Another skar 4500RP stuck in protection.

[Basshead1989]

I was saying you asked if I was running a signal into the amp and I was running a 100 Hertz signal into the amp and wasn't getting a square wave and the pins that I got signal on in the pics in #15 were because my probe was on 10us/1v probe x1 instead of 10us/1v probe x10

 

[Perry Babin]

On the output pins of the TL072 (pin 1) and the LM211, you should see a square wave swinging ±5v. You have to get there before you can check anything else.

 

[Basshead1989]

Ok Perry on the tl072 I'm not getting a square wave on the output (pin1) it's -3.5vdc.

On the lm211 output (pin 7) no square wave either it's -4vdc.

On pin 10 of driver board header the audio input pin it's 3.5vdc

 

[Perry Babin]

Install a 10k resistor between pins 1 and 2 of the TL072. Any yellow to any green in the attached image (from the type 3 page) will work.

 

[Basshead1989]

The output pin on the tl074c that feeds the audio input to pin 10 of driver board has 5vdc sitting there with or with out audio injected but all the other output pins have a little noise to them and the other tl074c op amps in the pre amp have the same noisy blur on there output pins.

 

[Basshead1989]

Ok Perry with the 10k ohm resistor jumped across
(pins 1&2 )of the tl072 I'm still not getting a signal it makes the line blurry like it's noisy but no audio singal theres now -2vdc on( pin 1) off the tl072 and 5vdc on (pin 7) of the lm211

 

[Perry Babin]

If you read 0v on pins 3 and 2 of the TL072, it's very likely defective.

 

[Basshead1989]

Ok with the 10k ohm resistor still on (pins 1&2) and a 100hz singal being injected threw the rca's both
(pins 2&3) are 0vdc

 

[Basshead1989]

Should I replace it and do test again with the 10kohm resistor in place or look for sqaurewave on output pins first

 

[Perry Babin]

You can check it with or without the resistor after replacing it.

The resistor allows you to test it to see if it's passing relatively clean audio when it has strange problems. Without the resistor, a signal of virtually any level will cause the output to go full square wave because there is no feedback.

 

[Basshead1989]

Oh ok I see what you mean I'll put one in and get back to you.

 

[Basshead1989]

Its still the same (pin1) of the tl072 is -3.5vdc and (pin 7) of the lm211 is -4vdc.

With the 10k ohm resistor on (pins 1&2) I get -1vdc on (pin 1) of the tl072.

Would the Mylar caps I have out effect this cause there still off the board.

 

[Perry Babin]

With 0v on pins 2 and 3 and the 10k resistor in place (and a good op-amp), there is virtually no way that it will have any significant offset.

Could you have a solder bridge?

Do you have both positive and negative 5v on the op-amp?

 

[Basshead1989]

I have -5vdc on pin4 and 5vdc on pin 8
I'll check for solder bridges.

 

[Basshead1989]

No solder bridges could I have a bad rail cap or output filter cap?

 

[Perry Babin]

I don't think so. This part of the circuit is very much isolated. The circuit beyond it is coupled via a 10k cap and the op-amp can easily drive anything that would be coming from something beyond the resistor. Are you sure that the resistor that you installed is connecting to pins 1 and 2?

 

[Basshead1989]

I'm pretty sure it is. pics below.

 

[Perry Babin]

I don't know what could be causing the offset. I'll think about it.

As a side note, on the 2-IC boards, you can simply solder a bridge across pins 2 and 3 to shut the ICs down.

 

[Basshead1989]

Ok Perry thanks.
I was going to solder pins 1 and 2 together but I figured it would be easier to do a small jumper wire so I didn't have to use solder Wick in case I had to disconnect and reconnect the pins multiple times.

I'll check back with you tomorrow have a good night thanks for the help as always.