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

  1. #71
    Supporting Member Jazz P Bass's Avatar
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    I still am not sure where that +35 volts is coming from on Q3 Base.

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    Is the amp currently back together, with the transistors (except q5), etc. back in place? If it is, jumper across Q5's Emitter and Collector and power up the amp with a light bulb limiter and report back what happens.

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    It is not back together but it can be. At this point am I done with all the trouble shooting that can be done in it's current stat?

    also.. I could temporarily solder on some jumpers to q5 to get it back in circuit in the short term if that is beneficial.

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    Quote Originally Posted by Stratapastor View Post
    It is not back together but it can be. At this point am I done with all the trouble shooting that can be done in it's current stat?

    also.. I could temporarily solder on some jumpers to q5 to get it back in circuit in the short term if that is beneficial.
    Done? I don't know, you have removed transistors and tested them. If you are positive that they are good then they need to be put back in and other things need to be tested.

    The usual reasons for an amp to blow fuses is shorted output transistors, shorted power supply rectifier, bad power transformer, shorted power supply capacitor and then assorted other problems. You have proven that the transformer and power supply parts are working with the correct voltage values. You have removed and tested the output and driver transistors. There are maybe three additional transistors in the power amp. Have you tested all of them?

    As was stated here before, the bias transistor is what keeps the two halves of the power amp in check. If it goes open or becomes disconnected from the circuit, the two halves of the power amp both turn on at the same time and will cause the amp to blow up or at least blow fuses. If you are certain that the broken leads only occurred after the initial tests, then don't bother trying the earlier suggested test or jumpering it in place. If Q5 has been doing its' job, then the problem with the amp is still to be found.

    We are all at a disadvantage here because we do not have the amp in front of us and we can only assume that you have tested things correctly and followed the directions that have been given correctly. So please understand that we are trying to help, but can only do so much from where we are.

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    Quote Originally Posted by Stratapastor View Post
    At this point am I done with all the trouble shooting that can be done in it's current stat?
    You can't be sure where the drive voltage will settle with the output/driver transistors removed and feedback inoperative so +35V on Q3 base may not be a problem. At this stage I'd reinstall Q3,4,9,10 and jumper Q5 C to E then with the speaker disconnected power it up with the bulb limiter in circuit. Measure the HT rails and the output voltage to the speaker. The output voltage should be close to zero.

    If the power rails and output voltage measure OK the next stage would be to check the values of R26,27,69 then install a new Q5 and repeat the above.

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    Last edited by Dave H; 12-06-2016 at 01:05 AM.

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    Quote Originally Posted by 52 Bill View Post
    Done? I don't know, you have removed transistors and tested them. If you are positive that they are good then they need to be put back in and other things need to be tested.

    The usual reasons for an amp to blow fuses is shorted output transistors, shorted power supply rectifier, bad power transformer, shorted power supply capacitor and then assorted other problems. You have proven that the transformer and power supply parts are working with the correct voltage values. You have removed and tested the output and driver transistors. There are maybe three additional transistors in the power amp. Have you tested all of them?

    As was stated here before, the bias transistor is what keeps the two halves of the power amp in check. If it goes open or becomes disconnected from the circuit, the two halves of the power amp both turn on at the same time and will cause the amp to blow up or at least blow fuses. If you are certain that the broken leads only occurred after the initial tests, then don't bother trying the earlier suggested test or jumpering it in place. If Q5 has been doing its' job, then the problem with the amp is still to be found.

    We are all at a disadvantage here because we do not have the amp in front of us and we can only assume that you have tested things correctly and followed the directions that have been given correctly. So please understand that we are trying to help, but can only do so much from where we are.
    I won't be able to work on this till tomorrow at the earliest. I just wanted to say that I'm really appreciative for all the help I've been given so far and for your patience with me. this has become a much bigger project than i first imagined but i'm learning a great deal from it. tomorrow I'll do as instructed in Dave's post and check the transistors 1 more time before I reinstall them.

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  7. #77
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    Quote Originally Posted by 52 Bill View Post
    Done? I don't know, you have removed transistors and tested them. If you are positive that they are good then they need to be put back in and other things need to be tested.

    The usual reasons for an amp to blow fuses is shorted output transistors, shorted power supply rectifier, bad power transformer, shorted power supply capacitor and then assorted other problems. You have proven that the transformer and power supply parts are working with the correct voltage values. You have removed and tested the output and driver transistors. There are maybe three additional transistors in the power amp. Have you tested all of them?

    As was stated here before, the bias transistor is what keeps the two halves of the power amp in check. If it goes open or becomes disconnected from the circuit, the two halves of the power amp both turn on at the same time and will cause the amp to blow up or at least blow fuses. If you are certain that the broken leads only occurred after the initial tests, then don't bother trying the earlier suggested test or jumpering it in place. If Q5 has been doing its' job, then the problem with the amp is still to be found.

    We are all at a disadvantage here because we do not have the amp in front of us and we can only assume that you have tested things correctly and followed the directions that have been given correctly. So please understand that we are trying to help, but can only do so much from where we are.
    Just so i'm 100% certain I'm understanding correctly. Testing the circuit with Q5 jumpered is still desired even though I have a new (actually several) resistors on hand to replace it?

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    Quote Originally Posted by Stratapastor View Post
    Just so i'm 100% certain I'm understanding correctly. Testing the circuit with Q5 jumpered is still desired even though I have a new (actually several) resistors on hand to replace it?
    i think you meant to type "Transistors"... Yes just replace Q5 if you have another one on hand. As Bill stated unless the broken leads were the fault and it tested good, then Q5 was doing it's job. The fault is still yet to be found. Jumpering the pads seemed to mean if you don't have another Q5 transistor to install.

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    Yes, if you have replacements for the transistor use them. I thought that you were waiting to find all of the parts you needed before placing an order.

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    I put the transistors all back in and replaced Q5. No change in voltages. I also get nothing from the headphones now... I'm hoping I didn't further break something.

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    Last edited by Stratapastor; 12-11-2016 at 08:58 PM. Reason: details

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    Quote Originally Posted by Stratapastor View Post
    I put the transistors all back in and replaced Q5. No change in voltages. I also get nothing from the headphones now... I'm hoping I didn't further break something.
    I just re-read this entire thread, unless I missed something, you never mentioned the headphones before did you? From what I remember, the headphones are driven by the power amp output. Were they working before?

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  12. #82
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    Quote Originally Posted by Stratapastor View Post
    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.
    This is the post that included a comment about the headphones. I think what made me think there could be oscillations was that he said the tone knobs had an effect on the sound. I took that mean no signal injected but actual noise (hum) generated by the amp. Stratapastor please explain what you meant about effected the sound? White noise hiss hum etc? So we need to know was it passing a signal from a bass guitar or other input, or was it noise of the amp.

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    Quote Originally Posted by Stratapastor View Post
    I put the transistors all back in and replaced Q5. No change in voltages. I also get nothing from the headphones now... I'm hoping I didn't further break something.
    What voltages? I'm not sure where you are up to. Are you saying that the transistors are replaced and the rail voltages are as they were in post #53 (i.e. correct) but it still isn't working?

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    Full update:

    OK. Full update on the current state of the project. I'll update this and repost it as more measurements are taken.

    Amp Function:
    Amp does not pass signal through the amp to a set of headphones. There is an hum much like an old monitor

    Preamp clip light does not come on except when the switch is initially flipped.

    The limiter light only lights momentarily as well.

    General Information:
    The speakers are currently removed from the unit.

    I'm currently using a 100w Light bulb limiter.

    I'm not very competent at this kind of trouble shooting or electrical work... but thanks to you all I am gaining some knowledge.

    Possible factors: (some of these are embarrassing.
    During the removal of the transistors which were tested out of circuit. On the circuit board the top pad of Q10 cath came off as well as Q9 base. These were both traces that are visible on the bottom of the board and so I did not repair them... and frankly I'm not certain how to do that other than bypassing the board with a jumper for that part of the circuit..

    When I removed Q5 the base and emitter legs were both broken flush with the board. This could have been from being jostled during trouble shooting.

    During trouble shooting I failed to re-secure the circuit board on one occasion and there was an ark from a mounting post to the post of R1 closest to C1. There were no changes in measurements from either component after this incident.

    Just now while inspecting traces I discovered that a ribbon cable is broken off at the board.... so I'm going to pause this update and fix that.


    [ B]Measurements:[/B]
    There is a DC voltage of 0.13 at the headphone jack when the unit is on and the selector set to headphones.

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    Last edited by Stratapastor; 12-14-2016 at 03:00 AM.

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    This is one of the reasons why I try to keep people from removing things to randomly test parts. There are just too many ways to damage the pc board and create more problems than you started with.

    All I can suggest is to go back and check every part that you have removed and test for continuity from the component pad to the next solder point along the copper trace to be certain that the pad has not been broken away from the trace. If you find any you will have to solder small jumper wires from the component pad to the trace where it is still solidly adhered to the board. You will need to scrape off the green solder mask from the copper in order to be able to solder the jumpers to the traces. Use as little heat as possible to keep from doing any additional damage to the board. I use the wire from cut off resistor and capacitor leads for the jumpers.

    Rather than measuring the voltage on the headphone jack, it would be more helpful to know the voltage at the main output point of the power amp. The part that would connect to the main speakers. I believe that there are pad resistors on the headphone jack, so the voltage will be reduced when read at the headphone jack.

    Now when you turn it on how bright does the limiter lamp light up?

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    Well, I finally took the time to re-solder the ribbon wires tonight and I can report that the amp works... unfortunately, the speakers both appear to be blown. Kind of a frustrating loss at this point. Now I have to decide if this cabinet is worth buying a decent 15 and horn for.

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    Quote Originally Posted by Stratapastor View Post
    Well, I finally took the time to re-solder the ribbon wires tonight and I can report that the amp works... unfortunately, the speakers both appear to be blown. Kind of a frustrating loss at this point. Now I have to decide if this cabinet is worth buying a decent 15 and horn for.
    How did you test the speakers? I think that the horn is just a simple Piezo tweeter.

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    Quote Originally Posted by 52 Bill View Post
    How did you test the speakers? I think that the horn is just a simple Piezo tweeter.
    Actually... I was so bumbed that I didn't think to plug the speaker into another amp. I'll try tonight and report back.

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    So, you were right!... so the amp is still broken but the speakers are good. I plugged them into an acoustic amp and the sounded just fine... so back to trouble shooting the amp... really i'm about ready to toss it and use the cab for use with a Kustom head a friend gave me.

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    Last edited by Stratapastor; 02-03-2017 at 05:16 AM.

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    Quote Originally Posted by Stratapastor View Post
    So, you were right!... so the amp is still broken but the speakers are good. I plugged them into an acoustic amp and the sounded just fine... so back to trouble shooting the amp... really i'm about ready to toss it and use the cab for use with a Kustom head a freind gave me.
    The sound of buzzing speakers is typical of an amp with crossover distortion. How did you adjust the bias of the output stage?

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    I've not adjusted the bias in any way... in fact. all i've really done once everything is said and done is replace a transistor that I broke and re solder the wires from the control board to the main board. At low volume it sounds ok... just very low. but if the volume is weak. i'm wondering if I don't have a bad solder joint somewhere. I'm still not an expert at soldering and user error seems like the most likely cause. I'll get out the tester and see what I can discover tomorrow. Right now I need some tylenol and a pillow.

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