode check mode and even resistance mode if you don't have a diode checking function.
    I'll go back and check. Should have measurements in 10.

    Would you be willing to video conference?

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    Old Timer J M Fahey's Avatar
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    The slow blow was in place but the 8A still blew first.
    You mean it was already blown.

    You said so in post #1.

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    Quote Originally Posted by J M Fahey View Post
    You mean it was already blown.

    You said so in post #1.
    The 8 amp blew after I turned it on the first time. I blew the slow blow last night after cleaning it up a bit and hoping that it was just a shorted wire... both are replaced and i'm currently chasing the short.


    Measurements of Transistors are as follows. Measuring with negative on the center post and positive on the outside leg. CL, CR, and then the outside posts measured either way because I don't know which is right.
    Q5 644, 1, 565/1
    Q3 585, 724, 271/1
    Q4 484, 589, 84/84
    Q9 1, 1, 713/1 (if I put the pos on the center it goes to 700,700)
    Q10 583, 546, 84/84

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    Okay lets makes more sense out of these readings. First off each transistor can be PNP or NPN and each transistor can have different pin arrangements BCE: Base, Collector, and Emitter. To figure out leg of the transistor is B,C, or E we will rely on the datasheet to show us the pin out of the chip. I have attached those on this posting for you. Do you have a Continuity tester function or Diode testing setting on your multimeter? I am also referring a few web pages about testing transistors too. Just think of a transistor as a couple of diodes (B to C) or (B to E) and you measure the diode that is between these junctions. Now it might be PNP or NPN and then you just have a difference in the polarity of your meter probes when testing. Read this entire page and think about everything I have typed already.
    Electrical Measurements Part 3 | RSR Electronix Express

    So find the base leg and measure positive or negative lead (depending on PNP or NPN) to Emitter or Collector looking out especially for shorts. A continuity tester will beep between legs to show a shorted connection.
    Another page to look at is this silly looking thing here>>>
    Transistor Testing

    Also, beware that sometimes you have to remove a transistor from the circuit to be able to get an accurate reading.
    Attached Files Attached Files

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    BTW looking at the measurements made on your last post Q9 seems a little suspect to me and that would be the first transistor I would pull to test out of circuit.

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    Just FYI. I'm swamped with work right now. I'm hoping I can shake free for a few hours tomorrow. Just don't want you to think I've given up. I've got those links open on my phone so that I can read when I get a second.
    My multi-meter is a levetron It has a diode test mode but I have a high frequency hearing impediment and I don't know if I always hear it over the Air conditioner.

    Thank you again for your help. I'll keep you updated. If I cant' get to it tomorrow then it will likely be at least next weekend before I can touch it again because I'll be out of town all week.

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    Old Timer J M Fahey's Avatar
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    Don't worry, you run this at your own pace.

    If sometimes we seem annoyingly insistent on some definition or measurement, it's because the way user writes it is confusing and does not help.

    As of continuity measurements, the beep is not important, we are interested in the screen displayed value, which although not labelled that way (I always wonder why) really shows voltage drop across what we are measuring, an important parameter when measuring semiconductors.

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    I completely understand your frustration... electronics isn't my wheelhouse but I deal with the same thing daily in my professional life. I'm working through learning this new vocabulary. Hopefully my next post will be a little more intelligent / helpful. Though, it would be wonderful if my next post just said "it's fixed".

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    To be honest your measurements so far were pretty well stated but just need some clarifications. Also, anyone that comes here for advice and asks for "Troubleshooting" help then they get A+ in my book right off the start. Too many times they have already started replacing this component and that component. Once they do that it may be even more difficult to track down the fault that caused the original problem.

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    I tested q9 again now that I know what I'm doing and it tested as follows BC - 720, BE - 720, EC-1, CE701. I believe that this would indicate that the transistor is OK.
    I also retested q10, 5, and 4 and found them all to be ok. Then I had an idea. I plugged in a pair of headphones I have with an inline volume knob and set the amp to headphone output. I then zero'd all the knobs and found that all the tone knobs affected the sound... but then I noticed that the preamp clip light was active. I THINK that this means that the signal is being boosted without any sort of limit... or just some limiting factor is not in place... I'm going to try to trace the input from the jack and see if I can find the voltage spike.

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    So I have few quick questions to catch back up on this thread. So is the amp still blowing the fuse? From what it sounds/reads like to me is that we over came that problem and the fuse is holding. Or are you set up with a light bulb limiter and that is why the fuse is not blowing? This is important to know where we stand on the fuse issue. As far as the clip light being lit up constantly that might give us a hint to as why only the 8 amp fuse blew and not the mains power fuse. I am not certain about it but I read some where that constant clipping of the signal can result in DC spikes on the output. In that case only with the speaker load connected would it have a negative effect on the amp. The clip light clipping the signal should have no negative effect on the circuit with the speaker disconnected. However, I am not too happy with that thought so don't go there yet unless some other members here can help confirm it. Especially since you said that just turning the unit on blew the fuse, so that makes no sense to my idea.

    With amp powered up and speaker removed & no signal measure for DC on the speaker output. Then inject the signal that is causing the clipping light to turn on and measure for DC volts again to see what we have.

    Also, here is a link to a fuller schematic representation of the amp. This is a hard schematic to follow since it has so many different connection points spliced together.
    http://music-electronics-forum.com/a...kingman-15.zip

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    Quote Originally Posted by DrGonz78 View Post
    So I have few quick questions to catch back up on this thread. So is the amp still blowing the fuse? From what it sounds/reads like to me is that we over came that problem and the fuse is holding. Or are you set up with a light bulb limiter and that is why the fuse is not blowing? This is important to know where we stand on the fuse issue. As far as the clip light being lit up constantly that might give us a hint to as why only the 8 amp fuse blew and not the mains power fuse. I am not certain about it but I read some where that constant clipping of the signal can result in DC spikes on the output. In that case only with the speaker load connected would it have a negative effect on the amp. The clip light clipping the signal should have no negative effect on the circuit with the speaker disconnected. However, I am not too happy with that thought so don't go there yet unless some other members here can help confirm it. Especially since you said that just turning the unit on blew the fuse, so that makes no sense to my idea.

    With amp powered up and speaker removed & no signal measure for DC on the speaker output. Then inject the signal that is causing the clipping light to turn on and measure for DC volts again to see what we have.

    Also, here is a link to a fuller schematic representation of the amp. This is a hard schematic to follow since it has so many different connection points spliced together.
    http://music-electronics-forum.com/a...kingman-15.zip
    I have a made a light bulb limiter. I'm sure that without it the fuse would blow.

    The clipping light is there with no input on my part at all. I'm assumed that whatever was causing the hum is causing the clipping.

    Also, it occurred to me tonight that I haven't tested my caps. C1 and C2 hav black plastic caps on them and I can't decide if they might be swelled ever so slightly... I'm considering ordering a multi tester with the capability to test caps.

    I'm happy that I'm learning through this project... but i certainly wish I wasn't still so ignorant about what I'm doing.

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    Okay great now I understand much better where you are at on the repair. We should verify our power supply voltages to see how good or bad those are looking. On the schematic you will see -/+HV and -/+15 supplies which need to be confirmed.
    Click image for larger version. 

Name:	PowerSupply.png 
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    Once we confirm that our power supplies are intact then we need to verify voltages on the op amps (reference "IC POWER CONNECTIONS" on the schematic) and preamp transistors. Such as transistors Q11 & Q12 and op amps like U1-U6. As far as the signal path we don't want there to be DC voltages at certain points of the signal path. It could be a case where DC is leaking into the signal chain thus amplifying this horrible noise. So you are correct in the idea to trace out the signal path. At some point in the signal path we then could ground out specific stages to see if that stops the noise and narrow it down to a specific area. Using the headphones might come in handy to monitor the noise as we ground out these sections of the signal to see if that makes the noise go away.

    Start with voltages to see where this is headed.

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    Quote Originally Posted by DrGonz78 View Post
    With amp powered up and speaker removed & no signal measure for DC on the speaker output.
    In addition to power supply measurements we also need to know what DC voltage is measured on the output. BTW completely ignore my comment about DC spikes on the output caused by oscillations/clipping, I was wrong.

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    I don't see the labels u#-c on the main board. Would they be on the smaller board that has all the pots on it? it's mounted upside down and I haven't pulled it yet.

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    I get 18v on the +/- HV but don't think i'm measuring the +/-15 correctly.

    however, in trying to learn more I found this... I don't know how to test what this poster is referring to but it sounds a LOT like what i'm experiencing. http://music-electronics-forum.com/t19506/

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    I would expect the + - HV to be more like 60 Vdc.

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    Quote Originally Posted by Stratapastor View Post
    I get 18v on the +/- HV but don't think i'm measuring the +/-15 correctly.
    How are you measuring the voltages? Black probe on chassis ground and red probe at one side of R1 & R2 for -/+HV. A good idea is to use a gator clip on the ground so it frees up one of your hands. Put your left hand behind your back(just a safety thought) and probe with right hand at the voltage points. Be careful to not slip and short anything out. D1 and D2 are where you will find a nice point to probe for the -/+15v rails and one end of R1 & R2 will be similar voltages too.

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    Quote Originally Posted by Jazz P Bass View Post
    I would expect the + - HV to be more like 60 Vdc.
    I think that he is using a light bulb limiter, so the voltages are low.

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    Also need to make sure what wattage bulb and that it is incandescent.

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    Quote Originally Posted by DrGonz78 View Post
    How are you measuring the voltages? Black probe on chassis ground and red probe at one side of R1 & R2 for -/+HV. A good idea is to use a gator clip on the ground so it frees up one of your hands. Put your left hand behind your back(just a safety thought) and probe with right hand at the voltage points. Be careful to not slip and short anything out. D1 and D2 are where you will find a nice point to probe for the -/+15v rails and one end of R1 & R2 will be similar voltages too.
    I just measured directly off of the rectifier and got 19.8v at D1 and d2 I got 2.9 and -2.9.

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    100w and yes incandescent. or at least hallogen but I'm 99% sure I bought incandescent because that's what was in the instructions. Also the bulb itself says 12v 100w on both the glass and the metal. i would assume that they would need to put actual voltage on the bulb but i'm no light bulb expert.

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    Last edited by Stratapastor; 11-24-2016 at 01:53 AM.

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    Quote Originally Posted by 52 Bill View Post
    I think that he is using a light bulb limiter, so the voltages are low.
    This would be correct. I could desolder the leads to the board if I need to test this out of circuit.

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    Quote Originally Posted by Stratapastor View Post
    the bulb itself says 12v 100w on both the glass and the metal. i would assume that they would need to put actual voltage on the bulb but i'm no light bulb expert.
    I have a trouble light that runs off a car battery. It looks exactly like a household bulb but give full brightness (100W) at 12V. It would also be used in a truck camper that runs off 12V. Is that what you have?

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    Quote Originally Posted by g1 View Post
    I have a trouble light that runs off a car battery. It looks exactly like a household bulb but give full brightness (100W) at 12V. It would also be used in a truck camper that runs off 12V. Is that what you have?
    No, I have a typo. It's 120v

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    Ah, good then. What threw me off was when you said you thought they would "put actual voltage on the bulb but i'm no light bulb expert".

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