Using FLS As A Test Signal Generator With DSO Nano

Using FLS As A Test Signal Generator With DSO Nano


DISCLAIMER: The following is NOT to be taken as a definitive procedure for repairing ANY electronic device and the author takes NO responsibility for any damage or injury that results from anyone using this guide. It is intended for educational purposes ONLY.

If you have ANY doubt about making modifications or repairs to your own equipment then seek advice from relevant qualified persons.

Valve amplifiers use and can store high AC and/or DC voltages that can KILL. You have been warned!


I have been waiting for the time to arrive to force me to get to grips with the DSO Nano oscilloscope I bought a while back, as I wasn’t particularly happy about its ease of use with badly translated and very sparse instructions it came with, or that I could find on the Web.
After a conversation with an ex TV engineer at the market today, who was also trained in valves, and has repaired guitar amps in his time, I realised I should get the Nano and Signal Generator functionality set up at home somehow.
He was telling me of basic methods for tracing a source signal (music even if no Tone Generator is available) using a “probe” type set up with a DC blocking capacitor attached to a small speaker or similar (headphones maybe?) that can obviously handle the range of voltages you may touch on a live circuit (DO YOUR RESEARCH!!!), to enable a homemade, bare bones test rig for tracing a signal through amp stages to help find the point of failure etc.
It may be worth researching this if you have minimal test equipment and want to test an amp or radio etc. but remember to TAKE RELEVANT PRECAUTIONS for working on any live equipment.
As the DSO Nano can only handle a maximum 80V with a 1 X probe, it should still be ok to use on most any valve amp with the 10 X probe switched in.
As I did a Post on synth Operators and waveforms a while back, I know FLS has a range of options for generating sine waves, so had a quick look again and chose the 3OCX unit in the Subtractive synth options of the Generators menu below:

I found that the A# (button 7 on QWERTY if menu option above is active) key gave a visually static waveform in Wavecandy.

This note can be triggered as I’ve shown before in older Posts, by clicking a beat note in the Sequencer, then R clicking the instruments and sending the note to the Piano Role window. This will cycle around to give an almost continuous signal. I used only the first oscillator of the 3xOSC unit by turning down the volumes of OSCs 2 and 3. You can choose other waveforms like saw or square if you wish.

As I have an M-Audio USB module, I used the headphone out socket with a guitar lead, as it has an independent volume control, and I can still hear the main output through my stereo. You will have to work out what options you have depending on your own equipment of course.
I unscrewed the guitar connector so I could attach the Nano earth croc clip and probe hook to the tip and ring wires of the lead.

Now the tricky bit if you are not familiar with the Nano – which was pretty much trial and error – when I first got this thing = random button pressing for ages to try work out what the menus do!
The bottom M button cycles through the right side page menu and the middle + and – buttons alter the values of that parameter. I got this set to 50mV/div, so you see I have a 100mV peak to peak sine wave coming out the headphone socket of the M-Audio, which would approximate a single coil guitar pick up output (humbucker around 200mV ac).
I used the X/Y offset menu to centre the waveform, and the XY/1 and 2 cursors to mark the peaks of the wave.
I am now in a position to tentatively probe a test circuit using the Nano with my FLS “Sig Gen” by plugging in the guitar lead into an amp – probably on the low DC voltage (26V) MOSFET amp I got a few weeks back – as this is safer to practice on than jumping straight in and firing up the 8 valve Ashdown Fallen Angel I got recently, which has a dead channel 1 and a dead output stage, at initial basic testing.
I’ll document that saga when and if I get the schematics from Ashdown – hopefully a reply tomorrow.