Gallien_Krueger 700 rb II short

[hlynge]

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 ????



Kind Regards
Henrik Lynge

[Enzo]

Output devices - probably shorted.

[hlynge]

Any guidance on where I find the output devices would be apprciated and also some hint on how to check them

[Enzo]

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]

Did you disconnect the speaker load before trying the bulb limiter?

[hlynge]

No speakers are connected - Can I test the power transistors when the board is not connected and how would I do that ???

[hlynge]

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]

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]

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.

You can unplug the AC Secondary connections (the 7-wire polarized connector) and then check with your meter in Diode Junction mode between the two AC~ terminals (middle ones) and the outer terminals (+) & (-)...remembering to reverse the leads when changing to measure the AC to (-) lead. I've found the 700RB II and 1000RB II fail there, and no failure in the output stage...but, to be thorough, you will need to remove the fan, small dia # 1 philips screwdriver thru the upper fan mtg holes to unthread the long screws that go thru the heat sink and into threaded fasteners below.

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]

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]

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 rectifier
Shorted capacitor
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]

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.

LM3886T.pdf

[g1]

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:

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

[hlynge]

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]

Dig your probes in. Mostly we are looking for shorts or low resistance between any legs of the output transistors.