DIY tube mic troubleshooting help..
 

Hey gang,

So about 14 years ago I built a Paia 2ch tube pre kit. I used it for a year or so, then two things happened; I started to notice that signals were only distorted in the negative swing on CH 1, and I accidentally applied phantom to the outputs (as the pre was connected to an analogue mixer that had PP across all channels). At the time I didn't have a function generator so I wasn't able to troubleshoot.

I did find a few blown caps from the 48v hit, changed those out years ago.

I have since become obsessed with this old project since I repaired a function generator kit. I've been trying to figure out what's going on with both channels for the last few days. Ch 1 mostly works; but it still only distorts in the neg swing of a 1kHz sine wave (through the tube drive section). Ch 2 on the other hand seems to be having an issue with the tube drive section not functioning at all; as well as no PP or Drive LED.

I know this is a weird thing and not many are likely familiar with this kit. I do know that there are some of you out there with more electronics training than myself. I have traced the signal through the circuits and can't quite isolate the issue.

On CH1, I'm guessing some form of DC offset is being introduced by a damage component. On CH2, I'm guessing the caps weren't the only thing that got damaged from the 48V hit to the output section.

Thanks guys...

Re: DIY tube mic troubleshooting help..
 

In the event that anyone ever has one of these. This is what I found out. Because of the +48V Phantom power that was applied accidentally to Ch 2 output. It blew the caps, and damaged the CMOS hex inverter.

Scott from Paia contacted me, although I'd already discovered the issue with Ch1. The thing that isn't mentioned in the build instructions is how the Symmetry trim on the PCB works. AFter fiddling with it and observing the signal. I found out that in order to set these pre's up properly you need to adjust this in order to get even distortion. To far one way or the other and the signal will start to distort on either the positive or negative swing of the waveform.

Re: DIY tube mic troubleshooting help..
 

Glad you are making some progress. Most of what you are doing is way too advanced for me. I have only replaced bulging/leaking caps in a few LED TV's power supplies and computer monitors.

Can you repair channel 2 or just hoping to use channel 1 which it looks like you figured out?

Re: DIY tube mic troubleshooting help..
 

Thanks, yah I'm gonna give ch 2 a try. Just trying to source that cmos hex inverter. Tough to do when you live in a remote area. Going to check out a place next week that may be able to help me out. I'm positive if I replace that, that it will work and I'll put back in the mix.

Working on another project too building a pedal that would be of use to engineers and performers.


Sent from my iPhone using Tapatalk

Re: DIY tube mic troubleshooting help..
 

You are in a remote area! Not too many roads there either.

Just make sure you don't apply phantom power again! My mixer (in my signature) allows me to turn on/off phantom power for pairs of channels. You should probably skip the mixer altogether and connect the mic pre directly to your interface (also making sure the phantom power is off, like the Digi002 rack that I have allows).

Re: DIY tube mic troubleshooting help..
 

I no longer use a mixer. That pre was damaged years ago, when all I had was a Delta1010LT. So in order to get it patched in it had to go through the mixer. And it was one of those things where I was rushing around and accidentally turned the phantom power on the wrong channel.

The way things are hooked up now with my Focusrite; I can use XLR to TRS to avoid and accidental PP hits to my Avalon or lunchbox.

:P

Re: DIY tube mic troubleshooting help..
 

I'm pretty damned good with electronics (retired EE) but for the life of me I'm trying to figure out what a CMOS hex inverter is doing in a mic pre? Especially on an output?

Re: DIY tube mic troubleshooting help..
 

I know it's a weird addition that PAIA did in order to produce phantom power. From my understanding, it's used to create a 60kHz 60VPP square wave; this is then put through a voltage divider circuit to provide 48V for phantom power and the Tube plate voltage.

Which is why my pre's phantom or tube section of that channel is not working. On Ch 1, I get about 57Vpp out of that hex inverter section; which is giving me about 47VDC for the phantom power. The Ch2 is giving me about 250mV; resulting in 0V in the PP section.

I bet the decision to use that CMOS is way cheaper than any kind of step up transformer or amp circuit with a rectifier circuit to convert the AC to provide those two voltages.

Re: DIY tube mic troubleshooting help..
 

Now I'm even more confused. Let me get this straight: you connected the output of the PAIA to a mixer that had phantom power on all it's inputs? And that's what blew the phantom power on the input in your PAIA?

Re: DIY tube mic troubleshooting help..
 

Well that PP circuit in the PAIA isn't on the output. My best guess is that stray voltage somehow made it's way into the circuit. The PP hit from the mixer blew a couple of caps. One coupling cap on the output and another I believe. Who knows, maybe it was something else that did the damage; it was just my best guess. All this happened about 14 years ago, so my memory may be a little foggy. :P. There might have been a few substances abused between now and then, LOL.

Eitherway, the CMOS chip isn't doing what it's supposed to be doing.