XXV Sampson power up issues

[beatnbronco]

understood, yes it does. Do the PS drivers get their voltage/signal from the driver IC on the driver board? The IC that's labelled KA7500BD? Digikey says the part is obsolete but the TL494 is a good replacement and I have some on hand if needed

 

[Perry Babin]

The 494 will work. The 594 is a bit better in some ways but I doubt that you will see a difference between the 494 and 594.

 

[beatnbronco]

do you think I should replace that KA7500B or what ways can I test it?

 

[Perry Babin]

Post the DCV on all pins. Copy and paste the following and fill in the blanks.


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

 

[beatnbronco]

Pin 1: 0
Pin 2: 2.3v
Pin 3: 0
Pin 4: 0.7v
Pin 5: 1.6v
Pin 6: 3.8v
Pin 7: 0
Pin 8: 13.8v
Pin 9: 0
Pin 10: 4.8v
Pin 11: 4.8v
Pin 12: 4.8v
Pin 13: 13.8v
Pin 14: 13.8v
Pin 15: 0
Pin 16: 5.1v

 

[Perry Babin]

The pin numbering is wrong. See attached.

Redo this and double-check 9 and 10. 9 looks OK but 10 doesn't.

 

[beatnbronco]

Pin #1 is near the indented dot correct?

 

[Perry Babin]

Yes. You have the first 8 right. It's the other half that are reversed.

 

[beatnbronco]

so i had 9-16 backwards
9. 5.1v
10. 0v
11. 13.8v
12. 13.8v
13. 4.8v
14. 4.8v
15. 4.8v
16. 0v

 

[Perry Babin]

!0 is either being dragged down by a short circuit to ground or the IC is defective.

No power applied, resistance from pin 9 to pin 7?

Same for pin 10 to pin 7?

 

[beatnbronco]

9-7 is 0.4 ohm and 10-7 is 0.5. Meter set to 200ohm

 

[Perry Babin]

That's unexpected. Can you post a high resolution, high quality photo of the area around the driver IC? I need to be able to read the component values.

Do you have another amp laying around to measure the same thing?

 

[beatnbronco]

best i can get without taking it off the board

 

[Perry Babin]

Measure the resistance across R30 and R31 on the main board. No power applied.

 

[beatnbronco]

R30 is 220 ohm and R31 is 000. So is R31 an open resistor?

 

[Perry Babin]

Open? There appears to be a short-circuit to ground. Lift (similar to the attached) pin 10 to see if the lifted pin 10 still reads 0 ohms to pin 7.

 

[beatnbronco]

answer is 1. I didn't think i would be able to get 1 leg loose from the board intact, which my assumption was correct. If the driver IC is bad then it won't matter

 

[beatnbronco]

1. on the DMM in resistance means pins 7-10 are shorted correct?

 

[Perry Babin]

You can do it. Since it had been many years since I had to do this, I made sure that it could be done with a center terminal today and it works. When doing this, I use a tiny jewelers screwdriver ground as shown below. The screwdriver I used was 1mm/0.04" in diameter. Try it on a junk board if you have one laying around.

 

[Perry Babin]

I don't know your meter but on some of my meters, a 1 is what's displayed with the meters are open (open, contacting nothing but air). It should read a number for a short (when you hold the probe tips tightly together).