GK power up circuit?

[Axtman]

Can you guys help me understand how the Gallien Krueger 400RB IV power up circuit works? (BTW, this power up circuit appears on many different GK models.)

When the amp is first turned on the LED lights up and is Red. There is no sound through the speaker even if you are playing. Then after a little while the LED turns Purple and sound comes out of the speaker.

Below are the pertinent portions of the schematic diagram. Thanks!

[Greg Robinson]

It's just a classic 555 bistable circuit.

[Axtman]

How does the fault detetion work? If the rails go above 60v (due to shorted speaker or output transistors) does the amp mute?

Thanks.

[Enzo]

No. Look in the output at the emitter circuit. Where it says BIAS. If the voltage drop across the pair of 0.1 ohm ballast resistors becomes large enough, it will turn on Q2. C2 is there so the condition must occur longer than just a signal peak - it has to charge up. If Q2 turns on, that in turn turns on Q11. Q11 on means the FAULT line will be pulled positive. Now look up to the 555 page. FAULT going positive turns on Q12, which in turn grounds pin 2 of the IC, the trigger pin. And that then turns off Q13, so the blue LED goes off and the amp mutes via the MUTE line. You did not include the actual muting circuit, so we will just assume it works.

[Axtman]

Thanks Enzo! I can always count on you as a wealth of information.

Here is the mute circuit.

[Enzo]

Sure, so when MUTE goes high, it turns on Q4,Q4 with completes the emitter path of the diffy pair.

[Axtman]

I'm trying to understand the circuit functions in order to diagnose the problem. I don't want to just randomly replace components....there is no learning in that!

The amp works just fine but there are no indicator LEDs. I do get voltage on the Blue Power On LED. The LED tests open. I used clip test leads and was able to get another LED to power on. I do need to buy a blue LED.

The amp Red Protect LED tests open and no voltage goes to it. Interestingly enough this LED is a red/green 3-legged LED. The green lead is soldered to the board but does not go anywhere. Either GK had a traincar load of these and wanted to use them up or the 3rd leg added some stability.

Also interesting enough the Red Protect LED is clear and in front/top of the Blue Power On LED. Red plus blue makes purple.

[Enzo]

Why over-think it. The schematic shows the green LED as unconnected. It does label it POWER, but I'd write that off as unused feature on the board. Some other model might make use of it. Or indeed, they might just use them because they stock them.

The red only gets voltage in the fault mode. If the LED si open, replace it.