Vox AD120VT Problem

[eschertron]

Is R84 even sized right? I calculate about 32mA is the failure point for this device. Why try to protect it with a 100mA fuse?

P = I2R
I2 = P/R
I = SQRT(P/R)
I = SQRT(1/1000)

Am I off here? What's R84 even doing there?

[The Dude]

FWIW, it's a T100mA (slow blow). I imagine the fuse is only there to protect the PT in case of catastrophic failure. Otherwise, there are lots of amps that don't have an HT fuse at all. Without measuring, we don't know what actual current is in the circuit.

[eschertron]

Is R84 even sized right? I calculate about 32mA is the failure point for this device. Why try to protect it with a 100mA fuse?

FWIW, it's a T100mA (slow blow). I imagine it's only there to protect the PT in case of catastrophic failure. Otherwise, there are lots of amps that don't have an HT fuse at all.

Worded poorly, I know. I guess I meant to ask, how is the fuse even going to blow first? I know the resistor will take a few seconds (or fractions of a second) to heat to damage, but so does the fuse. A direct short across the rectifiers, or some other catastrophic current failure, I see the fuse protecting the transformer winding. But a slower high-current failure (between 32 and 100mA) will always take out the resistor first.
So my point is, if the bean counters didn't want to pay for a resistor sized to live through the short (yup, needs 10W plus to do that), why not ditch the fuse and leave the resistor as the sacrificial element? Would it not blow fast enough / reliably enough to save the tranny?

[eschertron]

Would it not blow fast enough / reliably enough to save the tranny?

And having said that, since the resistor was probably collateral damage, are we any closer to finding the fault?

edit: OK, I read back and see that a foreign object was dropped into the chassis? Was that the root cause?

[imf]

And having said that, since the resistor was probably collateral damage, are we any closer to finding the fault?

Could it have anything to do with me dropping a metal pin onto the board (fell through the vents on top)? No idea where it could have hit.

[eschertron]

Likely, if that was the first tech's assessment.

[The Dude]

Sorry. I edited my post while you were typing. You're not the only one who "worded poorly".
Knowing actual circuit current would be valuable info in trying to figure out why it is the way it is. Since we don't know......
Maybe the resistor is in case somebody puts a 20A fuse in there?

As to finding the fault, I agree. There could be more wrong after the power supply is restored. That resistor went for a reason. The short could have been as simple as the pin dropped into the amp and everything fires up just fine. It could also have caused a short further down the line. That's why I suggested checking continuity from B+ to ground to see if there are any shorts still existing.

Edit:
In other words, I believe (and to simplify):

The fuse value was chosen based on transformer ratings regardless of what the circuit draws, which is presumably less than transformer ratings. The resistor is a fail safe in case someone installs a larger fuse.

[imf]

I have continuity after the rectifiers D13 and D14 but I guess I can't find out the true extent until I get current past D11 and D12 right?

[The Dude]

What I am suggesting is that you put your meter ground on chassis ground and meter positive lead on the 216V output and see if there are any shorts. If there are, you will take out the resistor and/or fuse again. You should verify that before powering up.

[imf]

What I am suggesting is that you put your meter ground on chassis ground and meter positive lead on the 216V output and see if there are any shorts. If there are, you will take out the resistor and/or fuse again. You should verify that before powering up.

Gotcha, makes sense.

[eschertron]

Was there a blown fuse? Do we know for certainty that the previous tech did or did not replace it?

Also for curiousity's sake:
E = IR
R = E/I = 216v/32mA = 6.7k resistance to ground will burn the resistor
R = E/I = 216v/100mA = 2.2k resistance to ground will NOT blow the fuse
anywhere in that area probing for 'shorts' will be the danger zone for the resistor. Of course, with the valve reactor technology, the tube elements won't conduct at all until fired up. So unlikely to find a "slow burn" fault this way. Only time will tell.

[imf]

Not sure if the tech replaced the fuse, but since there is a 125mA fuse in there instead of the 100mA may suggest he may have, unless it was common thing for them to do at the factory when they ran out of T100mA fuses.

eschertron, how did you get the 32mA? Trying to figure it out, am I missing this on the schematic?

[g1]

32mA would come from working backward from the 1watt rating (P=Isquared x R).

Could it have anything to do with me dropping a metal pin onto the board (fell through the vents on top)?

Hey, kids are supposed to do that for you.

[imf]

32mA would come from working backward from the 1watt rating (P=Isquared x R).


Hey, kids are supposed to do that for you.

Haha yes, well it was around 2002/3, so I was somewhat of an overgrown child at the time. Now I just feel like a dummy trying to learn this electronics stuff. I'm having fun though, I'd like to try and build small effects pedals or something to mess around. I was always interested in it, which is why I have some tools laying around for it. It doesn't help that this amp is quite a complicated schematic for a complete novice.

But I get the working backwards thing, thank you g1.

[imf]

What I don't get is... this was supplying the OT center taps out of C56 cap right? The tubes are glowing how much does it take for that to happen? Or is that from the green wires going to the heaters (which btw aren't even green, they're brown)... very confused.