PLEASE HELP! B-52 100A Blowing Fuses!

[JackRobot]

Hey guys,
A friend let me use his b-52 LG-100A amp head, and being my luck, I was playing it, low volume and it blew a fuse, I put a new one in, and it blew it again, I took it apart and didn't seem to see anything burnt, it never seemed to get hotter then normal temp, the only thing I saw, was where the rear circuit board connects to the aluminum in the back, there are 3 small hard metal connectors that go into it. And the metal around them seems to have melted away? Thats the only thing I could see? Any help would be great,  I really need to get this fixed, I know a shop will cost me an arm and a leg, If anything i'll just buy him another one as you can find them for around 150 bucks on e-bay, Should have just waited and not used someone else's stuff :grr

Thanks!
Jack

[joecool85]

Hey guys,
A friend let me use his b-52 LG-100A amp head, and being my luck, I was playing it, low volume and it blew a fuse, I put a new one in, and it blew it again, I took it apart and didn't seem to see anything burnt, it never seemed to get hotter then normal temp, the only thing I saw, was where the rear circuit board connects to the aluminum in the back, there are 3 small hard metal connectors that go into it. And the metal around them seems to have melted away? Thats the only thing I could see? Any help would be great,  I really need to get this fixed, I know a shop will cost me an arm and a leg, If anything i'll just buy him another one as you can find them for around 150 bucks on e-bay, Should have just waited and not used someone else's stuff :grr

Thanks!
Jack

It's true, you shouldn't have used someone elses stuff.  That said, you can probably fix this.  Could you possibly get a picture of what "melted away"?

[JackRobot]

Hes a good friend of mine, and i'd do the same for him, But as i'm broke as all hell, I don't have the cash to pay the repair bill, so if I can fix it that would be awesome.  :tu:

Heres a pic of where I think it burned through, Another thing, I was testing the wires from the power supply, and one had no power going through it, is it possible that the plug for the power is bad causing it to blow fuses?

[teemuk]

The transistor in the middle is a part of the "bias-servo" that tracks the temperature of the heatsink and adjust the bias voltage according to it. (It actually tries to track the temperature of the transistors attached to the sink). It does this to compensate the fact that transistors conduct more current when they get hot. It doesn't look like it's melted, though: There's a drilled hole in the sink and the transistor is stuffed inside it and covered with thermal paste to aid heat conduction. In fact, I see nothing in the picture that would look like melted.

The two transistors next to the heats sensor transistor are "drivers" and the big ones are the main output transistors that handle the highest currents. The picture does a bad job in showing whether they look burned/charred or not.

No offense but it seems that you are not exactly familiar with how these things work. You should realize that if you try to fix this thing by yourself you need to know how to effectively troubleshoot the circuit and how to fix it / test it without doing more damage in the process. That could be something that is currently way above your skill level. So, if you need to get that thing fixed ASAP then I suggest you bite the bullet and just take it to service.

If you try to fix that amp by yourself then I suggest you start reading stuff that helps you to get acquinted with things like how basic transistor amplifiers work, how various components are tested to find out if they are faulty or not and how to do that without causing further damage.

[JackRobot]

The more I looked at it, The more I saw that it was not burnt. Sorry for misinforming.

I agree, I am not the most skilled in electronics, but my dad and brother are, which helps.

Any answers with the power supply being the cause?

[teemuk]

There can be plenty of wires in the power supply. There are, for example, the primary (mains side) wires such as the Line, Neutral and safety ground wires that go from the mains socket to components like the switches, fuses, chassis and transformer. Then there are the secondary side wires from the transformer to the rectifier and common point. There can be even more wires from the rectifier to the filter capacitors and from the filter capacitors all over the circuit. All of these are considered supply wires. What wires exactly did you test? Then, when you say that "one had no power going through it" what did you mean by that: No voltage or no current? Are you sure there was even supposed to be? Based on your description, I have absolutely no idea what you measured and what readings you got.

You see, unless you have some basic knowledge about these things, that allows you to give us an informative and clear description about things like the fault symptoms or the procedures and measurements you've conducted, it is pretty difficult for us to help you at all.

Just by hunch I'd say that likely the output section has short-circuited (as that is a very common scenario that causes the fuses to blow rapidly). Another about equally common scenario is that a regulator circuit of the preamp supply rail has shorted out. Aside the regulators failing, faults in the power supply circuit are, in my experience, very rare. Anyway, these are just guesses and unless you can verify the problem by measuring I suggest you do nothing erratic.

If the fuses blow it indicates that you have a lot of current flowing in a wrong place. Unfortunately, fuses blow a lot slower than transistors and when a transistor fails it usually takes several others with it as well. In some cases visual examination should reveal some signs/causes of the fault but when it comes to transistors they often fail so rapidly that they do not even have a chance of burning badly before the fuses trip. Luckily it is quite easy to test if a certain transistor has failed.

I feel compelled to give some warnings: Swapping parts randomly can fix the amp if you are lucky but in the worst case you will replace just about everything before you fix the correct part. Also, unless you know how to power up the amp safely after doing some fixes there's a very good chance that the fault is still there and that it takes down the new parts you just installed. If you are not experienced with soldering iron you can easily damage the PC board and in the worst case ruin the amp almost beyond repair. What would have been a cheap repair at a tech now turned into a very expensive one. Also, many techs refuse to work on equipment if they see that some amateur has tried to fix it already (as these attempts very often just cause plenty of other faults or ruin the PC boards). Techs don't waste their time with those cases unless you pay them big time.

Anyway, since your brother and your dad are skilled in electronics then for starters it might be wise to consult them for assistance. They can likely assist you a lot in how to get started. They should know how to measure if the secondary side supply rails provide the correct voltage (one of the basic tests for an amp) or if one or several of the output stage transistors have failed. I will gladly help as well but I can't do that if the descriptions are as vague as they've been so far. I'm sure your relatives can provide some help in how to make yourself better understood when it comes to this sort of "tech talk" as well. Then, you can try contacting the manufacturer and see if they can provide you a schematic. It's not mandatory for fixing an amp but for a novice it can be a tremendous help since figuring out the circuit just by looking at the PC board can be extremely difficult if you don't have a certain idea of what the circuit most likely looks like and what are the functions of different components. For us who try to troubleshoot via forums and therefore can't be there to inspect the amplifier and its circuit board the schematic is pretty much the only effective way to figure out what's going on in the circuit.

At some point you likely need to power up the amp to do some basic tests; in that case you must limit its current somehow to prevent parts from failing, fuses from blowing etc. One way of doing this (using a light bulb limiter) has been discussed numerous times at this forum. There are also at least two threads discussing basics of amplifier troubleshooting. The "Amplifier Discussion" section has a permanent thread titled "Book about solid-state guitar amplifiers", from which you find a link to download my book about the topic for free. It's a lot of reading but hopefully can shed some light about the basics of transistor amplifier electronics.

That should get you started....