My Paia tube mic pre died in two weird places. 1. The SS part works fine, but the tube half does not. The tube filaments light up, and the tube works in other gear. 2. The phantom power LED does not light up, and +48V mics plugged into it will not work even when using the SS half of the channel.

This leads me to believe the 45-48V feeding the tube plate and the phantom power part of the circuit are kaput. Yes?.

Looking at the schematic, there's a CD4049 that's acting as a voltage-tripler or somesuch. That provides the higher voltage for the tube and the phantom power, no? Yes?

So if I replace the CD4049, all should be well?

Am I doing this right?

I should note that there are no obvious burnt parts or shorts or funny ozone smells.

That sounds like a good approach.

Is the voltage tripler a single device and not discrete I assume?

Check the voltages feeding it to find where in the path things have gone wrong. Obviously, if it is not receiving any voltage then you can assume that the problem lies before that component. Similarly, make sure that it is not putting out the correct voltage either!

Chris G! long time no see! hope all is well in DC!

your guess is my guess. sounds like you've got the 6.3V for the filament but not the higher voltage for the plate and phantom power. i'm not familiar with the IC you're talking about, but based on the fact that you're looking at a schematic, i can only surmise that there aren't separate taps on the power transformer for 6.3V and the plate voltage. if that's what your IC is doing, upping the voltage, that's exactly where i would look.

that IC appears to cost about $0.30 on the net. if that's what's wrong, you're in luck!

this might be totally guitarded of me to suggest, but is there by any chance a fuse somewhere in there that you've missed?

electronics tinkering rules!

can you post the schematics? or point me to a copy or something?

tangent wrote:can you post the schematics? or point me to a copy or something?

yeah, take a gander, and thanks in advance:

http://www.paia.com/fantusch.gif

the online schematic's a bit hard to read, but IC1, in the upper-right quarter of the diagram, is a CD4049.

toomanyhelicopters wrote:Chris G! long time no see! hope all is well in DC!

So far, so good. I've just got back from a months-long trip overseas.

As for yr other question, there ain't no fuses inside.

Chris G wrote:

tangent wrote:can you post the schematics? or point me to a copy or something?

yeah, take a gander, and thanks in advance:

http://www.paia.com/fantusch.gif

the online schematic's a bit hard to read, but IC1, in the upper-right quarter of the diagram, is a CD4049.

*ahem*

bump

So I replaced the CD4049, rechecked all my solder joints, made sure there were no shorts, put a known good tube back in ...

and still no +48V phantom power, and no tube sound.

any other possible culprits? anybody eyeball that schematic yet?

Chris G wrote:So I replaced the CD4049, rechecked all my solder joints, made sure there were no shorts, put a known good tube back in ...

and still no +48V phantom power, and no tube sound.

any other possible culprits? anybody eyeball that schematic yet?

i looked at it a bit, but am not really able to make too much sense out of it. the resolution is pretty bad, and i don't really understand what's doing what. sorry.

what's the deal with the circuit that seems to be floating in space, connected to nothing in the upper right section? and what's the deal with the circled letters? it seems like there should be a table associated with it that explains what they're for or why those locations are special. i dunno. there's no transformer, and a bunch of IC's, so i can't really make heads or tails of it.

toomanyhelicopters wrote:the resolution is pretty bad, so i can't really make heads or tails of it.

Same here. I tried magnifying it (not literally!) but it's a bit fuzzy to easily look at.