I sold off my giant oscilloscope a long while ago since I used it maybe once per 3 years. And I didn't want to mess around with software oscilloscopes for smartphones or computers due to the rare possibility of damaging the expensive device, so I went searching for low-cost, tiny oscilloscopes and found one that comes in DIY kit form.
It's called the Xprotolab (tiny) or XMinilab (slightly larger). I have no affiliation with this company--just a happy customer.
http://www.gabotronics.com/electronic-kits-modules/kit-to-build-xprotolab.htm
A nice feature is that it has a built-in waveform generator on separate pin-outs. This came in really handy during the biasing (explained below in the biasing setup procedure).
It also includes a free open source software interface (Win/Mac/LINUX) that works if the oscilloscope is attached to a computer via microUSB cable--I used this for my U87i JFET biasing. If I blew something up, it would be the cheap Xprotolab, not my laptop.
Things to note:
1) DIY on the Xprotolab or Xminilab is not for the faint of heart. Soldering SMT was a pain, especially the microUSB connector. (I went DIY because I am already over-budget on my U87 builds, but I would not do it a second time.) I didn't use enough solder on the microUSB shell and ended up breaking it off. Luckily, I was able to reattach it without having to buy another one, but even that was a pain because the solder ran inside the connector such that I couldn't insert the plug any more! I was able to fix it by draining the extra solder out.
2) The Xprotolab's screen is very, very small. It will work for biasing, but it is hard to see the sine wave beginning to flatten out on such a tiny screen (hence my choice to use the software interface). Next time I would buy the larger Xminilab DIY kit so that I won't have to attach the laptop at all.
3) The waveform generator's output is too low for biasing. I had to amplify it with a single mixer channel, but this was not hard.
4) You will destroy the microprocessor with too high input voltage (max 30V on analog inputs), so be sure to use attenuated probes for higher-voltage sources. You will not need attenuation for biasing the JFET and can use direct wires to the U87 board.
---
Here's what I did for the JFET biasing:
[list type=decimal]
[*]Xprotolab's waveform generator pins go to 1/4in plug, which goes into mixer channel. Observe tip and ring, where ring = ground. This matters--I had the oscilloscope pins flipped relative to the waveform generator and got weird results--they must share the same ground (and it says this in the manual).
[*]Mixer output goes through 1/4in plug to injection point on U87 board. Tip goes to R6, ring goes to ground.
[*]Hook oscilloscope ground pin to U87 board ground--the same ground as for the mixer output.
[*]Hook oscilloscope input pin to Bias point.
[*]Plug in Xprotolab to laptop with microUSB cable. It should power up the tiny screen and green LED
[*]Run xscope software. It should auto-detect the attached Xprotolab, but if it doesn't you can force it to on the Options tab using the Connect button.
[*]In xscope, go to the Waveform Generator tab and choose the Sine waveform and set the Frequency Range to 10k and set the Desired Frequency to 1000.00 (Hz).
[*]In xscope, go to the Oscilloscope tab and click the Auto button. This is pretty much the only button you need for the biasing process.
[*]On your mixer, zero out the knobs and sliders and turn it on. Attach the microphone (without transformer wired in) to a DIFFERENT channel than is being used for the waveform generator so that you can supply it 48V phantom power.
[*]On the mixer, slowly raise the outputs for the waveform generator channel and main outputs until you see the 1kHz sine wave show up on the xscope oscilloscope screen. You may need to hit the Auto button in xscope a few times to get it to show properly.
[*]Once you can see the sine wave, you can push the mixer slider up until you start to clip the waveform and then proceed with biasing. It was quite easy for me, I just cranked the trimmer clockwise until the clipped waveform was symmetric, then backed off the volume of the waveform generator using the mixer until the sine wave was smooth, then fine-tuned the trimmer a bit until clipping started at exactly the same time on both peak and trough. It went very fast.
[/list]
This setup was pretty easy to put together using male-female and female-female jumper wires meant for breadboarding. The hardest part was soldering a 1/4in plug to bare 22AWG solid wires and then plugging each bare wire into one end of a female-female jumper to connect the mixer and waveform generator together. Do the same with a male-female jumper on the output side of the mixer.
It's called the Xprotolab (tiny) or XMinilab (slightly larger). I have no affiliation with this company--just a happy customer.
http://www.gabotronics.com/electronic-kits-modules/kit-to-build-xprotolab.htm
A nice feature is that it has a built-in waveform generator on separate pin-outs. This came in really handy during the biasing (explained below in the biasing setup procedure).
It also includes a free open source software interface (Win/Mac/LINUX) that works if the oscilloscope is attached to a computer via microUSB cable--I used this for my U87i JFET biasing. If I blew something up, it would be the cheap Xprotolab, not my laptop.
Things to note:
1) DIY on the Xprotolab or Xminilab is not for the faint of heart. Soldering SMT was a pain, especially the microUSB connector. (I went DIY because I am already over-budget on my U87 builds, but I would not do it a second time.) I didn't use enough solder on the microUSB shell and ended up breaking it off. Luckily, I was able to reattach it without having to buy another one, but even that was a pain because the solder ran inside the connector such that I couldn't insert the plug any more! I was able to fix it by draining the extra solder out.
2) The Xprotolab's screen is very, very small. It will work for biasing, but it is hard to see the sine wave beginning to flatten out on such a tiny screen (hence my choice to use the software interface). Next time I would buy the larger Xminilab DIY kit so that I won't have to attach the laptop at all.
3) The waveform generator's output is too low for biasing. I had to amplify it with a single mixer channel, but this was not hard.
4) You will destroy the microprocessor with too high input voltage (max 30V on analog inputs), so be sure to use attenuated probes for higher-voltage sources. You will not need attenuation for biasing the JFET and can use direct wires to the U87 board.
---
Here's what I did for the JFET biasing:
[list type=decimal]
[*]Xprotolab's waveform generator pins go to 1/4in plug, which goes into mixer channel. Observe tip and ring, where ring = ground. This matters--I had the oscilloscope pins flipped relative to the waveform generator and got weird results--they must share the same ground (and it says this in the manual).
[*]Mixer output goes through 1/4in plug to injection point on U87 board. Tip goes to R6, ring goes to ground.
[*]Hook oscilloscope ground pin to U87 board ground--the same ground as for the mixer output.
[*]Hook oscilloscope input pin to Bias point.
[*]Plug in Xprotolab to laptop with microUSB cable. It should power up the tiny screen and green LED
[*]Run xscope software. It should auto-detect the attached Xprotolab, but if it doesn't you can force it to on the Options tab using the Connect button.
[*]In xscope, go to the Waveform Generator tab and choose the Sine waveform and set the Frequency Range to 10k and set the Desired Frequency to 1000.00 (Hz).
[*]In xscope, go to the Oscilloscope tab and click the Auto button. This is pretty much the only button you need for the biasing process.
[*]On your mixer, zero out the knobs and sliders and turn it on. Attach the microphone (without transformer wired in) to a DIFFERENT channel than is being used for the waveform generator so that you can supply it 48V phantom power.
[*]On the mixer, slowly raise the outputs for the waveform generator channel and main outputs until you see the 1kHz sine wave show up on the xscope oscilloscope screen. You may need to hit the Auto button in xscope a few times to get it to show properly.
[*]Once you can see the sine wave, you can push the mixer slider up until you start to clip the waveform and then proceed with biasing. It was quite easy for me, I just cranked the trimmer clockwise until the clipped waveform was symmetric, then backed off the volume of the waveform generator using the mixer until the sine wave was smooth, then fine-tuned the trimmer a bit until clipping started at exactly the same time on both peak and trough. It went very fast.
[/list]
This setup was pretty easy to put together using male-female and female-female jumper wires meant for breadboarding. The hardest part was soldering a 1/4in plug to bare 22AWG solid wires and then plugging each bare wire into one end of a female-female jumper to connect the mixer and waveform generator together. Do the same with a male-female jumper on the output side of the mixer.
