Autotek MM 4000.1

[S Merlington]

I have a strange issue with theis amp..

If i install the audio driverboards amp will not power up no power light no nothing its dead..

If i take both of the audio driverboards out of the amp amp will power up and idle perfectly ..

I checked all regulated voltages and all are there and correct..

I get rail voltages also..

But with the audio driverboards in i get nothing..


Any ideas what might cause this or what to check??

 

[Perry Babin]

You need to compare the DC voltage on all pins of the KA7500 and the LM293 with the driver boards in the circuit and out of the circuit.

Did you try installing one board at a time?

 

[S Merlington]

voltage on the KA7500 with no audio driverboards in the amp..

Pin 1:0.00
Pin 2:0.00
Pin 3:0.00
Pin 4:0.00
Pin 5:0.00
Pin 6:3.80
Pin 7:0.00
Pin 8:13.80
Pin 9:5.33
Pin 10:0.00
Pin 11:13.80
Pin 12:13.80
Pin 13:4.95
Pin 14:4.95
Pin 15:4.95
Pin 16:0.00

with them in

Pin 1:6.32
Pin 2:0.00
Pin 3:0.00
Pin 4:0.00
Pin 5:0.00
Pin 6:0.00
Pin 7:0.00
Pin 8:13.22
Pin 9:0.00
Pin 10:0.00
Pin 11:13.22
Pin 12:13.22
Pin 13:4.95
Pin 14:4.95
Pin 15:4.95
Pin 16:0.00

LM393 with boards in the amp
Pin 1:6.91
Pin 2:0.01
Pin 3: starts at 6.00 and slowly drops down to 5.00
Pin 4:0.00
Pin 5:1.03
Pin 6:3.25
Pin 7:0.10
Pin 8:0.06

With the audio driverboards removed

LM393
Pin 1:0.10
Pin 2:0.00
Pin 3:0.29
Pin 4:0.00
Pin 5:1.02
Pin 6:3.32
Pin 7:0.11
Pin 8:13.22

I tryed installing 1 driverboard at a time in either location amp does the same thing

 

[Perry Babin]

There are too many pins with 0.00v. Various problems:

Pin 5 should be at approximately 1.6v. The IC wouldn't produce any pulses that could be considered normal with that pin at 0v.

Pin 2 should likely be near 2.5v. At 0v, the comparator (internal to the IC) can't function.

Pins 9 and 10 are generally the same voltage (±0.05v)

When it was idling perfectly, did you confirm that all FETs were being driven?

 

[S Merlington]

Ok with the audio driverboards out of the amp .. and the amp powerd up..

I get nice square waves on all of the gate legs of all power supply fets..

KA7500
Pin 1:0.01
Pin 2:2.41
Pin 3:0.08
Pin 4:0.00
Pin 5:1.46
Pin 6:3.70
Pin 7:0.00
Pin 8:13.02
Pin 9:5.34
Pin 10:5.34
Pin 11:13.08
Pin 12:13.08
Pin 13:4.96
Pin 14:4.96
Pin 15:4.96
Pin 16:0.00

LM393

Pin 1:0.10
Pin 2:2.49
Pin 3:0.30
Pin 4:0.00
Pin 5:1.03
Pin 6:3.23
Pin 7:0.10
Pin 8:13.06

This is with no audio driverboards in the amp..

Do the voltages appear to be correct or do i have a problem somewhere?


I guess i wasent making good contact with the pins on the first attempt

 

[Perry Babin]

That looks OK.

 

[S Merlington]

Before i install the driverboards and post the voltages I have a question ..

Is this normal for the trace to be cut in this amp or did someone cut it?

 

[Perry Babin]

I don't know if it's normal for that amp but it's not uncommon for there to be a mistake in the layout or a need to make a modification to improve the design. The depth and width of the cut would tend to indicate that it was done intentionally.

 

[S Merlington]

Ok i will re-install the audio driverboards and post voltages once they are in

 

[S Merlington]

KA7500

Pin 1:6.30
Pin 2:2.54
Pin 3:4.74
Pin 4:3.96
Pin 5:1.56
Pin 6:3.66
Pin 7:0.00
Pin 8:13.22
Pin 9:0.00
Pin 10:0.00
Pin 11:13.22
Pin 12:13.22
Pin 13:4.95
Pin 14:4.95
Pin 15:4.95
Pin 16:0.00

LM393

Pin 1:6.91
Pin 2:2.48
Pin 3:starts out at 6.80 and keeps falling down towards 0.00
Pin 4:0.00
Pin 5:1.02
Pin 6:3.40
Pin 7:0.10
Pin 8:13.22

 

[Perry Babin]

When the voltage on pin 3 is above the voltage on pin 2, pin 1 should be near the positive supply voltage.

When the voltage on pin 3 is below the voltage on pin 2, the voltage on pin 1 should be near ground.

When pin 1 on the 293 is near ground, pin 1 on the 494/7500 should be near ground and the IC should produce output.

 

[S Merlington]

Is there something on that board bad or something on the main board defective?

 

[S Merlington]

can i short pin 3 to 4 together on the 393 and see if it will power up or is that not a good idea?

 

[Perry Babin]

When the voltage on pin 3 drops below the voltage on pin 2 of the 293, the voltage on pin 1 of both ICs should be near 0v. You have to realize that the voltage on pin 1 changes as pin 3 drops below 2.5v.

 

[S Merlington]

ok i resolerd the power supply driverboard to the main board

Heres what i get on the 393 now

Pin 1:6.91
Pin 2:2.48
pin 3:starts out at 3.00 then drops to 2.59 volts then spikes back up to 2.70 volts and continues to do this ..
Pin 4:0.00
Pin 5:1.02
Pin 6:3.37
Pin 7:0.10
Pin 8:13.22

Voltage on PIN 1 of the 7500 remains at 6.91 volts..

 

[Perry Babin]

Are you sure that you're watching the voltage on pin 1 of the 7500 closely enough?

I would expect it to pulse down to near 0v when pin 3 of the 293 spikes back up.

 

[S Merlington]

Yes i tested it again voltage never moves from 6.91 volts on Pin 1 of the 7500.. Used 2 multimeters to do this test and sat there for 5 mins

 

[Perry Babin]

It may be too fast for a multimeter to catch it. Use your scope. Set the vertical amp to 5v/div and set the coupling to DC. Align the trace to the reference. Set the timebase so that the trace turns to a slow moving dot and watch the voltage on pin of the 7500 closely. You should see it pulse at the point where the voltage on pin 3 of the 293 jumps back up.

 

[S Merlington]

DO not see the pulse when the voltage jumps back up..

I set scope according to you message and then messed with the settings a lil still did not see a pulse when the voltage jumps

 

[Perry Babin]

Does the rail voltage pulse up/down when the voltage on 293-3 jumps back up?