Gallien Kruger ML [S] Guitar head : Power supply

[Rob Denton]

I am attempting to resurrect a GK stereo guitar amp. It's been sitting in my closet for about four years. Before that it was molested by at least one technician who was unable (unwilling) to solve the problem. The nice folks at GK sent me the schematics for the PS. Unfortunately the numbering isn't quite the same on the schematic as on the PCB. Also a lot of the component values are different. So, there's been a lot of head scratching. I've tried just replacing the components that had obviously exploded... there seems to be more to it than that. My next step is to dissect it piece by piece and replace everything that isn't working.

Anybody know where I can get my hands on a +/- 45VDC and +/- 22VDC power supply?

The PS part # is 406-0071e, if anyone has a GK amp that's sitting in the basement (or attic) I might be willing to take it off your hands.

[teemuk]

I think that not many people are willing to give those amps away. The "shotgun" method of fixing is likely no good - there's a big chance that once you fix something, something else that is faulty just destroys things again. You have better chances if you try to troubleshoot that device and locate the problem. It likely will be the cheaper option as well. What kind of fault does it have?

[Rob Denton]

I started by replacing a TIP50 that had obviously exploded and a couple of the caps a diodes that were in range. When I fired it up again the new diodes exploded. The filter caps and coils all seem to be fine. I think it might be the pair of powerMOS transistors (BUK456) or the Current PWM controller (UC3843).  I can't get the thing to stay on long enough to test anything out.

[teemuk]

http://www.about-guitar-amps.com/guitar_amps_schematics/gallien_krueger.htm
(at http://www.about-guitar-amps.com/free_guitar_amplifier_schematics.html)

Contains very excessive amount of service information for G&K amps, including M/L series. You'll find complete schematics and layout diagrams (for many revisions of that amp), documentation of circuit changes, scope screen caps, etc. One of the documents even has something written about basic troubleshooting of those amps. I suggest you download all that stuff. The size of the documents is about 15 Meg so I can't really post that stuff to anywhere. You need to register to get the password to access the schematics but it's free and you won't be receiving spam afterwards.

As for keeping the amp running and preventing damage, when lacking expensive lab equipment, the usual trick is to hook a light bulb in series with the device (amp) in the mains side. Here's a diagram of the circuit at R.G. Keen's Geofex site:
http://www.geofex.com/Article_Folders/SPO_Test.htm
Personally, I found it a bit obscure at first (unfortunately, the site with simpler schematic seems to be down) but it starts to make sense when you realise that the light bulb is simply plugged in to one of those outlets. So, just think of a light bulb wired in series with a mains outlet. It's a simple, cheap and effective "device" that will likely prevent a lot of blow-ups.

Beware, with those amps (and I assume in the particular area of fault as well) you're dealing with a switch-mode power supply that is directly connected to mains (no galvanic isolation) and can be very lethal. But I think you knew that already.

[Rob Denton]

HEY teemuk,
Thanks for the links. I see what you mean about the light bulb safety circuit being a little cryptic, but a bulb in series with the PS makes a whole lot of sense. The revisions of the PS for the GK seem to deal with stuff that doesn't really pertain to the one I have. The layout for rev E may help me decode the schematic. Thanks again for your WoW (words of wisdom). It's been almost 15 years since I did any serious electronics; I guess that makes me a born-again newbie ;-) Provided I don't accidentally defibrillate myself, I'll let you know what progress I make.

Peace.

[Rob Denton]

Thanks again for the book. You rock!

[J M Fahey]

Dear Rob: a "Plan B" solution that I´ve sometimes used with those beasts, after I got tired of replacing blown parts,and learning that the switching power transformer was absolutely unavailable, was to simply ditch the switching board and convert it to a "regular" power supply (which is the *other* version of the same amp). The 50/60 Hz power transformer is slightly too thick to fit fully into the original chassis, so I cut a "window" in the aluminum bottom panel , let less than 1 cm of (properly insulated) windings show through it and used 4 oversize rubber feet yo avoid it touching the speaker box. In a MB150 combo, that wasn´t even necessary. That "solution", by the way, is used by GK themselves . Cheers.