Hifonics Problem

[swordfish689]

I have a hifonics that had the protection light on. I Fixed this problem, Now when the protection light goes off the amp doesn't turn on. Here's my quick question, it uses the Ka7500bd smps controller can i use a tl494 as a replacement. The amp is a zx7500.

[Perry Babin]

I think the 7500 is the same as the TLx94 but I'd suggest using the 594 instead of the 494. The 594 has always been a good replacement for the 494 but the 494 was not always a good replacement for the 594. I don't think this is a problem in the newer amp designs but it was in some of the older designs so I always use the 594 because I know it's not going to cause any unforeseen problems.

[swordfish689]

I finally changed the pwm chip, thinking this is my problem, I ordered 594's but first i wanted to see if it worked with the 494. It works, but I'm not having nice square waves on the inputs, and pin 9 and 10 are only 1.8v. I find that a little low, I'm guessing 5v is the norm on this amp. Also the outputs have 21volts on one side and only 17volts on the other. Then i put in the 594 to see maybe the chip is not strong enough to run 4 drive transistors, the 594 is even worse i get only 17 and 11 volts on the outputs and the light is barely on.

p.s. I learned with the original chip the ka7600bd, the supply would start up then shut down.


Here are voltages on the chip


p1 :1.2mv
p2 :5v
p3 :62mv
p4 :186mv
p5 :1.71v
p6 :3.94v
p7 :gnd 0v
p8 :11.28v p.s.
p9 :1.789v
p10 :1.799v
p11 :11.28v
p12 :10.64v
p13-14-15 :5v
p15:.872v

[Perry Babin]

This is very strange. With only 186mv of deadtime, you should be very near full duty cycle.

I breadboarded a chip and set the deadtime to ~190mv and the output was ~4.5v.

Have you measured a square wave gate drive before with that meter? Do you know if it will read ~1/2 B+ with a 50% duty cycle square wave?

[swordfish689]

Here's what it looks coming from pin 9 and 10



the 5volts/div
and 5uS/div

I should be seeing around 20uS, it seem very compact.Also the chip runs very warm.

Sorry perry, i don't understand what you mean.

[Perry Babin]

If that's 5us/div, the operating frequency is too high. The timing capacitor is likely the cause but you should also check the timing resistor.

Ct and Rt are connected to pins 5 and 6 of the 594.