|hlynge||12/27/2017 7:31 AM|
|Gallien_Krueger 700 rb II short|
Need some guidance on where to start my error finding. Have a used Gallien - Krueger that when connected to a 100 W bulb limiter lights up the entire room so there is a short some where. I got it used and the seller explained that it blew the fuse when turned on and that is correct.
Visual inspection does not provide any good clue. where should I start ????
|Enzo||12/27/2017 9:25 AM|
|Output devices - probably shorted.|
|hlynge||12/27/2017 9:51 AM|
|Any guidance on where I find the output devices would be apprciated and also some hint on how to check them|
|Enzo||12/27/2017 10:31 AM|
|There are a dozen large powr transistors under the heat sink assembly. Those are the output transistors. We look at each one with an ohm meter to determine if any two legs are shorted together on each. That is where we start.|
Other less likely failures might be the main rectifiers - the large rectangles nestled between the main filter caps.
|Mick Bailey||12/27/2017 11:09 AM|
|Did you disconnect the speaker load before trying the bulb limiter?|
|hlynge||12/27/2017 11:53 AM|
|No speakers are connected - Can I test the power transistors when the board is not connected and how would I do that ???|
|hlynge||12/27/2017 2:21 PM|
|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|
|J M Fahey||12/27/2017 7:22 PM|
|Power transistors seem to be sandwiched between PCB and heatsink.|
The contact surface will be coated in white thermal grease, which is sticky.
12 of them will hold the heat sink quite tightly to the 12 power transistors, but I hesitate to suggest you simply jimmy them away, because there might still be some other element joining them and you might destroy the PCB.
With due respect, I think you are trying to start with a very complex piece of equipment.
To boot, GK has long made very compact amplifiers, so they choose complex assemblies jusb to make it a little smaller.
Not good for any Service Tech and even more for somebody starting to learn..
A jimmy tool:
|nevetslab||12/27/2017 9:35 PM|
I also find the 50W HF AMP IC fail on the opposite end from the AC power supply end.
As JM Fahey said.....tackling repair on one of these amps is not a place to start learning how to repair an amp. They are very tedious and challenging, densely packed in their component placements. Still, you might be lucky and the problem is a failed bridge rectifier..
|hlynge||12/28/2017 1:30 AM|
|I am trying to be methodical and start by testing all the transistors to confirm that they are not gone. would you be kind and give me a hint on how to identify the "50W HF AMP IC" - google is not my friend on that one )|
|Mick Bailey||12/28/2017 4:27 AM|
|I recommend that you do not attempt to disassemble the heatshink but instead check the transistors from the underside of the board. Each transistor has a single mounting screw which is central, and three terminals. Page 28 of the service manual (Electrotanya) shows how they are located. There is no need to disrupt the assembly and if the transistors are OK you've disrupted them for nothing and will have to go to the expense of buying new thermal pads.|
One check I make with a 'fuse blower' is to see if there's DC on the main speaker output when the amp is powered with no load attached. Sometimes the amp is so badly shorted the test is meaningless, but very often if only one side is affected one of the output rail voltages will appear at the output.
There are a number of faults that are possibilities. One or more shorted transistors are a common occurrence, but less likely are;
Shorted mains transformer
The 50W HF amp IC is shown on the schematic as the horn driver amp IC (LM3886). A short here will cause your limiter bulb to light up. If you are unable to read a schematic or identify the output transistors then you have an extremely difficult job ahead.
|nevetslab||12/28/2017 11:22 AM|
|I didn't have a GK 700RB II in the shop, so I went to our Rental inventory, opened up a 1001RB which mechanically is nearly the same thing & took some photos of it inside. You'll find the LM3886T 50W Power Amp IC on the left end of the amps' heat sink assy.:|
I've also attached the Data Sheet of the LM3886T. When these parts fail, there is usually a short between the Output terminal Pin 3 to either the V+ Pins 1 & 5 or to V- Pin 4. Replacement IC's are avaiable in both the Metal Tab or an isolated plastic case. The metal tab part required sn insulator sheet between the heat sink and the part. GK often uses a greas-less rubberized heat transfer pad in place of greased Mica. The grease-less part usually has thermally bonded to the part, and in removal, it damages the insulation, making it unusable, requiring it to be replaced. If that part has shorted, and no shots are found on the power transistors underneath the heat sink, sandwiched between the hat sink and PCB, then you'd need to either buy the isolated plastic part or buy the mating Mica insulator, and would also need silicon thermal grease for instaling the new one.
I've also attached the National Semiconductor data sheet on the LM3886T.
[ATTACH=CONFIG]46301[/ATTACH] [ATTACH=CONFIG]46302[/ATTACH] [ATTACH=CONFIG]46303[/ATTACH] [ATTACH=CONFIG]46304[/ATTACH]
|g1||12/28/2017 12:13 PM|
|I think Henrik is able to read schematics and we have more of a language barrier than lack of ability.|
The service manual Mick referred to is available here:
|hlynge||12/28/2017 2:04 PM|
|I confirm that I am fairly able to read schematics but a little rusty, and yes there is a little bit of a language barrier. I am finding it a little hard to get good readings on the transistors in circuit. I will report back when I have been through all the transistors.|
|g1||12/28/2017 2:45 PM|
|Dig your probes in. Mostly we are looking for shorts or low resistance between any legs of the output transistors.|
|Enzo||12/28/2017 2:54 PM|
|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||12/30/2017 6:21 PM|
|J M Fahey||12/31/2017 7:53 AM|
|Thatīs 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||2/11/2018 3:49 AM|
|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||2/11/2018 10:00 AM|
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||2/11/2018 10:26 AM|
|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.
|Sowhat||2/11/2018 11:04 AM|
Maybe this can help.
|g1||2/11/2018 11:49 AM|
|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||2/11/2018 12:36 PM|
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||2/11/2018 1:34 PM|
|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||2/11/2018 10:56 PM|
|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
|Sowhat||2/12/2018 12:29 PM|
|g1||2/12/2018 12:34 PM|
|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||2/12/2018 12:56 PM|
|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||2/12/2018 1:07 PM|
|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.
|g1||2/12/2018 3:06 PM|
Does the labelling on the board state that it is for 230V operation?
If not, you should assume the labelling is for 120V as GK is a US based company. That is why I asked you to compare your wiring to the picture of the transformer wiring.
The power switch can not make the fuse blow unless it shorts to the chassis.
|Enzo||2/12/2018 4:24 PM|
|g1, look at the photo in post #26.|
The power switch is before the fuse, so if you remove the fuse and it no longer blows fuses, then the switch is not involved.
|g1||2/12/2018 5:48 PM|
|Ok, thanks for pointing out the pic Enzo, I had posted the wrong revision drawing in post #28 which I have corrected.|
His picture matches the drawing.
As far as I can tell it can only be a bad power transformer.
|The Dude||2/12/2018 6:06 PM|
|This is a bit difficult to explain, so hopefully you can follow.|
There is a metal shield between the transformer and the preamp board bolted to the chassis and sticking up vertically inside the unit. Often times, in this amp series, the transformer slides into one of the screws sticking up into the chassis that holds the shield in place nicking into the transformer and shorting to the chassis via the screw. Sometimes, it wrecks the transformer. Other times, it's just a small nick. Once you remove the transformer and cover the scratch, it will remove the short to the chassis. If that is the case, either replace the screw with a shorter one or grind it off so it's not sticking up into the chassis as far. Then, make sure you tighten the transformer back down well so that it can't slide into the area where the screw sticks up. It's worth having a look before ordering an expensive transformer. You just need to remove the transformer and have a look at the underside to see if what I've described is what's happening.
Edit: I edited your picture to better explain.