Glitching Wasp keyboard fix
Bastard thing. It mostly works now, except when it doesn’t.
Ages ago, after moving house, I got my Wasp out to have a go at MIDI-fying it, and found that it would glitch when holding a note, as if it was flickering quickly between the held note and one much lower down.
Since then the Jasper clone was announced and released, I’ve got mine up and running in the last week, and that seems to work ok (although the VCA envelope sustain gets louder as a key is held down, which isn’t quite right). But the keyboard triggers absolutely perfectly, so it gives me a chance to compare it to my old Wasp.
I noticed irregular pulses on the 4013 at IC44 on both Q and Qbar (labelled NOTE_READ and VCA_ENV_TRIG on the helpfully labelled Jasper schematic). I didn’t trust the Q-to-reset link, so I recreated that on a breadboard and linked to it, still the same. Checked the connections, my soldering might have been wonky – all OK. I replaced the CD4013 with a HCF, supposedly has Schmitt trigger inputs, so could be more forgiving of noisy inputs. Nope.
After much fretting all I had left was to strap a ceramic 0.1uF capaacitor across ground and +5V on IC44, and… it was much less glitchy. I added a couple more (fairly randomly) in the keyboard circuit, and then the glitches stopped.
And only then did I have a look at the 5V line on one of the non-decoupled ICs:
…which doesn’t look like it would much help. The Wasp uses a couple of NE555 timers at heart of the oscillators, and they have a tendency to smash the power rail. Only as far as I could tell the glitch wasn’t at its loudest next to the 555s, so I suspected something else.
Now I’ve put it all back together, I realise that 3.36µs works out to about 297KHz, which is in the same ballpark as the frequency of the master oscillator, which is supposed to run at 250KHz. Next time I open it up I’d be tempted to put a capacitor across 5V and ground on IC11 and see if that quietens it down.
Anyway with the glitch gone, I dared to start celebrating, and then found a different, intermittent problem – G and G# would play the notes B and C. Pressing the keyboard slightly harder seemed to bring the problem on, so it seem likely it was a dodgy solder joint, and I tracked it down to a dodgy connection between IC31 and IC30 – reflowing the joint on IC31 fixed it.
Very occasionally still F# will play a slightly inbetween lower note, but I’ve spent way too long staring at this as it is – I’m calling it done for now.
As a postscript, if you have a knackered or badly behaving Wasp, I had a look around for some help on fixing the keyboard and there was some interesting stuff in this post and comments on the Clacktronics post on the Wasp, but that’s about it. Now there’s a new generation of Wasps springing up in the form of the Jasper clone, the Muffwiggler DIY section might throw up some new help.
