Maxon Payne 2

Well, I tested my original hypothesis of how to fix the Maxon Autofilter tonight, with a roll of duct tape from a $2 shop. Would layers of duct tape on the overly-soft sponge buffer between the miniature switch and the pedal movement get the Maxon Autofilter switching again? Time to cut them up and see!

I first tried applying a 6-layer thick square onto the pedal foam to add enough contact pressure to switch the pedal. No dice. Then I added a further 4 layers of duct tape.

Success!! The pedal switched over - just- at the bottom of its travel. Screwed the switch in, clicked it a few times-then it stopped clicking. Time for more tape. I added another 9 layers, when it started working, then another three more for good measure. It worked, but again, it eventually stopped changing over the switch as the foam block "adapted". Argh.

Another 8 layers of duct tape later, the pedal is now changing over nicely. As I predicted in my earlier post, the foot pedal movement barely moves far enough to disengage the pedal (the deepest travel of the switch) so alignment is critical. I think I will be keeping the pedal in the engaged mode when not in use to minimise switch pressure on the foam.

And of course with all of this insertion and removal of the switch, one of the flimsy wires to the switch carrier board snapped! Murphy's Law in action, and time for more soldering.....

The Joy of Forgetting

About a year ago, I was browsing a pawn shop when I saw a guitar pedal that immediately drew my attention for a few reasons. It was big, it was colourful, it was made by Electroharmonix (who have a legendary rep in their own right and also employed former EMS VCS3 designer David Cockerell in the 1970s), and it had the word synth in it!!!

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 :)

Hey Mr. Tampurine Man

You never know what you find in an "uncurated" antiques shop. In this case, something that looked like a vintage answering machine that had been hacked to be an analogue preset rhythm box-for about 50 bucks after negotiation with the seller. Woo-hoo!!!

Getting it home, it sounded very much like a poor man's DR-55-claves, open and closed high hats, snare and kick and that's it, but all voices were nice and punchy.

On the back was a label which made it appear that it had been part of the holdings of an educational institution-presumably in the US given the date format from when it entered inventory in 1980 -  T. something College of the Arts?

The label is interesting for a couple of reasons-firstly, it dates it from the brief period when analogue synths were moving into the consumer space before the Casiotone onslaught, and indeed the quirky design brings to mind Paul Schreiber's recollections of how he was told to design the Moog Concertmate for Radio Shack around the same time. For example, they required all line outputs be on phono jacks for connection to domestic hifis-as is the case here. It has a 2.5mm start/stop footswitch input that screams "dictation machine"-and indeed it was sold bundled with a plain-looking footswitch that just reeked of typing pool anti-chic, and was probably excess stock from a Realistic dictaphone.

Secondly is what appears to be the college's intended use of it-a "tamura". In South Asian music, an electronic tampura or tanpura box is an electro-acoustic accompanist for live acoustic musicians (it was originally invented in India in 1979). When I got the Realistic home and turned it on I was surprised-the built-in speaker is loud-really loud, easily sufficient to accompany a small acoustic performance. In these days, when we are conditioned to expect inbuilt speakers in portable instruments are usually toys or at best sufficient for personal practice, this came as quite a surprise.

There was also something else that was really loud-a continuous tone that would not stop unless the input power voltage was dropped to 4.5V. Fortunately the schematic was not hard to find online. It appeared that the "tuned" voices (bass drum, claves and snare) were produced by the "pinging" of four pole single-transistor hipass filters generating tones from decaying feedback from the collector, and the emitter of each transistor is connected to a trimmer which sets the gain on the collector resistor and therefore controls feedback. Could it be that one of the voice trimmers was out of alignment and feeding back too much so that the sound never decayed?

Time to open it up!

The biggest surprise is that the speaker takes up almost half the space inside the instrument. The rhythms are driven by a Sanyo preset rhythm generator chip called the LM8972 (bottom left of the main board). Many other devices used this, and many of them used a remarkably similar circuit to this one, but in stomp box format (such as the Sound Master Rhythm 1). Transistors were all the bog-standard NPN 2SC1815 except for the reverse-biased noise generator, which was a 2SC1923, presumably because it was noisier! Note the big red inductor-a single transistor LC cymbal noise filter is a big part of the DR55's sound, and a similar circuit technique is present here too.

Any way, to work. After all the effort it was almost anticlimactic. A slight twist on one of the trimmers and the droning stopped, with the snare now decaying nicely!

I tried turning the noise trimmer, and it did modulate the level of the noise source pretty effectively.

So to wrap up, it's a fun instrument with a powerful voice that is hamstrung by its limited rhythm selection. Also its primitive RC power bypassing puts the sensitive analogue circuitry at the mercy of interference from the digital circuitry, so that even when stopped a very faint ticking is audible from clock interference "pinging" the clave circuit. Nonetheless, it has great potential as a sound module. Adding trigger inputs would be easy as it already has pulse extractors on all voices, likewise adding individual outs, with plenty of room in that huge case.

Another thought-remember those pedals based on the same circuit we mentioned earlier?  Thanks to prominent Youtubers featuring them, they are now quite expensive, although they offer very little of the customisation potential of this unit due to their cramped size. This one on the other hand seems to have slipped under the collectors' radar courtesy of its odd appearance and the Realistic brand name and thus is undervalued accordingly. But then again, so was the Moog Concertmate synth once upon a time, for exactly the same reasons...

A SAD day for Mr. Roland (with bonus: Bitscope or Bitnope?)

Once upon a time, back in the early 90s, I bought something from the old Smithys shop in Camperdown, and they had four dinky little Roland RS09 keyboards with clunky old fashioned-looking grey rocker tabs on their panels sitting unloved and unwanted with $100 price tags, right in the middle of the workstation keyboard era. I didn't give them much thought.

Shortly after that I went into another music store (might have been Venue) and found a slightly more modern-looking example of the same model with coloured buttons, and played it through headphones. It sounded magnificent, with its simple controls belying its ability to control textures by balancing an organ and string section, and by selecting whether either, neither, or both of the two voicings went through the rich-sounding ensemble circuit. I subsequently got distracted by other things, and by the time at the end of the decade when I started hankering for an RS09, the prices being asked in music stores had seriously rocketed skyward, to $400 and beyond, so I thought that was that.

In 2009 I saw Stereolab at the Metro theatre supported by Crayon Fields. Crayon Fields used an RS09, which sounded great through a PA! Stereolab had had keyboard troubles that night-Laetitia banged her Little Phatty several times against its stand to keep it working, and near the end of the set Joe Watson's Wurlitzer piano stopped working altogether. The support act obligingly set up their RS09 for him, and it stormed through French Disko, prompting Laetitia to exclaim "The Roland Rocks!" By this point RS09 prices were climbing towards $A1000.

Then one fateful day in 2017 I walked into a shop and found a grey rocker-tab RS09 sitting there with a $240 price tag. A couple of sliders appeared a bit bent, but all knobs were present and it powered on. This one was not going to get away from me, so I bought it immediately and left the shop.

All the keys and controls worked, and it was still a great sounding keyboard, if maybe a bit more trebly through headphones than I remembered, and there also seemed to be more chorus animation through one of the channels. Maybe it could be adjusted? Still well worth the price. In line with what I had heard at the Stereolab gig, the RS-09's "thin" sound was well-balanced for a keyboard combo amp, and sounded top-notch in mono. Plus-full polyphony-all 44 keys at once!!!

One day I tried isolating the output channels into my combo amp, and discovered to my consternation that while one channel sounded rich, the other only gave a slow pitch variation effect. Something was wrong, and it was time to investigate...

The original 1979 RS09 (which I own) has physical rocker-tab and slider-toggle switches for organ cancel, organ filter, string footages, and ensemble in/out, plus its ensemble circuit uses four SAD-512D BBD chips, two for each output channel, with each channel's delay lines modulated by slow anti-phase triangle LFO pairs, running at different speeds per channel. One delay line per channel can also switch in a fast sine wave LFO (again, slightly different speeds per channel) in addition to the slow LFO-much richer than the Juno or even Dimension D circuits, which only ever use a single LFO and one delay line per output.

In 1980 the RS09 was updated with Jupiter-8/TR808 style colourful keyswitches to electronically control registrations and ensemble switching, as well as changing some legending from light grey to a garish orange (gotta move with the times), plus changing the nice solid-looking "waterfall" piano keys to so-called "diving board" types, which are more vulnerable to accidents if they are ripped upwards. The electronic switches are not as reliable as physical switches due to dust contamination, however as a positive they moved to Panasonic BBD chips, which are somewhat easier to find than the Reticon SAD BBD chips used in the old version.  

Luckily, both the official owners' manual (from Roland) and the unofficial scan of the service manual (from elsewhere) both refer to my earlier version, so time to dive in.

After unscrewing the top and propping it up gently, I found a very well-laid out circuit board (typical of Roland in the analogue era), with the four modulation circuits converging to a central dual op-amp that mixed the final stereo signal. As I keep the keyboard at home, and my "real" scope is in my workshop, I thought I would check it out with the aid of a Bitscope Micro USB oscilloscope I bought in an Element14 runout sale two years ago.

Bitscope monitoring the A440 test on Mod1 and Mod2. Note the 44 decay generator capacitors on the voicing board at bottom left. The joys of full analogue polyphony!

The BitScope is a cute little device, Australian-developed and made with a good software suite thrown in. Its dependence on an FTDI driver makes it finicky, however-I eventually worked out that my Mojave-running MacBook needed to be started up with the scope attached to its working USB port for it to work.

I quickly found that its performance noticeably suffered in two channel mode, and it was very noisy with low-level (ie small-signal audio) signals. The 500ns pulses that controlled the BBD delay time also looked very slewed on the display, and almost seemed too much for its (nominally) 40MSPS performance, but on the other hand its limited storage memory made it struggle with very slow signals. Nonetheless, it helped solve my puzzle.

I had been hoping that the dead part was not one of the rare and expensive SAD512D BBD chips-maybe the LFO circuit to one delay pair had packed up? Roland conveniently provided LFO test points and all four LFO outputs appeared to be working perfectly. OK.

I then checked the BBD clock test points. All BBD clocks appeared to be working OK, and modulating in response to their LFOs. Hmmm, maybe it's the output circuitry.

At the final mixer opamp, all the delayed audio signals were present except one-Modulator 4. Maybe it's the BBD output noise filter? Fingers crossed.

First I checked the bases of Mods 1 and 2 initial single-transistor output noise filters. Nice signals moving in opposite directions to each other, showing clock noise ready to be cleaned.

Checked the bases of Mod 3 and Mod 4's initial single-transistor output noise filters. No signal on Mod 4-not even clock output. Starting to worry.

Finally checked the signal into Mod 4's BBD. It was fine. Dammit! New BBD needed! Or I could cut my losses and continue to enjoy it as-is. Hmmm, let's see our options...

(to be continued)

Maxon Payne

Once upon a time in the 1990s, a dear friend and former university classmate, who had been playing guitar since the 70s, gave me an old guitar pedal he had no use for any more. The pedal, an Ibanez auto filter, was really cool - sturdy and bright-sounding. Then one day circa 2001, I lent it to another friend who packed it up when they moved house, never bothered unpacking the box it was in, then threw out the unpacked box three years later when they moved house again. Grrrr...

Cue forward to 2020. My former classmate is enjoying his retirement, the pedal-dumping friend is now a former friend I haven't spoken to in years (because of Facebook radicalisation, I hasten to add), and on my first visit to a favourite pawnshop after the easing of Covid-19 lockdown I saw this on display:

It was a faithful-looking reissue of the Ibanez auto filter, sold and branded by the pedal's original manufacturer! The shop only wanted $99 for it, but it was clear that it had at least one issue...

That issue would be very easy to fix with a nut and washer from a donor 1/4" jack, and the sales person offering it to me for $80 made the deal a no-brainer. 

I couldn't wait to get it home and test it. When I plugged it in, however, I found that the circuit (and the indicator light) would only engage when the pedal foot switch was pressed to its limits, and would not latch on so that releasing foot pressure would cause the pedal to revert to bypass. I figured that the pedal's switching logic had failed, but what I found demonstrated both that some things are best left in the past, and that messing with the formula can be a terrible idea.

Doing a Google search of forums showed that modern Maxon pedals tended to have switching failure which locked them into the engaged mode, and that unlike their antecedents they were true bypass (ie mechanical switching, not logic). Where people had not hacked conventional heavy duty foot switches onto the enclosures as a solution, the recommendation was to replace the switch. Maybe the switch's return spring had broken? I plucked up the courage to take a look.

I found that despite their tank-like casing, the build quality of this recently-built pedal is still very much 1980 second-tier Japanese manufacturer, with phenolic boards containing through-hole carbon film resistors sitting on their ends connected by a mess of light-duty wire. Fittingly the battery compartment is only separated from the foot switch carrier board by a flimsy piece of plastic covered in a thin layer of foam.

(Note the hilariously un-1980 main body screws, not just Torx, but tamper-proof Torx! FFS-did Apple refuse to sell them pentalobe screws?? Memo to fellow manufacturers-this sort of shenanigans is not going to stop anyone from ordering a suitable driver set from Amazon to hack the product, just make life inconvenient for anyone attempting field repairs)

I unscrewed the switch panel and pulled it back through the mount hole to reveal the switch. Not only did the switch click in and out nicely when I pressed it directly, but the pedal switched between engaged and bypass modes without any problems. Clearly there had to be a problem somewhere else.

Hmmm, let's look at the mechanical linkage to the foot pedal movement then. A block of foam rubber on the underside of the pedal surface. OMG! Foam rubber, which can permanently compress under sustained pressure, and loses resilience with age. Seriously??? 

Well, that explained the switch not pushing in far enough to latch, but what of the switch not budging at all when engaged? Like a lot of changeover pushbuttons, the switch shaft stays lower when actuated as a visual indicator-which means that disengaging it requires a deeper push, which the foam block was not capable of. This would not have been so much a design concern in the 80s original, where a simple momentary pushbutton was used for electronic switching, as the action would have been shallow and identical either direction, but the current mania for "true bypass" circuits necessitates a changeover switch with relatively deep, asymmetrical action, and the foam block is not up to the task. Evidently no-one at Maxon thought of that when "improving" the design. 

I chose to re-insert the switch in "on" mode so at least the pedal is useful for now, but I will be trying to layer gaffa tape onto the foam block to better distribute pressure across its surface and to try and get the foot switch working. More to come!

Hello world! Welcome to my blog which will be a vaguely coherent insight into things crossing my path. Hope you like it! Justin B-H