Gallien_Krueger 700 rb II short

[Enzo]

If a transistor appears to be shorted, it needs to come out for testing further. But be aware, that when a part is shorted, there is nothing in the surrounding circuitry that can make it appear not shorted.

[Sowhat]

How is it possible to remove the cooling grid from the heat sink. I have removed all screws and it is still solid stuck. Not moving at all. Any advice appreciated

Yeah, all the thermal pad insulators are acting like glue. They're not glue but all compressed together by the screws makes them stick. Some gentle but firm prying can start the ball rolling and before you know it the heatsink comes off. This is not without some danger however. It would probably be great if you didn't have to remove the heatsink at all, but if you got shorted output devices, you will have to do the deed and hope that you don't break or rip anything in the process. Shorted GK's always give me heartburn.

[J M Fahey]

That´s what I meant, including the prying tools pictures and warnings.

I always say that each Manufacturer has his own pet ideas which he applies all over the place; in this case GK has always made VERY compact and tightly packaged amplifiers which has made them famous.

Not *really* needed here where the head is roughly conventional size but they did it anyway.

You would have an easier task if repairing an equivalent Peavey or Hartke head, simply because of more available space.

Now some Ampeg ones are also tough to disassemble and reassemble, specially some SVT Pro versions.

What has been suggested so far is fine, you must identify power transistor pins which are accessible on the exposed (solder) side of PCB, and with no need of disassembly (yet) you can test for shorts between Base, Collector and Emitter legs (I assume they use Bipolar Transistors).
Use the Diode test setting, resistance scales are not very useful here.

If using a cheap Multimeter , you might want to sharpen probe tips, so they "bite" better on the solder pad you are making contact with.

Please measure and post results.
The main red flag being a short between any 2 transistor legs.

[hlynge]

I was not brave enough to start prying free the cooling paste so i tried to buy a replacement Gallien-Krueger 207-0250-B 700RB-II Replacement Amp Assembly from full compass. It has now arrived but it is still blowing fuses. Now I suspect that it is the trafo that has a short. it continues lighting up my voltage limiter tilll I pull the purple wire. What is the best way of testing a trafo ???

Kind regards Henrik Lynge

[Sowhat]

it continues lighting up my voltage limiter tilll I pull the purple wire.

Kind of doubt the transformer is bad... purple wire??? I don't have a 700RB power supply schematic but I do have an 800RB and they should be very similar. In that case the purple wire is -15 volts, so... your short my be in the preamp or any low signal circuitry using op amps, not the power amplifier. You need to short test the -15 rail and the +15 volt one for that matter (one of the red wires?).

Locating a short in a power rail is time consuming and troublesome which usually winds up causing you to methodically remove components from the power rail one by one until the short disappears, or you can spot it with an infrared camera or see an evaporation pattern using alcohol, but this is all a bit futile when the fuse blows right off the bat... nothing has time to get hot... Fortunately you have a limiter so things can get hot without blowing the fuse. Use a multimeter and check the resistance between the purple wire and chassis ground, if it's zero ohms you got a rail short... then it's just the task of locating it, good luck!

[g1]

Is the purple on the primary or the secondary? The schematic in the link shows a violet wire on the primary. If that's the one, disconnecting it does not tell you anything.
To test the transformer, disconnect all the secondary wires and leave the primary connected. If the fuse does not blow, the transformer should be ok.

https://elektrotanya.com/gallien-kru.../download.html

[Sowhat]

Is the purple on the primary or the secondary?

Yeah, good thought but I bet it's the secondary side. I just remembered that I posted a similar problem with an 800RB a few months ago...

http://music-electronics-forum.com/t45285/

Maybe this can help.

[g1]

I would tend to agree but the 700RBII schematic I linked shows the violet on the primary. pg. 25 of the PDF
No purple or violet shown on the secondary.

[Sowhat]

I would tend to agree but the 700RBII schematic I linked shows the violet on the primary. pg. 25 of the PDF
No purple or violet shown on the secondary.

The 800RB is physically quite different from the 700RB to my dismay, I thought they were basically different wattage versions of the same thing... whoops! I assume from your location, hlynge, your GK is wired 230V.

Now that I see the 700RB schematic all that's being done there is disconnecting the mains with the purple wire, yeah, not very helpful in troubleshooting but sure saves fuses.

To disconnect the secondaries while leaving the primaries connected, unplug J20 which is the 7 pin molex between the big filter capacitors... oh yeah, and reconnect that purple wire (J15?).

[Sowhat]

Just a little follow up here... once you have J20 disconnected and J15 reconnected, power up with your limiter and if there's no or little glow your transformer is probably OK. If you pass this test, power off and reconnect J20 and then disconnect J8 which is the smaller 7 pin connector that powers the preamp... it will have wires going to the preamp, it's easy to spot. Power up again on the limiter and do the check, if you're still good (little glow if any) you have a short in the preamp, if not it's your brand new store bought amplifier module that has a short, and who knows what could be up with that?

[hlynge]

Yes I am from denmark and the amp is wired to 230 V. Having disconnected J20 (Large white connector) and reconnecting J15 (violet led) results in a full glow on the limiter. What to try now.
Your help and guidance is highly appreciated
/Henrik lynge

[Sowhat]

Yes I am from denmark and the amp is wired to 230 V. Having disconnected J20 (Large white connector) and reconnecting J15 (violet led) results in a full glow on the limiter. What to try now.
Your help and guidance is highly appreciated
/Henrik lynge
[ATTACH=CONFIG]47055[/ATTACH]

Well then Henrik, you got a short in your power transformer, just as you suspected and nevetslab pointed out. The only other things in that circuit is the IEC socket, power switch and the fuse which are part of the amplifier module you just purchased, I don't think anything can be amiss there... examine it visually, but I don't think you will find anything wrong.

[g1]

When you got the amp it was blowing fuses so you still need to verify the PT primary is wired correctly.
Look down the 230V column in the picture and check that the correct wires are going to the connections.
VIO to J15 etc.

[hlynge]

The board that I have got is of the series 206-0250-B2 and there is cable coloring printed on the board. This is what i have followed and I i have now confirmed that the cabling is correct according to the printed color coding on the board.

Is it possible that the switch can short-cut as this is the only component that seeem a litte questionable ???

[Sowhat]

I have heard some advise to you on this forum about the complexity of this piece of equipment for a novice, and they are very right about this however I applaud your tenacity with this issue. A power input forward approach is the way all of us should analyze a problem like this, and we do except for when we don't. This last test involving J20 should have been the most logical first step in this diagnosis after checking the fuse. Here however, it was the last.

Can't cry over spilled milk but it sure would have been nice to avoid the extra expense of a 700RB amplifier module plus a power transformer. On the other hand, nevetslab explained that these power transformers fail for a reason which usually lies in a failure in the amplifier module power section, so, maybe you have no choice on the matter here and maybe replacing it entirely with a new, good, used, working 700RB would have been the winning move.