Presonus TubePre fault

A friend of mine has this small tube preamp with the following problem:

as soon as you plug the unit in and wait a couple of seconds (drive + output are set to 12 o'clock) the vu meter gets on full deflection and the only output signal is low-frequency hum, no Mic-Signal whatsoever.

Checked the walwart which seems fine, also replaced the tube, no change. as soon as I pull the tube out, the meter stays at zero.

any help or hints much appreciated
christoph

Can you find a schematic?

No chance - couldn't find one.

cheers
cri

Ask for a schematic from the company?

Jakob E.

Unfortunately they don't give them out

regards,
christoph

Then you need to draw it by yourselfe.

damn

christoph

Presonus service is usually quite helpful.

In the meantime, you could look at the output of each opamp with a meter and see if any of them are slammed to the rail. Could just be a bad opamp or possible a shorted cap.

Best Wishes!
Charlie

Strangely enough, the fault is only appearing on the mic input - as soon as i plug in an unbalanced plug on the DI in, everything is back to normal. I couldn't spot any obvious anormalities in the Mic in section, and I also didn't get how the switching (Mic / DI in) works, but will check all the caps for shorting...

thanks
christoph

have you tried different microphones?

It actually doesn't matter if a mic is plugged in or not (the fault is always audible/noticable as long as there's nothing plugged in the DI input), but yes I connected a dynamic and a condenser, with no difference.

thanks
christoph

Have you checked the DI connector/soldering?
It seems logical that the fault must be there or somewhere immediatly connected to that spot

Either gotta be a bad solder joint or a defective/broken connector.

HTH!

Have you checked the DI connector/soldering?

Click to expand...

That's what my first thoughts were, but strangely enough is the connector perfectly fine, and so is the soldering?!

I don't get the DI/Mic switching, because there is no knob for changin the input - how else could this work?

Many thanks!
Christoph

Typically the jack has a swinging arm connected mechanically to the tip connector. The swinging arm and a couple of stationary arms form a switch; when you plug something into the jack, it flips the switch to DI mode instead of mic preamp mode.

Peace,
Paul

The Jacks contacts and "switches" are working fine, I couldn't spot any sort of a relay switcher - but I'm quite new to all this :?

thanks
christoph

All the caps are fine as well, nothing is shorted. There are three chips in there, one TL084c and two "MC330790 XAF328" Every Chips has 30V between Gnd and Vcc which seems fine accoring to the datasheet I found for the TL084.

Still don't get how the thing switches the inputs, cause this seems where tho problem is located...

thanks
christoph

If it's anything like the usual setup, it switches the inputs using internal connections in the jack.

Peace,
Paul

yes, that's what I thought as well (since there are three contacts) but how the system works in detail? a relay of some sort? there are just two conducting paths coming from the tip of the plug - so one circuit can be "opened" for switching.

cheers
christoph

I don't know if the picture helps, but I couldn't spot anything. All the voltages are fine, there's nothing changing internally when I plug a guitar in the DI input, except that the unit starts working normally?!

any more ideas?
christoph