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

[J M Fahey]

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.

[Stratapastor]

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".

[DrGonz78]

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.

[Stratapastor]

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.

[DrGonz78]

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

[Stratapastor]

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.

[DrGonz78]

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.


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.

[DrGonz78]

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.

[Stratapastor]

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.

[Stratapastor]

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/

[Jazz P Bass]

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

[DrGonz78]

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.

[52 Bill]

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.

[DrGonz78]

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

[Stratapastor]

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.