R40 Smoking

[NuSkoolTone]

After testing my J99's I was about to put them in the chassis and then it appeared something touched (I left the chassis power on by mistake) and R40 smoked!

I had another 150 resistor on hand (5%) so I snipped out the old and put that in. When I powered up the J99 (removed from the Chassis), R40 Immediately burnt up violently as if it were going to catch on fire!

It was fine 30 min ago. What the heck happened? What should I check?

Also is putting a 5% resistor at R40 OK? The ones I have measure about 147-148.

Thanks!

[NuSkoolTone]

Replaced Q1 because one of the readings was off and appeared to be bad, even when removed from the circuit. However R40 still smokes.

A couple harness Wires seemed out of wack (Orange and Grey were disconnecting) so I made sure they were good. NO change.



What's odd are the Q2 measurements on the "working" J99.

BE measures (Pos on Base) is OL with what seems a fluctuation to 2.1v, but EB (Neg on Base) gives .688 Is this OK? On the "Broken" J99 the BE values are in the .6 range for Q1 AND Q2 Now that Q1 has been replaced.

Also tested C24 and C25 on both Modules. I don't know if these measurements in the circuit help, but C24/C25 on the "working" J99 are both around 120 while C24 is around 180 and C25 in around 330 in the "Broken" one.

Also reflowed R41, R42, and SW2 with no change. R40 still smokes.

D11 Seems to give the same measurements on each Module. With negative on the white band they both give about .577

WIth the Pos on the white band of D11, the broken on will give a reading of 1.9 volts and VERY slowly climb and stabilize at 2.347V On the Working one, D11 shows 2.1v for a split second then goes to OL.

Had a thought this morning, and I think I'm on to something. When I flip the Phase on SW2 it doesn't smoke. When I flip it back, it smokes again.

Edit: I'm not flipping phase, but turning phantom power on and off.

[tpryan]

Look at the schematic. If R40 smokes, there's too much voltage across it. That means that the supply side voltage has become way too high (unlikely) or the load side voltage has gotten too low. How can that happen? For starters, C24 could short, R41 could change to a much lower value, there may be a short between pins 1 and 3 of SW2, R42 could change to a much lower value AND C25 could short, or there could be a low resistance path across Q1 and Q2 like you get when they're swapped PNP for NPN. There could also be shorts caused by poor soldering and untrimmed component leads. Whatever it is, it's right in front of you. All you have to do is make some measurements and figure it out.

[NuSkoolTone]

Look at the schematic. If R40 smokes, there's too much voltage across it. That means that the supply side voltage has become way too high (unlikely) or the load side voltage has gotten too low. How can that happen? For starters, C24 could short, R41 could change to a much lower value, there may be a short between pins 1 and 3 of SW2, R42 could change to a much lower value AND C25 could short, or there could be a low resistance path across Q1 and Q2 like you get when they're swapped PNP for NPN. There could also be shorts caused by poor soldering and untrimmed component leads. Whatever it is, it's right in front of you. All you have to do is make some measurements and figure it out.

Ok I check C24 and C25 out of the circuit. Both test about 104 and megaohms across the leads. So I assume they're good.

R41 and R42 are correct values out of circuit.

Q1 and Q2 are the correct parts and orientation.


NO short on SW2 when checking between pins.

Did I miss anything? If the problem is obvious please tell me. I feel like I'm beating up the PCB doing this.

Checked for solder bridges on teh back side did not see any

R40 SHows the following voltage looking down form top part of PCB:

Phantom OFF:
ABout 48V on each side of R40

Phantom ON:
Left side drops to about 13V and right side drops to 0 and smokes

FWIW R1 and R2 aren't getting phantom either.

[NuSkoolTone]

What exactly do you mean by the supply/load side. I get the gyst, but I don't know how to approach this.

Assuming my tests above for the caps not shorting on their own and being the right value, do I still need to check if they are shorting in teh circuit? If so, how do I do that?

If R41 and R42 measure OK outside of the they should be fine too right?

SW2 doesn't short with it self in the circuit, though I don't think pin2 on it was getting 48v when switched on. (Not sure if that's normal).

IS there much likelihood D11 is bad?

Please help, I'm kinda stumped.

[NuSkoolTone]

OK, I'm traced back to the Phantom power switch and Pin 2 tops out at about 13V.

Any idea why that would happen?

I know something was said about the load side, but the component values tested ok when I checked them.

Is it possible the switch is bad? I haven't considered it really before, because the switches seem pretty heavy duty and improbable to fail.

What else should I look at?

The schematic shows this block of components terminating at J8 after D11 and Ground. So is it ONLY these 10 components and their construction that could be causing the problem?

PLEASE HELP.

Also, let me mention this is NOT the J99 that I had to replace the rotary switch on.

[tpryan]

Assuming my tests above for the caps not shorting on their own and being the right value, do I still need to check if they are shorting in teh circuit? If so, how do I do that?

If they're shorted, the voltage across them will be 0 (or very close to it) all the time.

If R41 and R42 measure OK outside of the they should be fine too right?

Yes.

SW2 doesn't short with it self in the circuit, though I don't think pin2 on it was getting 48v when switched on. (Not sure if that's normal).

? Pin 2 should be connected to pin 1 in one direction and pin 3 in the other. Make sure it does that, and only that.

IS there much likelihood D11 is bad?

No, but it's easy to check with your DMM.

What else should I look at?

Q1 and Q2. Also easy to check for shorts with your DMM.

The schematic shows this block of components terminating at J8 after D11 and Ground. So is it ONLY these 10 components and their construction that could be causing the problem?

Yes.

[NuSkoolTone]

Well R40 has stopped smoking after replacing Q2.

However I'm not out of the woods yet! Still no phantom power.

The voltage appears to stop at the base of Q1 where it's .473v.

Not surprisingly, the same voltage is on the base of Q2, + of C25, and R42 connected to it.

The other side of R42 has 47v, sw2 has 47 volts, + of C24 has 47v, r40 has 47v when phantom off, 0v when phantom on. (On the side towards the front of the PCB) The other side of R40 has 48v regardless of SW2.

I replaced Q1 and reflowed R42, R41 and don't see a difference.

Also where does the "+48v" connecting Q1/Q2 in the schematic come from?

Any ideas on the problem?

[tpryan]

What happens if you remove C25?

[NuSkoolTone]

What happens if you remove C25?

When I flipped the switch R40 smoked again (I need to get more, I'm out) The one side of R40 SLOWLY drops voltage, and R1/R2 start to gain voltage.

When I measured where it was 473v before it now has 0v. The problem here is, I don't know if it's AFTER I hit the switch or not.

The closest I have to 150 on hand is 180ohms for R40. Can I use these to further troubleshoot this?

Does this info help at all?

[tpryan]

Send it in, I'm curious to see it.

[Meyer]

I have the exact same problem: When I connect power to the j99 R40 burns up violently. I checked everything suggested in this tread but no luck.