Guitar Gear Forum

Hey guys. Forewarning... I'm very new to electrical stuff in general... not to mention guitar amps. I can hold my own with most things mechanical, but I'm at the mercy of the electronics gods when I enter this territory...

With that being said, I need a little bit of help. I found an old Gallien-Krueger 250ml in my attic the other day. I remembered getting it from a family member years ago, with the report that it was "Blown out". I figured I should try to get it working, being that I never really gave it a shot at all in the past. I found a few simple things wrong... The fuse was missing, and the thing that held the fuse was destroyed. Bypassed the fuse (FOR NOW... if I get it working I'll put one in), and just kinda cleaned it all up inside and mounted everything back where it went. Still no sound... so I started pulling apart the top. I found on the back of the board two burn marks, along with two solder lines (I don't know what they're called...) that were burned apart. Being the genius I am, I figured they just spontaneously blew out, so I went right to work soldering in some wire. After that was done, I plugged it in, and still no sound. Well, it made a little bit more sound than before, but nothing more than just a slight sign of existence. No response to my guitar, and none of the lights worked either. I also began to smell something burning... so I unplugged it. Looking more closely at the board, I found that the two pins I had soldered were connected to a sort of square thing, which appeared to have some sort of bubbles on the top. I think it's pretty safe to say that that piece is shot. I looked up the number on the side, and ordered a few replacements (Because I'm going to mess at least a couple up...). The number was KBPC102, and the part is called a "Bridge Rectifier". I don't know what that means, but I did notice it was connected to two of the three pins coming from the power supply down below, so I figure it's important...

So my question is... what else should I look for knowing that the bridge rectifier is blown? Nothing else appears to be burned or bubbled or anything, and there are no more burns on the bottom. I just want to make sure that nothing else blew out when the bridge rectifier went.

So long story short for those of you too lazy to read... The bridge rectifier is blown. What might it have taken with it?

I really appreciate any help on the topic. I'd love to get this old amp working again.

Going with the path you are taking - replace the bridge rectifier (converts AC power to DC) - then see what happens. Although when the rectifier went, it could have taken other parts oug with it.

Other than that take it to a tech.

Rule 1 - DO NOT bypass the fuse. There's a reason it was blown. Something on the other side is fucked and drawing too much current. You're allowing MUCH MORE harm to come to the amp internals.

I wouldn't plug the amp in without something to protect against short circuits in the power supply, which the bridge rectifier is part of. There's a reason the rectifier (which, as mamberg stated, converts AC to DC) blew out. Could very well be something to do with all that current you're allowing into the amp by bypassing the fuse.

A current limiter should be used to provide that short circuit protection. Like this...



PS - being mechanically inclined doesn't apply in any way to electronics. Honestly, it doesn't sound like you have the basic understanding necessary to fix the amp without a lot of trial and error, and errors can really make things much worse.

GRIMESPACE wrote:Rule 1 - DO NOT bypass the fuse. There's a reason it was blown. Something on the other side is fucked and drawing too much current. You're allowing MUCH MORE harm to come to the amp internals. .

^^^THIS!!!!!!!!!!!!!!! 1000Kx^^^

Do not byass the fuse on any amp - you could blow a $200 to $300 transformer or worse... if there is loose wire, LETHAL voltage could leak to the chassis.
Order a new fuse socket and buy a few extra fuses.
I know you want to work on it right now but, you need to get the fuse issue fixed FIRST!!!

Rule 2 - Capacitors retain LETHAL charges for several hours (or longer) after an amp has been turned off.
If you do not know how to discharge the capacitors STOP FUCKING AROUND WITH THE AMP AND TAKE IT TO A PRO OR YOU COULD DIE!!!!!!!!
Seriously!!!

Alright...

So having not seen this until just now, I went ahead and put in a new bridge rectifier... I didn't die, but after reading this I don't think I'm going to try again...

The amp made noise for about 15 seconds, then the new bridge rectifier made a puffing sound and started making smoke...

Probably just gonna take it to the music store and see what they can do for me. I really don't feel like dying at the moment, so...

So for the record, I'm not even gonna touch the amp for a few days, cause I don't want to get shocked by one of those whatever-es. But just out of curiosity... does anyone know what could cause the bridge rectifier to go like that?

edit: Called the music store. Their guy only does tube amps. They gave me a place to go to, but I'm curious. How difficult is this stuff to learn? I'm good at just about anything mechanical (Yes, I understand that this in no way correlates to electrical), and I'd love to learn electrical stuff, as the extent of my electrical knowledge is that I just learned what the third pin on a wall socket does... I've never had any reason to try to learn more than simple car electronics though... Can I learn enough to fix this safely by researching the interweb? Or is this like professional electrician stuff?

For this, a good start would be to find (ie search - I'm teaching you to fish, not feeding you ) a schematic for the amp, and post it, along with some good gut shots of the internals - especially said rectifier (or is that rectifrier? ).

Ajax is right about the capacitors, especially any filter caps (they usually are big & round). They may be safe, but they can do more than just kick your ass - they can hold hundreds of volts, and if you hit it, you (or your next of kin) probably won't be too happy. I always use my voltmeter to check them, but that's for another discussion.

We might be able to see something that's not jumping out at you - there's some seriously sharp muthafuggas here that could spot a spec of fly shit on components (especially mamberg). I'm thinking you may have a filter cap that's shorting out - might be burned or otherwise non-functional. That, or some other wiring short could definitely pull the extra current that's causing your rectifier diodes to fry.

Alright, so I found what I think is a schematic...

http://bmamps.com/Schematics/Gallien-Kr ... Manual.pdf

This is what I got. Pages like 10-14 are what we're looking for. I think anyways...

Now, I have no idea how to read this, so I'm going to get started on some evening reading to see if I can start learning what all those symbols mean. If anyone has a good guide to teach a beginner like me to read those... that'd be nice. If not, I'll just keep searchin. I did see the one schematic on page 13 though that had the KBPC102 Bridge rectifier written on there, so that seems like a good start. Now I just need to figure out where everything goes and how to measure/test each thing. I have a decent volt meter somewhere. I just need to find it...

Pictures of the board are coming. I think I may see an issue... There seems to be a brown spot on the capacitor (is that one?) above the bridge rectifier, but I'm not sure if that is just a mark that the bridge rectifier made when it blew. But yeah, I have to get on my phone to post the pictures, so those should be here in a few minutes if all goes well.

Okay, the pictures didn't want to load to the forum, so I uploaded them to Imgur. here's a link to my "album". I feel like a professional photographer using this...

http://imgur.com/a/uY2pJ

Most of the pictures are up, though a couple are still finishing uploading as of 9:45

Anybody have any thoughts on that thing that I think is a capacitor directly above the Bridge rectifier in the pictures? That brown spot?

Anything else off that I didn't notice? The only thing my amateur eyes noticed was that capacitor.

The purple can with the brown spot is definitely a capacitor. Not sure if that's normal discoloration or not - weird, anyways. If you can get some shots of the other side of that, we can narrow down which one it is (I can't see the value in the shot). The more shots around the rectifier, the better.

The hole in the rectifier looks weird, too. You said this one overheated, right? Are you sure you soldered it in the right orientation (there should have been 4 pins - easy to get it mixed up if not careful). Was there a heat sink (finned metal) or something else bolted onto the old rectifier?

The schematics look like they cover several different models. Which one is yours?

And, I'm curious about the leads on the black 2200uf cap - it looks like there's a purple wire connected to one side, going into shrink wrap. It almost looks like the lead from the other side folds around to meet it. Is that right? Don't poke at it - the caps are the things that can give you a jolt.

In the future, consider using photobucket. Real easy to post pics there and get links that embed into your post so the pic shows inline.

I think the purple cap with the brown spot is C49, which does look like it could cause problems with the power supply if it were shorting out, since it has a fairly direct path from +45v to -45v through R122 (the purple resistor directly above C49) and D20 (hard to describe location). It's in one of the channel outputs (upper left corner in the schematic). I'm really wondering if that might be it. How long did this thing sit unused?

<edit - uploading amended pic to show location of components>

GRIMESPACE wrote:The purple can with the brown spot is definitely a capacitor. Not sure if that's normal discoloration or not - weird, anyways. If you can get some shots of the other side of that, we can narrow down which one it is (I can't see the value in the shot). The more shots around the rectifier, the better.

The hole in the rectifier looks weird, too. You said this one overheated, right? Are you sure you soldered it in the right orientation (there should have been 4 pins - easy to get it mixed up if not careful). Was there a heat sink (finned metal) or something else bolted onto the old rectifier?

The schematics look like they cover several different models. Which one is yours?

And, I'm curious about the leads on the black 2200uf cap - it looks like there's a purple wire connected to one side, going into shrink wrap. It almost looks like the lead from the other side folds around to meet it. Is that right? Don't poke at it - the caps are the things that can give you a jolt.

The rectifier just kinda blew up... yeah. The hole was always there though. I got 14 new ones (Smallest quantity I could find in the US) and every one has that hole in the middle. I just double checked, and it was soldered in right. It's got markings on each of the 4 pins that line up with the old one in the pictures that I took prior to taking it off. No, there was no heat sink on anything, though it's possible somebody else messed with the amp before I got it.

Mine is a 250ml.

The big 2200 leads go to different places. The purple wire goes over to the back, and then down into the board, and the other lead just goes straight into the board. The lead to different places underneath.

In the future, consider using photobucket. Real easy to post pics there and get links that embed into your post so the pic shows inline.

GRIMESPACE wrote:I think the purple cap with the brown spot is C49, which does look like it could cause problems with the power supply if it were shorting out, since it has a fairly direct path from +45v to -45v through R122 (the purple resistor directly above C49) and D20 (hard to describe location). It's in one of the channel outputs (upper left corner in the schematic). I'm really wondering if that might be it. How long did this thing sit unused?

<edit - uploading amended pic to show location of components>

I've had it in my attic for at least 5 or 6 years. Not sure how long it was sitting before that though.

More pictures...







I'll look more into that small purple capacitor tonight. Gotta figure out how to test it.

edit: Sorry, the picutres didn't show up here. They still work if you click though. I'll try to fix them today, but I don't have time at the moment.

Only way to test a cap is to remove it (at least one of the legs) from circuit - other components connected by circuit board will influence the value across the leads. You should be able to safely unsolder that cap with the amp unplugged, since that appears to be the lower voltage side of the power supply (hence the 25v rating), and then take your reading with a capacitance meter. If your meter doesn't check capacitance, you might be able to test what we're looking for by reading resistance between those leads - if it's 0 ohms, it's shorting out, but if not, it still may behave different when powered up.

For extra safety, you might want to discharge the big black 2200 cap by shorting the leads (use an insulated jumper cable with alligator clips at the ends, or something like that). With this being a solid state amp, you don't have the 400-500 volts present in tube amps, but you've still got 90 volts potentially ready to bite.

As for the pics, easiest way to post from photobucket is to click the 4th (bottom) IMG box, which will copy the link to the clipboard. Just directly paste that into your posting here, and you're done. I'm pretty sure the ones above didn't embed because of the redundant tags.

Okay, I fixed the pictures.

Thanks for the advice. I'll check that capacitor tonight when I get home.

I really appreciate the help.

edit: While I'm here, does anyone have a suggestion on soldering irons to use for stuff like this? I'm pretty sure the one I'm using now is way too big, but I'm not sure what I should be using.

And one more question... If I need to replace the capacitor, does the number on the top matter? Or is it the 220uf 50v that matter? It has a couple letters and numbers on the top metal part, and I can't seem to find them with that marking. However, I did find these:

https://www.amazon.com/uxcell%C2%AE-220 ... B005MN2TJK

which seem to be the same rating.

220uf 50v Radial Electrolytic is probably all you need to know

For soldering irons, budget will determine how much you can buy. Variable heat control (with a dial, rather than changing tips) is important when soldering things like circuit boards and small components versus grounding the back of a pot.

I've seen pretty good recommendations for these: https://www.amazon.com/Weller-WLC100-40 ... B000AS28UC



I spent more money and got this one, which I'm really happy with: https://amzn.com/B00ANZRT4M



And if I didn't say before,

So I tested the one capacitor, and on ohms set to the biggest number (My meter doesn't have capacitance), it started at like .6 and then started going up. From what I read online, I believe that means it's functioning properly. Time to start checking all the capacitors I can find, then figure out how to check other things...

Edit: I'm wondering if I should put in a fuse and solder in another Rectifier and just try again. There is a possibility that doing it the first time, as I'm not the best at soldering, I could've heated it up too much. That can kill it, right?

Another edit...
I found something strange on the back. Pictures below... It seems there's a wire soldered in there. It doesn't look like that's how it came, based on the soldering joint... I feel like it's supposed to be there, because one pin on a resistor (I think...) Only connects to that wire. Nothing else, and I don't know why it would just trail off like that. But that solder joint definitely doesn't look like a new amp...

Any thoughts?




One more edit....

After printing off the schematic that's really just a picture with the symbols put on it, I found the solder joint in question to lead to what I believe to be a MOSFET. Now to figure out how to test that...


Another edit:

I'm like... speechless.... I think I found it. When touching the middle pin and either side one (I know there's one I'm supposed to check, and I checked that one first, then the other just cause why not) I get a reading saying that they're connected. I also get that they're connected when testing Ohms. That means it's shorted, right? I'm gonna test some other MOSFETs to see

Okay, so I'm not really sure quite how to do the reading. I looked up a video, and with the mosfet "powered off" It all goes how the video describes. With the "Powered on" test, I can't get it to do what he says on any MOSFETs. I don't get power from the drain after touching the gate or whatever.

However, with the powered off test, the one in question has connection between drain and both other pins, while the others do not. I think this one is bad.

Disregard my enlightened "I think I got it!" post... I have no idea what's going on... I can't figure out which pin is which, and none of my tests on any mosfets go like the ones in the videos... is it possible for all of them to be bad...?

It's quite possible that the output transistors have failed and taken the rectifier with it. I don't know if we have many people here who know a great deal about SS amp repair but I can fully recommend asking here: http://music-electronics-forum.com/f22/