Gallien-Krueger 800RB

[Janky]

Hi all,

Brand New here, and not a Forums guy, so I apologize if this is not the place, or it's already been answered.

I re-acquired my old bass amp from a friend whose had it for around 20 years. It's a vertical switch GK 800RB. It won't power up at all. No lights, nothing. I poked my Fluke meter around to rule out the basics...conductivity through the fuse, the power switch...Power goes all the way to the transformer, but that's where I run out of knowledge. There is no signs of damage or burning or even cracked solder joints anywhere. I'm assuming its the transformer, or even the power supply board... Can someone either point me in the direction of a troubleshooting schematic to know what voltages should be where, so I know what to rule out? Or does anyone have intimate knowledge of this amp that could walk me through a few checks so I don't shotgun parts at it?

Thanks
Chris

[Leo_Gnardo]

Welcome aboard!

Power goes TO the transformer leads, you've "rung out" the AC cable, power switch & fuse, that is measured continuity thru all these. So next, do you measure any continuity thru the power transformer's primary? Given the cable, switch & fuse are OK, you should be able to do this by simply putting ohm meter probes on the power cable's hot & neutral blades. If the PT's primary is OK you'll see a couple ohms resistance, maybe 3 give or take. If you read open circuit then there's one more thing: IIRC these amps have a thermal shutoff switch which is supposed to shut off the power if the output transistors get too hot. If that switch is broken, stays open circuit then you'll get no power. It's been a while since I've been in an '800, but I think the switch is on the aluminum heatsink bar that the output transistors are bolted to. Should look like a cylinder maybe 1 cm or 3/8 inch with a pair of quik-connect blades on it, and AC power wires stuck on them. Check for that, ohm it out & let's hear back after that.

[Jazz P Bass]

I will have to agree with Leo.
To prove out the AC input all the way to the power transformer primary winding, I find it convenient to simply turn on the power switch & measure the resistance of the two blades of the power cord.

If that tests good, then you have to dive in to the PT secondary circuit & the rest of the power supply & the complete amp.

That will take a wee bit of knowledge & skill.

[Enzo]

But unplug that cord from the wall first...

[Janky]

Ok...so the two terminals on the primary side of the transformer show open.
So if I understand correctly, it's toast. Is the "Toroidal Transformer kit" what I want to throw at it?

[Janky]

HOLD THE PHONE!!
So now that I decided the transformer is shot, I cut the insulation tape back and followed the primary coil wires ways in, and see what must be an inline thermal switch....on the other side of the switch, I get continuity!

[Janky]

[Leo_Gnardo]

HOLD THE PHONE!!
So now that I decided the transformer is shot, I cut the insulation tape back and followed the primary coil wires ways in, and see what must be an inline thermal switch....on the other side of the switch, I get continuity!

EDIT: Now I see the photo, that's an internal fuse. This type is often heat-activated, opens if it gets too hot. Now there's some decisions to be made. There are replacements available, I'm sure someone here can suggest where to find one. Since it's your amp and you will be the one using it, you could chance just leaving out the fuse and getting on with whatever other problems the amp may present you. Possibly somebody ran the amp into a 2 ohm load for a while, thinking "it's rated for it, why not?" and cooked the transformer. It's also a good choice to replace the transformer if GK still offers it. Best to prove the rest of the amp is still working before spending on a fresh tranny though.

[Janky]

Yeah, I will track one down so I can sell it someday with a clear conscience...but for now, I'll just bypass it to see if everything else works fine.

Thank you all for what you've helped me with so far!

[Leo_Gnardo]

Yeah, I will track one down so I can sell it someday with a clear conscience...but for now, I'll just bypass it to see if everything else works fine.

Thank you all for what you've helped me with so far!

Before you just go plug in & hope for the best, I'd like to suggest 1) Bring the power source up slowly using a Variac or similar (not a standard light dimmer) or B) use a "light bulb limiter" AKA "dim bulb tester." I thought we had a sticky somewheres around here, but the same item is described over at our friends "The Amp Garage."

http://ampgarage.com/forum/viewtopic...2a7c39f0c401f5

Note TAG has had a problem lately with being identified as a malware distribution site. It isn't at all, just blast past the well meaning warnings, have a read, and build yourself this handy shop tool for a couple bucks, or maybe just leftover parts.

Failing that, would some helpful MEFster please post a link to the MEF light bulb limiter sticky.

[Janky]

Thanks

[J M Fahey]

1) Digikey has them:
Thermal Cutoffs, Cutouts (TCO) | Circuit Protection | DigiKey
if the link sends you to a generic Digikey page but not the specific one (sometimes happens when open in another machine) search for: "thermal cutoffs" , most popular brand is "Cantherm" altough there are others, 250VAC rating, most are 10A (20A rating does not hurt though) and price is cheap less than 2 bucks.

Temperature: get one between 82C and 92C ; any lower will drive you crazy (cutoff in the middle of a hot solo) and any higher, 100C and above does not provide real protection, because we reach plastic bobbin, tape and cable plasticmelting point (just significant softening is dangerous)

I suggest this **only** becausen I see not even the slightest meltdown signs in bobbin or yellow tape, and no toasting on insulating paper.

Any of these 3 signs in a transformer makes it unusable, even if it "works".

Please post what does the original (blown) one says, a picture closeup even better.

Just to guess "what were GK people thinking".

EDIT: just looked again at the picture before closing it: the visible enamelled wire shows the *dreaded* "fluo red/pink" colour of "self stripping" enamel , also see the last inch of it smoothly tinned, that enamel which has NO place in a MI amplifier (was invented to save some seconds assembly time in cheap wall warts) is *designed* to evaporate, leaving no trace behind, just nice shiny copper, at solder temperatures.

Or what's seen here: submerge the end for 10 seconds in a small "tinning pot" , with molten plain solder inside.

It's designed so you do NOT need to "peel" it off, just wrap it wherever you want, apply soldering iron and it evaporates out of the way.

So you save 10 seconds per connection (a grand total of 5 or 7 in this transformer).

Big problem is that overheating turns coil into a 2 pound block of solid copper ... even a grape seed sized hot spot destroys the transformer.
So yes, it's quite advisable that you replace it with a working one, it might die on you (no big deal) but burn your home or kill people (BIG deal) .

Personally I *hate* that wire, I use heavy duty electric motor rated wire, "Class F" , guaranteed to 180C and really stands some 250 C .
Only "problem" : a bitch to solder, I have to burn it on a hot gas flame (match or lighter or alcohol burner not hot enough) and then scratch or sand it off.

Must be a masochist because I actually prefer that

Have gotten amps back for servicing showing transformers with toasted paper tape and twisted molted bobbins (rolled aluminum paper "fuse" + loading the amp with 4 or 5 cabinets in parallel and using it in an 8 hour long Heavy Metal festival ) ... and after repairing the dead power transistors and blown supply diodes the amp started working still with the overheated transformer

[Leo_Gnardo]

1) Digikey has them:
Thermal Cutoffs, Cutouts (TCO) | Circuit Protection | DigiKey

Should you go the replacement route, be aware of a problem that's been noted on MEF pages: TCO's are temp sensitive, and can be wrecked by the heat of soldering-in. You'll want to use a heat sink clip on the TCO leads, and get in & out fast with your solder iron. Might even make sense to buy a couple TCO's in case you melt one the first time around. Cheap insurance.

[Janky]

250V
121°C
10amp
" UMI J121"

[J M Fahey]

Ok, thanks.
Personally, given the weak wire enamel they used as seen on the picture, I'd buy a 100C tops for next time.

They are close to the edge with the 121C they had used, self soldering wire is rated 130C *but* has no safety margin (it's designed to evaporate with no residue left) and the hottest point on a winding is neither at the outer layer (where you found it) which has a short path to air nor the closest to the core, but somewhere in between.