Hi, I'm trying to repair an old Acoustic 450 amplifier.
I got it after someone plugged the amplifier to 230V mains with the power transformer was set to 115V and someone in the past bypassed the 7A fuse so many components went faulty. The bridge rectifier and the main filter caps was faulty as well as the original drivers and two original output caps. I replaced all the output transistors with 2N3773G and the drivers with 2N5320 and 2N5322 as well as all the electrolytic caps and diodes on the power amp circuit.
With a variac supplying a low operating voltage and a bulb limiter with the output transistors and a 8 Ohms speaker load connected both drivers are getting extremely hot and after 20 seconds or so both getting burnt.
I've already tested all the small transistors out of circuit with a Chinese LCR TC1 and all tested well.
As far as the PSU, the bridge rectifier and the main filter caps were faulty and were replaced with new ones.
With new drivers and with a 100ACV from the variac (power transformer wired for 230V) and without the power transistors the DC voltage at the speaker output is varying from 7.5V on startup to 5V after 1 minute.
How can I make this amplifier to work without blowing the driver transistors?
I got it after someone plugged the amplifier to 230V mains with the power transformer was set to 115V and someone in the past bypassed the 7A fuse so many components went faulty. The bridge rectifier and the main filter caps was faulty as well as the original drivers and two original output caps. I replaced all the output transistors with 2N3773G and the drivers with 2N5320 and 2N5322 as well as all the electrolytic caps and diodes on the power amp circuit.
With a variac supplying a low operating voltage and a bulb limiter with the output transistors and a 8 Ohms speaker load connected both drivers are getting extremely hot and after 20 seconds or so both getting burnt.
I've already tested all the small transistors out of circuit with a Chinese LCR TC1 and all tested well.
As far as the PSU, the bridge rectifier and the main filter caps were faulty and were replaced with new ones.
With new drivers and with a 100ACV from the variac (power transformer wired for 230V) and without the power transistors the DC voltage at the speaker output is varying from 7.5V on startup to 5V after 1 minute.
How can I make this amplifier to work without blowing the driver transistors?
It's safer to do your testing without a load. This will put less stress on the output transistors and drivers should anything be amiss. Then with your setup, check the outputs are mid-rail. Then connect an oscilloscope and look for oscillation. If there's oscillation, let us know what the frequency is...