Voltage regulator question

[Ice cream man]

Hi, I am trying to narrow down the cause of a fault on a power circuit board of a Laney RB9 bass amp which has lost it's +15v rail. The amp had originally blown the +15v regulator and I replaced it with a new one. When I test the regulator in circuit it shows only about 0.7v but when I remove the +15v leg from the circuit board it tests good.

Would any of you's know what the probable cause could be? I have tested only the resistors and diodes that are directly connected to that +15v rail and they are ok, my instinct is now pointing toward a faulty capacitor or op-amp that is pulling down the voltage.

I have included the schematic circuit diagram here which has the power and pre-amp sections on the first two pages and the +15 regulator is labelled U2 7815 at the bottom of the power board on the second page.

Thank you for any comments.

[The Dude]

Yes, something on the +15v line is likely shorted. Measure continuity from the +15v to ground and you'll probably get pretty low resistance. I'm not familiar with this amp, but if you can unplug power to any ancillary boards, maybe you can isolate the problem to a given area. From there, check with your fingers and see if you can detect anything getting hot. You might be able to narrow it down to a particular part.

[Enzo]

While it is on, does that new 15v regulator get hot?

Dude is right, something is loading down that power rail. An IC could be bad, do any feel hot to the touch? A filter cap on that supply could be shorted, plus at every or at least most of the ICs ther will be two small bypass caps to ground from each 15v rail, right by the IC. One of those shorts and the supply is grounded. yes it is difficult to find one when the whole board is involved.

[Enzo]

And unplug the preamp from the system, does the +15 come back up?

[Ice cream man]

Thanks guys, OK,

1. With the two PCB's connected I measured the resistance from +15 to ground and I got about 337 ohms.
2. I removed the graphic equalizer IC LA3607 U2 from the pre-amp PCB, and I read 3.4K ohms.
3. I have now disconnected the pre-amp PCB from the power PCB and measured the resistance from +15 to GND and I get 4K ohms.
4. I powered up the amp with the power PCB connected only and I am still not getting +15v on the output leg of the regulator to ground

The old +15v regulator that I replaced was fried as it smoked and cracked but this new one seems cool enough.

[Jazz P Bass]

Have you measured the voltage going Into the regulator?

[Ice cream man]

Update

Hey fellas this is a weird one for me,

Yes Jazz P Bass, I have measured a good healthy 23Vdc going into the regulator.

This is what happened, with the pre-amp re-connected and IC LA3607 U2PCB back in and everything as it should, I accidentally shorted out the ground leg and the +15 output leg of the regulator with my meter probe and relay RLY1A clicked and the +15v came back up surprisingly.
I also checked the heat of the regulator with my fingers and it is slowly getting very hot and beginning to smell after about 10 minutes. any more clues fellas?, I'm getting badly beat up on this one !!!

[g1]

Did you put the same IC back in for U2? Leave it out till the supply is ok, or just replace it.

[The Dude]

Also check for AC on the supply to the regulator and compare to the - supply. It's possible you have a bad filter cap pre regulator and the regulator doesn't like it.