Marshall 5215 preamp issues

[Axtman]

I am getting about -11.5 volts DC (relative to ground) on pin 2 and 3 of the first IC 1A. I can't seem to find where that voltage is coming from. Help!

[The Dude]

I would either unsolder those pins of the IC or remove it and see if the voltage is still there. You may have a bad op amp.

[g1]

Also, check that you have proper supply voltages at pin 4 & 8.

[Axtman]

I removed the op amp and no voltage on those pins. I put different op amp in and the voltage returned.

The voltage supply on pins 4 and 8 is -15.5 v and + 15.5 v.

[The Dude]

g1, "Also, check that you have proper supply voltages at pin 4 & 8."

^^^^^^ Yes, that first. I just assume that is done (step one- check op amp supply). But I shouldn't assume things. I have appropriately slapped myself.

[g1]

Don't see anywhere that voltage could be coming in schematically, have you looked around for solder bridges or other errors?
What is the history of this unit, was it working and just stopped, or anyone else tried to fix, or ebay purchase etc.?

[Axtman]

Okay I just remeasured with the op amp in (I installed a socket.).

Pin 1 = -11v
Pin 2 = -10v
Pin 3 = -1v
Pin 4 = -14v
Pin 5 = -1v
Pin 6 = -12v
Pin 7 = -12v
Pin 8 = +15v

[The Dude]

Since you installed a socket, what are the voltages on the pins with the op amp removed?

[Axtman]

With the IC removed, this is the voltages that I get just from the socket.

Pin 1 = 0v
Pin 2 = 0v
Pin 3 = 0v
Pin 4 = -15v
Pin 5 = 0v
Pin 6 = 0v
Pin 7 = 0v
Pin 8 = +16v

Here is the really crazy thing. It passes a signal with the op amp REMOVED! If I put a signal in the input jack I get a signal out of the speaker output jack!

[The Dude]

Those voltages look good.

Although many odd voltages, the puzzler in your previous voltages (IC in) is pin 5 at -1V. According to the schematic, that should be ground. Maybe you're missing a ground somewhere? Check for continuity from pin 5 to ground and make sure it is indeed ground.

What is the symptom of the amp? Aside from strange voltages, what are we fixing?

The op amp you substituted is good and not maybe a used/bad/wrong one? In your first post you said pins 2&3 both had -11.5V. In post #7, these voltages changed making me wonder if the op amp you sub'd is either defective or the wrong part. Part orientation is correct?

[Axtman]

Thanks The Dude! Due to your tip I noticed that the 5 pin was bent under and not soldered to the board. I unsoldered the whole socket and resoldered it and now I get correct voltages! I still have the original problem of low distorted output.

[The Dude]

I would run a signal into the effects loop return and see if you still get distortion. In other words, determine if it's a preamp or power amp problem.

[Axtman]

Yes the signal into the effects out works fine so the problem is in the preamp board. In poking around with a signal generator I think the problem is around IC 1. I will do a more thorough investigation tomorrow.

Are op amps with the same pin out interchangeable? I don't have an exact replacement for the original IC but have a few other op amps on hand.

[Enzo]

Most any of them would work in place of that TL072. certainly at least, well enough to see if that is your issue. The other couple that are 1458s, I think should stay 1458. The newer 4558 tend to make this old marginal design unstable.

[Axtman]

I think I may have found a broken trace going into the first IC.

BTW are Diodes D1 and D2 between pins 2 and 3 of the IC1 op amp clipping diodes? Can I just remove them for a cleaner sound?

Thanks.