Reverb problems

[spy]

I'm trying to repair the reverb circuit in a tube amp. The amp is a Fame tube amp. The reverb circuit is with opamps but ...it doesn't work.

The pan seems ok as I measure the resistance at the input and my meter shows me around 26 Ohms. The output is ok, too. I can hear the springs when crashing at the output of the amplifier.

With a sine wave at the input of the amplifier and the pan connected to the circuit I don't have any signal at the input of the pan or at the output of the opamp that drives the pan. But, when I un-plug the pan there is a signal at the output of the opamp! What does this mean? The opamp is the MC1458P...

BTW, I'm sure that it's not the pan because I tried another one I have around with the same results.

Thanks!

[guitician]

When you disconnect the pan and the signal comes back from the op-amp, is there any DC voltage on that signal too? You may have a bad coupling cap.

[spy]

I have no idea, I'll check it!
But as I remember there's no coupling cap at the output of the opamp. There are two 47 Ohm resistors, one at each output of the opamp, driving the pan.

[guitician]

OK, a DC differential type arrangement. Do you get a signal from both outputs?

[spy]

Hmm, I must unsolder the resistors to check it out as the ends are connected together to drive the pan! I'll do it and let you know.... thanks, guitician! Hope it's the opamp!

[guitician]

You could just check the inputs too. There should be signal on either the positive or the negative of either op-amp half. One side inverting, and the other non-inverting.

[spy]

I've checked the inputs! And I forgot to mention that when the pan is connected the signal disappears from the inputs too. There's something like noise...

[guitician]

Seems to be the MC1458. But, if the drive circuit isn't ground referenced, you may be seeing something else. Is the input to the pan grounded to the chassis?

You should measure the output signal from pin#1 to pin#7, or output-to-output.

[spy]

The input is not grounded to the pan's chassis, but it's the output.

With the pan unplugged and the oscilloscope at pins 1 and 7 I don't have anything. I thought about what you said of dc at the output of the opamp and I measured with the meter and I have -12Vdc! Any idea?

[guitician]

Is the -12 volts at the point were the two 47 ohm resistors meet, or did you de-solder them? What DC voltage do you have at the inputs of the opamp pins# 2,3-6,8? If your inputs are at close to 0v then your opamp is bad.

If you see output signal on only one of the two outputs then the differential drive wont work, and one of the two halves of the opamp is dead.

[spy]

Yes it's where the resistors meet. I didn't de-solder them!

Here are the voltages I measure at the opamp with the ground at the chassis or at the ground of the pcb.

1: -12V
2: 0V
3: 0V
4: -14.50V
5: -12V
6: -12V
7: -12V
8: 14.50V

[guitician]

If you don't see a source for the -12 on pins 5 and 6, like a shorted cap etc, the opamp is dead.

[spy]

First of all thanks for your help guitician!

Pin 1 is connected with pin 5 and one 47 Ohm and pin 6 is connected with pin 7 and the other 47 Ohm. Nothing else is connected with these pins!

[guitician]

Swap out the MC1458P. This is a general purpose dual opamp and has many cross reference part#'s. It should be available most anywhere you can find IC's.

[spy]

That's not easy as it sounds!

It's soldered on the pcb! So I have to take off the PCB first! But I'll use a socket for the next one! You never know....

I'll let you know what the results will be....