Wow. What a frustrating stage this was. I hit a brick wall and the project languished as a result. No point in adding day numbers to my titles anymore!
I put the VCF together shortly after the last blog. I took my time just as I had with the other sections of the x0x and thought I has put it together successfully. Once I began testing it I couldn't get a signal to confirm it was working though. I didn't have scope so I tried to whip up the
diy audio probe that's suggested on the x0x build page. I fudged my through it but never really got it working properly or to my satisfaction.
Because this section of the x0x has several rare parts, I started to wonder if one or many of them were faulty. So I spent tons of time checking resistor values, voltages, betas, and the like, all with mixed results. I was sure something was wrong. At the same time, I thought it was high time to get a scope but I couldn't justify hundreds or thousands of dollars on a new one, and frankly, I didn't want yet another 50 pound tool in my arsenal. My hope was to track down a proper soundcard probe and use free software. Between these two challenges it took 6 months. It was really discouraging.
The GOOD news is that I had success with both of these challenges. First things first: I found a
probe at a great price by Virtins. $30 US, shipping included to Canada. Can't argue with that. Once I plugged it into my line input on one of my low end sound cards, I wired it all up to the x0x, got testing, and low and behold: the damn thing seems to be working just fine. I just couldn't measure it properly.
I can't begin to express how relieved and elated I am to have past this step. I feel like I can continue and know that I don't have to revisit a problem.
On a side note, the OTHER good news is that I managed to track down 2SA733-APs as well so I'm that much closer to making an authentic 303. The critical components are right in the VCF: Q8, Q9, & Q10. It turns out my hFE reader on my multimeter is also crap so the component log component log isn't accurate either. (I always thought they were unusually low.) I'm going to leave them in and revisit this later. I need time to find a better way to measure these too but this less important than starting to solder again!!
A real Sight for sore eyes:
The VCF test with the cutoff and resonance at 0%.
The VCF test with the cutoff at 100% and resonance at 0%.
The VCF test with the cutoff at 100% and resonance at 100%.
This was another painless step in the process. There's nothing to really test just yet according the documentation. That will follow once the I/O is in place. It's also nice to see the volume pot in place.I couldn't resist snapping a better pic of the BA662 I have too. I'm looking forward to hearing this amplified. The LA4140 also looks ancient despite being new old stock. It just looks vintage alongside some of the other old ICs. Note that I'll also be building the wiring for the jumpers.
Q33's hFE: 200 mfm*
