Earthquake 1000bx excessive current while playing

[damage]

The amp draws excessive current when playing. At idle, only draws 1 amp. But when just one channel plays (left or right) it draws 4 amps just to make 1 watt. All the outputs test good, I know one channel is because I replaced all the outputs. Also replaced all the PS FETS with IRFZ44 and 125 ohm gate resistor.

Any ideas?

TIA!
Tristan

 

[Perry Babin]

Can you reduce the input voltage to the amp to about 10v? If you can, do so. Does the current draw increase when you reduce the voltage?

 

[damage]

Well my variable PS is not regulated. But if I input 10V, then input the sine wave, the voltage goes down and the current goes up pretty rapidly.

 

[Perry Babin]

If it's adjustable, it's regulated. When you decrease the input voltage to 10v (no signal, no load), does the idle current increase?

 

[damage]

Starting at 13.8V and descreasing, the current goes up from 1.3A to 1.6A until 11V then the current continues to go down fairly rapidly.

I dunno about the PS. I thought it was a variac type. It's a Brodhead Garrett. But I've never opened it so I dunno. Seems to get dragged down fairly easily. It'll pump out about 9 amps at ~13.6 knob turned to full power. Unloaded it will do 25V. I only use it during repairs, not during completion testing as it's the only adjustable one I have.

 

[Perry Babin]

I could be wrong about the power supply regulation. I've never seen one that does what you described.

Post a photo of the gate drive signal on the power supply FETs.

 

[damage]

Here is a shot. Not exactly perfect.

 

[damage]

Gate resistors are 68 ohms and FETs are IRFZ44N.

 

[Perry Babin]

Set the DC voltage lower (near 10 or 11v) and post another image.

 

[damage]

Looks the same at 11V as it does at 10V.

 

[Perry Babin]

The drivers don't appear to be pulling the gate voltage down properly. What are the PNP driver transistors?

Does the gate signal look the same on all of the FETs?

 

[damage]

As far as I can tell, it's a single 2SC3227 for each channel. Yes, the signal looks the same on all, both sides. Signal coming off the TL594 looks cleaner, though it does have the ringing on it.

 

[Perry Babin]

There should be a PNP driver. The 2SC is an NPN transistor.

 

[damage]

Here is a schematic I've drawn.

 

[damage]

Opps... there is 12V connected between R36 and R34.

 

[Perry Babin]

Can you post a photo of this area of the board?

 

[damage]

Here's the 594 and to the lower right of it one of the drivers.

 

[Perry Babin]

Can you post a photo of the bottom of the board of the same area?

If the original photo is larger than you can post here, please email it to me.

babin_perry@yahoo.com

 

[damage]

Sent. Thanks!

 

[Perry Babin]

This is like some of the older Autotek and Hifonics amps. It uses the 56 ohm resistor to pulldown the gate voltage.

What is the total number of power supply FETs?

What were the original FETs?