Although it appeared a bit beaten-up and well-used, it seemed to be in one piece, so I asked the staff what the price was. $29. Instant buy.
The original pedal was indeed designed by David Cockerell in 1978, and was very interesting. It employed an obscure RCA transconducting op-amp (CA3094) extensively, and had some innovative ideas. For example, the "subharmonic" generator was actually a chopper timed by what the pedal's circuitry saw as the fundamental harmonic waveform. The chopper signal silenced the original signal during every second cycle for a subharmonic with traces of the original harmonic structure. Its square wave output (a pre-filtered saturation distortion) had the input signal's amplitude envelope re-imposed on it. The octave output was not time-based, but actually a full-wave rectifier with high-pass filtering. There was a threshold-triggered variable attack generator controlling a VCA.
Icing on the cake, it had an 18dB lowpass VCF with resonance whose control source defied both received guitar pedal wisdom (from the 70s until now, preferred logic is that filters should follow the input signal amplitude envelope ie Moogerfooger LPF and Maxon Autofilter) and received synth-head wisdom (filters should be controlled by an envelope generator with defined rise then fall times ie Moogerfooger MuRF). Instead, input signal threshold triggers a linear ramp generator with arbitrary start and stop levels, and a time control.
So, I got it home and turned it on. All seemed to work. The slider travel seemed a little rough but there was no noise or dropouts when I moved them. However, there was no filter motion at all-no matter what the settings, the filter was stuck tracking the stop level slider. Oh well, maybe it broke and that's why it was hocked for so little. Let's take a look.
The board inside is (nearly) all surface mount, with the prominent exception of tantalum caps (Y tho????), and looked very different from the 1978 schematic-and not just in the absence of CA3094s. The board did not seem to be very logically laid out, either. I put it aside to work on other things.
Yesterday I found that a brave soul (Bernard d'Uur) had actually made a trace of the schematic for this revised modern reissue. Yay! A few things were apparent. Not just the OTAs switching to 13700s, but much of the circuit had been revised. Gone were the nice dual 12V power supplies, now there was a cheap and nasty op-amp rail-split of the 9V signal to give virtual ground at 4.5V (c'mon EHX, even hobbyists know to use the LT1054 charge pump IC to get dual 9V supplies!), and the circuits appeared to have been cheapened a lot-gone were the zener references and a lot of the OTAs, replaced by generic transistors, generic signal diodes and resistors in paralleled configurations. In short it looked like a design that had the hallmarks of a synth designer (whose design language was typical late-1970s) had been re-imagined by guitar pedal guys (whose design language often seems stuck in 1967!). Still, the filter circuit seemed intact, despite the current design cheaping out on ceramic caps rather than film.
So, I got to work. Still working on the assumption that the pedal had broken after purchase, I suspected that one of the many signal vias (courtesy of the haphazard-seeming layout) was to blame. With expansion of the many fibreglass layers in a PCB, signal via traces running between board layers can be vulnerable to failure, which is why they they are often filled with epoxy, solder (better), or soldered solid wire contacting the top and bottom pads of the board (NASA standard!) to guarantee longevity.
I noticed that there were empty part positions, largely to accommodate through-hole versions of parts where no SMT equivalent was available at time of design, such as JFETs and some capacitors, and set to getting the board out of the case.
At this point it was apparent that it was not designed for field servicing, as the jack sockets would not allow the board to be easily removed. I eventually pried each side by increments with a small screwdriver, and it was free.
Looking at the front, it was clear how dirty the board was from use, probably due to a total absence of dust filters on the slider runs (cmon, EHX!).
I returned to Bernard's schematic, and decided to start looking around the ramp generator, starting from the timing capacitor C6. Ummm, what timing capacitor????
The filter ramp timing cap had never been fitted at the factory!!! This explained why the filter was static alright, and it must have been the case that the guitarist who bought this just shrugged their shoulders and simply used it as a glorified fuzzbox. Still, it begs the question of what sort of quality control something that new currently costs $A600 deserves-there were no quality control stickers on the board or inside the unit.....
The solution was a nice fat tantalum cap scrounged from a prototype I had to hand and soldered in seconds. First switch on-yay!
The 18dB filter squelched beautifully, and with ye old Concertmate drum machine I was in heaven.
Conclusions: Quality control matters. Having any parts such as tantalum caps that can be assembled by machines, assembled by machines, is A Good Idea. Not leaving multiple empty part positions on your boards so that hand assemblers don't get confused when visually checking the parts that should be there, is A Good Idea. Designing your PCBs so the layout is logical and aids service, like Roland analogue synths, is A Good Idea. And when troubleshooting, don't ignore the obvious, assume cost equals quality, or discount human stupidity :)
No comments:
Post a Comment