Genz Benz GBE 600 DC Fault Protection

[disorder]

I have a GBE 600 amp on the bench that had it's positive rail fuses blown. After replacing the fuses the amp will stay on without blowing them, however the PROTECT LED comes on. In this amp if it comes on it switches a relay open so nothing goes to the output jacks. I have since removed all the positive side power transistors and the protect led stays on. Not sure if its related but when I remove IC1 the protect LED turns off and the relay switches close. However replacing IC1 with a known working chip turns the amp back into protect mode.

Does anyone have any experience on these amps? I'm trying to figure out this protect feature is initiated so that I can then find whats turning it on. I have the schematics from Genz and will PM them if it helps. Thanks.

[MarkusBass]

I don't have experience with this amp but there are general rules, especially if you have a schematic. I would start with a check whether there is DC on the output and what are the power supply rails.
If this is OK, you can continue with the protection circuitry; if the LED is on, why not check what are the signals on the input of this circuit. Then you most probably would see that replacing the IC1 was not needed (because the circuit works correctly). You have to find out what signal cause that the LED is on. And with schematic and few simple measurements this shouldn't be difficult. Don't think that sending PM to Genz is needed.
Is this schematic somewhere on the net?

Mark

[Enzo]

The protect circuit disconnectsw the speakers when it detects DC on teh output. You cannot operate the amp without the positive output transistors, that will guarantee the thing protects itself. WIthout them, there is nothing to counterbalance the negative side parts, so the output can never center around zero.

The relay opens the connection to the speaker, so explore the circuit on the amp side of the relay. ANy DC offset there?

[mozwell]

With the power off, do the negative side power transistors measure ok. What part number are they. Are they Bipolar, or Mosfet.
Do the positive side transistors you removed measure ok.
Are the power supply rails all ok, what voltages are they.
Replace the positive side transistors if they are ok.
What is the output DC voltage, before the speaker relay.

If any output transistors are blown, replace their driver transistors as well.
If only one transistor is blown on the positive or negative side, replace them all on that side just in case the others have been stressed.

Can you post the sch, so we can see what we are talking about here

[disorder]

Here is the schematic...

GBE-NEO 400-600 WATT POWER AMP SCHEMATIC 1-12-06.pdf

I have pulled all of the positive output transistors... When I disconnect VCC from the power amp board, the fault protection LED turns off, the relay switches closed, and the preamp portion of the amp still functions as normal. I can connect or remove -VCC and the fault protection stays on so I assume the issue lies in the positive rail.

Now all the positive output transistors and their driver transistors measure ok with a meter and with a Peak Transistor Meter. To try and source where the DC on the output is coming from I lifted the following components.. D8, D7, D6, and D12 along with R48, R54, and R36. The exact transistors removed are Q29, Q27, Q25, Q23, Q21, Q19, and Q14. The problem still remains.

When I go to measure the DC on the output it constantly goes from 0VDC to a few millivolts to my meter reading OL (overload) then it repeats.

Any ideas? This is where my technical abilities leave me lost... a little over my head here.

[MarkusBass]

I'm sorry but what you are doing is completely wrong. If you pulled out all of the positive output transistors, why haven't you pulled out the ones from negative side? Do you expect that without those transistors it will start working correctly? Also, lifting or pulling out other components is a bad idea. For example Q14 - I wonder why you haven't seen "magical smoke" yet? If the amp works (nothing gets hot, etc), I would leave it as it is and start measuring voltages (e.g. on Q12, Q13, Q15, Q16). But first put Q14 back (maybe a new one) and decide what to do with output transistors. I think that with so heavily modified amp no one will tell you what is wrong with it. Pulling random parts from positive side/rail of the amp is not a good idea. You need to understand the schematic.
Edit: also check whether you have +/-15V power supplies.

Mark

[disorder]

The amp does NOT work, I was pulling those components to try and find where the DC was sneaking to the output. I thought I eliminated the negative power supply as a cause because disconnecting -VCC from the power amp board did not cause the PROTECT led from turning off. Disconnecting VCC turns the PROTECT led off and it also closes the relay... thus proving that the DC is sneaking to the output from the positive side... right?

[MarkusBass]

I understand that it does not work. But pulling out components is not a proper way to fix it. A much better way is to measure voltages on (not pulled out) components. The prove that DC is sneaking to the output from the positive side is not needed. You need to find out how does it happen. And this can be done much easier with components in the circuit. I would measure voltages on Q12, Q15, Q14 transistors and R37, R38, R43, R41 and R43 resistors.

Mark

[Enzo]

Positive voltage on the output can ONLY come from the positive supply, but the question is WHY is it getting there.

There is a good chance those parts were being told to conduct, and they were just doing their job. Removing them leaves you with a missing circuit and no way to determine why it was not working correctly.

If your automobile engine is running poorly, you can't remove the pistons to find the trouble.

[Jazz P Bass]

It may assist you in the repair, if you keep in mind that the output section is a Class B topology.
Class B meaning all sections of the output circuit are (basically) off.
In a properly working output section, the components are off, merrily waiting for the preamp signal , going high & then low, to "drive" the circuit into conduction.
If a rail is stuck on, something is conducting (as Enzo stated).
An ammeter on the mains, a lamp limiter, a static check of all transistors & resistors, a few basic voltage measurements, will assist you in of finding the offending component(s).

[disorder]

I replaced Q17 after it measured bad and the amp now does not enter protect mode. However, I am wondering should the trimmer at Q14's base be able to balance out the voltage there?

[Jazz P Bass]

Q14's trimmer is for setting the idle bias.
It will determine how "hard" the output section is turned on at idle.
Remember, it is a Class B output section.
By adjusting VR2, you are adjusting the output section into "just" Class AB.
You could monitor the voltage across Q23 Emitter to Q24's emitter.
50 mv's sounds like a good number to start.
What you want to do is minimize the crossover distortion, by going slightly into Class A/B.

[mozwell]

Disorder
Just to be sure, do your supplies measure correctly ? (all measurements reference to pcb 0V, anode of D15)
+Vcc (cathode of D12)
-Vcc (anode of D13)
+15V (cathode of D14)
-15V (anode of D16)
measure the speaker output, it shoudl be within +/-0.1VDC of 0V
It is good that the protect led is not coming on now

If you want to disable DC fault protection, temporarily short R19

Generally, if an output transistor is blown, it is best to replace all devices on the same rail as the blown device, and also its driver transistor. Even if the driver transistor hasnt "gone" it may have been stressed & not be reliable. for the small cost of the extra transistor, and as the amp is open anyway, its a small price to pay for extra reliability.

If you can adjust the bias for the output transistors, it looks like all is good.
Leave the amp on for half an hour, is anything getting overly hot ?
Recheck the bias & adjust VR2 again.
If you have a load bank, run the amp at a decent load for a few hours to make sure it all works ok.

[romackr]

Would anyone happen to still have a copy of the schematic. I have clicked the accompanying link but it just hangs. Thanks.

[Enzo]

Someone here may be able to repost it, but the simple thing to do is call Fender and ask them for the drawings.