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

[Stratapastor]

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.

[Stratapastor]

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.

[g1]

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?

[Stratapastor]

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

[g1]

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".

[J M Fahey]

1) Also would expect around +/-60V rails, but that would be straight to mains.
With a 100W bulb in series, I´d expect somewhat less but still reasonble, think +/- 40 to 50V DC

Quite low +/-19V means amp is eating way over normal, it might either be oscillating or way overbiased.

Check oscillation with a scope , in the example you mentioned Zobel network was open so amp was unstable, you might have the same but that´s just one possibility among many.

Discarding that, you can check for overbiasing by measuring voltage drop across power transistor emitter ballast resistors, also measure and post voltage across biasing transistor .

2) if you have +/- 19V main rails, you have nowhere to pull +/-15V from , so those +/-2.9V do not surprise me at all.

[Stratapastor]

1) Also would expect around +/-60V rails, but that would be straight to mains.
With a 100W bulb in series, I´d expect somewhat less but still reasonble, think +/- 40 to 50V DC

Quite low +/-19V means amp is eating way over normal, it might either be oscillating or way overbiased.

Check oscillation with a scope , in the example you mentioned Zobel network was open so amp was unstable, you might have the same but that´s just one possibility among many.

Discarding that, you can check for overbiasing by measuring voltage drop across power transistor emitter ballast resistors, also measure and post voltage across biasing transistor .

2) if you have +/- 19V main rails, you have nowhere to pull +/-15V from , so those +/-2.9V do not surprise me at all.

I understood some of those words... but not enough to know what to do next. I don't know what oscillating or over-biased are in the context of amp circuitry, or a zobel network. I don't know what "power transistor emitter ballasts resistors are or a biasing transistor.

Is there a book you'd recommend for me to learn some of this stuff? Also, any lines on a cheap oscilloscope?

[Stratapastor]

It's been a long time since I checked all the diodes, resistors, and transistors. And to be honest, I didn't really know what I was doing for a lot of it. I'm thinking I might take the time to test each one and write the measurement on my schematic.

I tested the DC volts at the sub. 11.8 VDC.

[Stratapastor]

11.8 dcv

[DrGonz78]

I understood some of those words... but not enough to know what to do next. I don't know what oscillating or over-biased are in the context of amp circuitry, or a zobel network. I don't know what "power transistor emitter ballasts resistors are or a biasing transistor.

Is there a book you'd recommend for me to learn some of this stuff? Also, any lines on a cheap oscilloscope?

Oscillation is something you will need observe with a scope. However, you had mentioned that the red clipping light stays on continuously without a signal applied and that can imply that the amp is self oscillating.
Read about oscillation on this page and just take it all in with a grain of salt. Get the jest of it all. Do some google searches to find other sources of information explaining it.
Amplifier Stability and Instability | Audio Undone

Also watch this little video here on youtube to show you someone observing oscillations on an amplifier using a scope.

(I will try to put this in lay man's terms, probably all that I am capable of explaining anyway)
The main power transistors in your amp and all the transistors for that matter will be biased. This is similar to setting the idle in your car, but in the case of the amplifier it is "idle current" that we need to set. Setting the overall bias of the power amp circuit is done while the amp is idle and just sitting there powered on with no input signal injected through the circuit. If set too low you get cross over distortion (cold bias) and set to high (hot bias) then you get smoke. Main point about the bias of the power amp is that a hot bias will cause the components to draw high amounts of current which will dissipate as heat through the amp components. Too much heat and the transistors will fry. If you look at Q3 and Q4 on the schematic you will see B C and E listed on each of the legs of the transistor. This is Base, Collector and Emitter. The Base pin of the transistor is where the bias voltage sits on those components perhaps just a couple volts. This turns on the transistor so it is barely on and at idle. When audio signal runs through the amp it will increase and amplify the current pull as well as the output signal, similar to hitting the gas pedal in a car.

When the amp has a large current pull it will drop the overall voltages of the power rails. One thought here is that your voltages on your are very low and this oscillation through the circuit is pulling a lot of current. So even with the light bulb limiter (which will limit voltages too) it appears that your voltages are really really low. So we asked you to measure across the emitter resistors for DC volts... Those are R29 & R42 on the power amp schematic. Really R29 is an Emitter resistor but this looks like a Quasi-Complementary output amp. Which don't get too hung up on all the terminology all at once and just try to follow the main thoughts. Q4 is a 2SC3264 and so is Q10 which are you main output transistors which are both NPN types. So you see Q4 has the Emitter leg connected to the R29("emitter" ballast resistor) but Q10 Collector leg leads to the R42 ("emitter" Ballast resistor). Many times you will see an amp have a true complimentary pair in those locations of the output transistors. Other versions may have NPN and PNP as pairs or many many other arrangements. So there is no simple way to explain all of it. It will take a long time to absorb and learning never ends. Main thing is measuring across the emitter ballast resistors will help us figure out if the bias is cranked up way too high. Honestly with 11vdc on the output we do need to find out if there are any shorted transistors in the amp or other bad components.

The Zobel network is a resistor and capacitor going to ground right at the output before the speaker. There is R44 & C21 and I think R43 and C20 are there too as part of the Zobel network. Go research on google what that does in relation to the stability of an amplifier.

A great book to browse is Teemuk's Solid State amp book:
https://www.google.com/url?sa=t&rct=...39782543,d.cGw

[Stratapastor]

[/QUOTE}\]

The Zobel network is a resistor and capacitor going to ground right at the output before the speaker. There is R44 & C21 and I think R43 and C20 are there too as part of the Zobel network. Go research on google what that does in relation to the stability of an amplifier.

[/QUOTE]

You are right, that's a lot of info. I won't be able to decidate all the hours i'd like to understanding it this week but I'll continue to ask questions as they come to me. The book by Teemu is VERY appreciated. looking forward to going through it while i'm out of town.

RE: the Zobel Network.
If the Zobel Network is there to neutralize the effects of the driver's voice coil inductance wouldn't it cease to be an issue once the sub was removed? Or can it create it's own problems in the absence of a voice coil?

[52 Bill]

This thread has gotten way out of hand. So, the amp was blowing fuses. With a light bulb limiter the amp will power on but is still drawing too much current.

Unplug the speaker if it has a plug or use the speaker switch to turn off the speaker for now. Use your meter to see if there is any dc voltage at the speaker output.

Does your meter have a diode test function? If it does, then use it to test the two output transistors and the bridge rectifier. Please post specific results here. You should not have any readings that are close to zero.

Don't get distracted with extraneous information until you are certain that the basic tests are done and that the usual suspects are ruled out. The clipping light may only be turned on due to the power supply problem.

[Stratapastor]

I should mention that I have a new multimeter that seems to be much more consistent in it's measurements.
Both Q4 and Q10 showed B->C of .528 and only with the + on be and - on C. But the measurement for BE was .100 in with leads +to - or -to +.

Should I desolder these and test off of the board?

[Stratapastor]

with a lack of anything else to do at the moment I checked all of the diodes in circuit and, with the exception of D8 and D10 they all measured correctly. I'm assuming it's because D8 and D10 are both in parallel with other components.

[DrGonz78]

I should mention that I have a new multimeter that seems to be much more consistent in it's measurements.
Both Q4 and Q10 showed B->C of .528 and only with the + on be and - on C. But the measurement for BE was .100 in with leads +to - or -to +.

Should I desolder these and test off of the board?

At some point pulling the transistors (Q4 & Q10) off the board for testing may be beneficial. Also in cases where a component can't be accurately measured in circuit de-soldering one leg off the board will be all it takes to measure it correctly. Sorry for my long winded post last night, but hopefully a few bits of it helped. Just focus like Bill said on the problem at hand and get at the root of the problem. Also, to point out that there is 11.8vdc on the speaker output.