GK 400 rb

[catstrat]

Hi Markus. I probably didn't do something right on posting the schematic. Let me try again. For a long time i would remove every suspect transistor or diode from the circuit to test it. That's the reason i tried the variac. The only way to find the health of a circuit is voltage checks and an o-scope. The other thing i didn't do is replace both drivers, i only replaced one. This whole thing is probably my fault. While i'm on the subject of drivers, do you know a suitable substitution for mje 15030 and mje15031. I thought about the TIP family but they're rated for 100 volts at the collector. The stock drivers are rated for 150 volts. The rail voltage on this amp is about 60 volts. I wouldn't want to push it though. thanks.

[MarkusBass]

For a long time i would remove every suspect transistor or diode from the circuit to test it.

The key issue here is the meaning of the phrase "suspected transistor". Do you mean any transistor that possible could have failed, or (this is how I understand it) a transistor that has incorect voltages between its pins and incorrect current/voltages on any component connected to it? And when you do it for living, desoldering components that did not fail is a waste of time.

The other thing i didn't do is replace both drivers, i only replaced one.

You should most probably replace both drivers. When output transistors fail, both drivers work in extreme conditions and in many cases they fail too.

do you know a suitable substitution for mje 15030 and mje15031?

I wonder why would you want to use substitution when the transistors are available almost in every shop and the cost is less than $2? Where are you located?

Mark

[Enzo]

I agree, the MJE15030 and family are cheap, readily available, why sub them?


If you set the bias so the crossover notch JUST disappears, then back off a hair so the notch just starts to appear, you should be biased about right, and the outputs should not get hot at idle.

using a variac to see if transistors are bad is not how I would use the tool. Not by seeing if they get hot anyway. You can't use a variac on an SMPS anyway. But on linear supplies, what you need is an AC current meter to monitor the mains draw of the amp under test. If teh current starts to ramp up at low mains voltage, stop and back down, something is wrong. Don;t turn it up until things get hot, that will damage parts. I look for shorted semiconductors with a hand meter while the circuit is off. I also look for open resistors at the same time.

[J M Fahey]

build and use a lamp bulb limiter to stop blowing expensive parts

[catstrat]

400 rb

Good morning. Well Markus, i'm thankful the majority, about 90 to 95 per cent, of my income are tube guitar amps. I only take in solid state when things are slow, like now. Tube amps i have no problem most of the time. It's always been my understanding when a solid state unit is blowing fuses check the output transistors along with the drivers. Same with tubes. If it's blowing fuses check the o/p tubes. If it's not any o/p devices then chances are either the rectifiers are bad or ,worst case, a power transformer. I'll be the first to admit, i don't fully trust my meters. I'll also admit i'm a bit intimidated by solid state circuits. If i have the transistors in stock i usually replace everything. I had one of the drivers but not the other and that may be my mistake. I just ordered all the drivers anyway. My variac has an amp meter on it so i can see what the current draw is. Of course you can't get any meaningful readings unless you're at full 115 volts. I don't let things get too hot. At idle if i'm drawing much over 1/2 amp then i then i suspect a problem. Then i back off of the voltage.My approach, if it's getting hot,remove from the circuit and try the variac again. If then the amperage draw is stable, check trans with your meter. I've done that before and had the trans. check good only to put it back into the circuit only to have overheat again, but that may be a driver that's bad. If i have everything in stock, i generally rebuild the whole O/P. People say a light bulb limiter, but honestly, i don't know how that would allow you to get the unit up to full voltage. But again maybe i don't understand fully how it's supposed to work. And Enzo, the method i used to set the bias on the 400 rb is exactly what you suggested. I played a guitar through the thing and it was fine. BUt the o/p transistors were getting too hot too fast, even at idle and that's what concerned me most. I try to use what i have in stock even if it's a substitute. Looking back on it and the fact i did only replace one driver, i'll bet you dime to a donut that where i screwed up. So when everything gets here and i get it installed in the amp, i'll let everyone know the outcome and i'll bet that fixes it. Believe or not, of the solid state stuff i do get,which isn't much, i have about a 75 per cent rate on fixing it. So i'm not a complete big 'ol dummy.lol.

[DrGonz78]

People are saying light bulb limiter as it will help limit current draw if it is excessive. The current is what will destroy all your important components that you just replaced if you still have a short, resulting in excessive current drawn. It's like wearing an amp condom. Everytime you fix a solid state amp and then power it up for the first time, then use the condom. The light bulb acts or reacts in a resistive nature to the current that is being pulled by the piece of equipment you are testing. For one it is acting like a fuse that will not break and even will absorb the current flow. If you don't use a bulb limiter then you run the risk of never being able to safely fix the amp. It is a must...

[The Dude]

Firstly and generally speaking, you should not run an amp with parts removed. Often, you can cause more problems. There are of course exceptions. Secondly, using the criteria of heat only to decide if parts should be replaced is not an effective way of troubleshooting. Parts that are open will not conduct and will therefore not get hot. Often parts get warm because some other part is bad and bias is off. There are a multitude of reasons you should not troubleshoot this way. There are reasons we use accepted means of repairing electronics and "standard" ways of doing things. It's because they have been proven to work and make logical sense. Respectfully, my advice is to learn how to check parts and take the guess work out of the equation. Don't replace something simply because it got hot. Don't fail to replace something because it didn't get hot. Don't randomly replace parts because you think they COULD be bad. Actually check them and find out.

[catstrat]

400 rb

Hello. You guys definitely make convincing arguments. After all, you have a lot more experiance on transistor stuff than i do. Question, how do i go about making a light bulb type current limiter? If i remember right, isn't it a 75 watt or so incandescent household bulb wired in series with the power cable? If so is it wired into the neutral leg or the hot leg? Well i guess i could check the internet for this info. And will the current limiter save the amp from damage while i check voltages? Fortunately for me, the expected voltages are either on the schematic or tech data somewhere. They're so many questions i have. I had a QSC 3200 power amp come to me yesterday afternoon. With a schematic and about 2 hours troubleshooting i found a shorted zener on one of the low voltage power rails. I just replaced both as a safety precaution. I hate customer returns and i don't have many. When i make a light bulb limiter should i keep the ground of the power cable? Thanks everyone for putting up with me.

[The Dude]

Here's a link to one of many.

http://music-electronics-forum.com/a...lb_limiter.jpg

If you Google search "light bulb limiter" you'll find much information.

[catstrat]

400 rb

Thanks Dude. I just checked on google and sure nuff, there it was. I'll get over to Home Depot tomorrow and get the stuff i need and build one, but i'll only pay cash,lol. Fact of the matter, i'm turning away a lot of solid state work which adds up to turning away money. With me, i learn the best when i jump in with both feet and just tackle it. I'll be getting the new drivers in this week for the GK and i should have the limiter built by then so i can test it safely. I'm sure i'll have more questions. Again thanks everyone.

[The Dude]

I'm glad you didn't take offense to my earlier post. Many would have. The fact that you didn't shows your willingness to learn. Troubleshooting solid state amps is a process not unlike tube amps in many respects. You have a power supply a preamp and a power amp. Start with the forest and work your way down to specific trees. Good luck!

[Enzo]

Besides, what is the most common repair? Replacing or resoldering jacks and controls. Do we turn away such simple, quick, and profitable work just because it is solid state?

[J M Fahey]

Light Bulb Limiter

[The Dude]

If it's not already somewhere, that link should be made a "sticky" on the forum. It's something that gets mentioned and asked about a lot.

[catstrat]

400 rb

Actually Dude, i didn't see anything to really get offended by. I do ask myself though, i'm retiring as soon as i hit 62 years of age which is about 1 year away. I've got tube amps pretty much mastered. I do custom tube amp builds from time to time and all kinds of mods and repairs. But solid state has always been a challenge but i'll master it to. I like to think i'm in the process of mastering solid state stuff now. Even after i retire i'll never stop working, basically i'd go crazy if i sat around here doing nothing. I think it was Jazz P told me about a year ago, if i get stumped on any Solid state stuff, just ask. All of you have been more than helpful and believe me, i do appreciate it.