Return of G K 400 RB.

[catstrat]

I'll check the opamp but it seems like i scoped the o/p from the preamp with a signal in and i got a pure sine wave with nothing out of the ordinary that i could see. But i'll check voltage at opamp and see what the are. On 4558 family of chips usually pin 8 is + DC and pin 4 is -15 or something like that. I've found bad chips by checking voltage at all the pins. Sometimes if an opamp is shorted you'll have voltage at pins other than 4 and 8. But all opamps are different and this amp has an opamp i'm not familiar with. I'll "Alldatasheet " it and see what's the pin out. Thanks.

[Dave H]

On 4558 family of chips usually pin 8 is + DC and pin 4 is -15 or something like that.

Schematic says the opamp is LF351N which if I remember correctly is pin 7 +15 and pin 4 -15 but check the datasheet. My memory is no good these days.

[catstrat]

400 rb

Hello everyone. I went ahead and changed the insulators on all the drivers and the to TIP transistors. Could a transistor test good on the bench with a meter but breakdown under the amps voltage? Now testing with my meter i get no indication of any type of a short at all as far as any beeps from my meter. Now i have -57 volts at the o/p. BUt i have no limiter bulb glowing. I still have -57 volts on the emitter and base of all o/p transitors. The bias trim pot has no affect on the o/p negative voltage at all. I checked the op amp on the o/p board an i do some voltages that don't add up. It seems like i have + 13 volts at pin 7 but nothing at pin 4. I'll check those again to be sure and check the data sheet also. I'd have to order that chip. I put a socket in for the opamp so i removed it from the circuit and my voltages still aren't right. could that chip be the cause for this problem? or maybe a voltage supply issue? The o/p transistors are all new, the drivers are all new and all of the TO 92s are new. I removed all the diodes from the board and they all check good. I'll look closely at the schematic again and see if i'm missing something. Thanks

[g1]

Look at points B and C in the power supply area of the schematic. You should have + and - 15V (or a little less with the bulb limiter).
What do you measure at B and C ?

[Dave H]

No limiter bulb glowing is good. The bias trimpot won’t affect the output voltage as it’s there to set the output transistor bias current. Can you set the bias current as I said above in post #43? If you have +13V on pin 7 and 0V on pin 4 of the opamp then the power amp output will be at -57V. The opamp has no negative supply so it can’t drive its output to -7V which is has to do to set the power amp output at 0V. Check the +15V and -15V supplies with the opamp removed if you still have +13V and 0V on opamp pins 7 and 4 check the +/-15V supply at points B and C. If the supply isn’t +/-15V you will have to repair that first before replacing the opamp. If the +/-15V supply is faulty the opamp may still be OK. As it’s now in a socket you could try it after repairing the +/-15V supply. When you have the power amp output at 0V be sure to set the bias current.