Peavey CS800 keeps blowing same power transistor

[g1]

You didn't answer whether you have a driver installed or not?
Your bad voltages are not showing up when the driver is removed, so it needs to be installed to take measurements.
If you do have it installed, somehow we skipped Q10 measurements anyway.
Please re-do Q6 & Q10 measurements with good driver installed.

[veganxxx]

Here are more accurate results. Everything is plugged in except for the shorted power transistors-

Q2 (power board)
B- -30.79VDC
E- -17.06VDC
C- .01VDC

Q10 (driver board)
B- -1.365VDC
E- -.796VDC
C- -30.55VDC

Q6 (driver board) I think I got the legs correct based upon the schematic, couldn't find a datasheet to verify
B- -20.97VDC
E- -30.31VDC
C- -1.353VDC

[g1]

Yes you must verify according to the schematic. Q6 base connects to R38, Q6 emitter connects to R39.
Q6 is an NPN transistor. If it is good, the base can only be maximum .7V more positive than the emitter. Your readings say the base is 10V more positive than the emitter. Either the transistor is bad or you are making an error.

Q2, your emitter should be tied to negative rail by R7, which is only 5.6ohms. Yet you measure around 15V across R7. So either R7 is open or Q2 has a problem.

[veganxxx]

Ok, so first, I mistyped and it isn't 20VDC, it's 30VDC so they are actually a similar number.

Second, I put a meter on R7. It isn't open but.... it does read over 1 megaohms not 5.6 ohms. I had tested all these resistors before, but I probably saw the reading as around 1 ohm and didn't notice the "M" on my meter. Since it gave a reading that seemed low I thought that's what it should read in circuit and wasn't open. Big whoops! I ordered some 5.6ohm 5 watt resistors and will report back when I swap the bad one out.

Thanks for the help so far!

[g1]

Sometimes when we loosely use the term 'open' we just mean high resistance relative to what should be.
So even if the meter doesn't say "OL", 1M for a 5.6R resistor can be considered 'open'.
By the same token, we might call a low resistance a 'short' even if it is not measuring near zero. For example we might call a cap 'shorted' even if it measures 53 ohms or some low resistance.

[Enzo]

Measure it again once you remove it. I bet that 1 meg is from parallel paths in the rest of the circuit.

[veganxxx]

Doesn’t current follow the path of least resistance? As in, if there were multiple paths in a circuit that were being measured at the same time wouldn’t the lowest resistance one be the reading that would come up on the meter?

[The Dude]

Doesn’t current follow the path of least resistance? As in, if there were multiple paths in a circuit that were being measured at the same time wouldn’t the lowest resistance one be the reading that would come up on the meter?

Yep. I think Enzo is speculating that the resistor might be completely open and that the 1 meg reading is from something else.

[Enzo]

Vegan: Nope.

Try this, hook your meter to a 100k resistor, or whatever relatively large value you have. Measures about 100k. Now clip a 220k resistor in parallel. See how the reading goes down? It doesn't say 100k any more. Probably more like 69k.

It is a common quiz question in electronics classes: VArious resistors in series and parallel, and you have to figure out the total reading.

Current doesn't exclusively follow the lowest resistance path, it distributes itself proportionally to the resistance in each path.

It the resistor is open, you would be measuring the resistance through whatever circuit is left.

[The Dude]

Of course, what Enzo says is right.
For clarity, I was (poorly) answering the inferred question. I think Vegan was wondering about post #36 and thinking it could mean that the resistor, out of circuit might measure less or test as good. The point is that a resistor will not measure lower resistance out of circuit, but it will measure higher when/if a parallel resistance is removed from the equation.

[Enzo]

Sure.

If it measures 1 meg in circuit, it will never measure lower out of circuit.

[The Dude]

Righto. It's just that someone who doesn't know that could interpret something else from post #36.
I know it's not what you meant or said, but I might think, measure it again- maybe it's good. Otherwise, why would I bother to measure it again? but i digress.

[veganxxx]

I get it. And yes, I did think it was implied that it might be ok out of circuit, which is is why I made an incorrect but in my mind a similar notion to what happens with multiple paths and multiple resistors.

The better analogy for me (a layman in electronics but someone who is a mechanic/builder) is that if you have one pipe that goes to a boiler manifold that distributes hot water to multiple paths of various diameters (resistors), the water won’t all go to one with the least resistance, it will distribute to all circuits, but of course more will flow where there is the least resistance. If there’s a pressure gauge (meter) where they all join back together and one at the incoming main pipe you can see the pressure drop of all the paths combined. If there’s only one path you measure that drop. Each path (resistor) that’s added in parralel, the drop increases.

[veganxxx]

Ok I replaced R7 and double checked the rest of resistors and ended up replacing R12 as well.

Now with or without transistors plugged in I get the following on Q4 through Q12 (through the light bulb)

Base- -30VDC
Emitter- -30VDC
Collector- 0VDC