Strange issue: osc1 intermittently "appearing" on voice 4

[axmonti]

Hi everyone,

Long-time PEK owner and (other than the endless encoder swap), haven't had any issues until yesterday when osc1 decided to disappear from voice 4. Very odd issue, and I've opened a ticket, but thought I'd post here as well in case someone has any ideas while I wait.

Not to retype the entire support request, but essentially I discovered that osc1 (L) intermittently appears in voice 4, and since the PEK uses DSPs, I'm at a loss to explain why - especially since osc3 works perfectly on voice 4. I don't think anyone but DSI has schematics for the PEK, but I can only assume that the oscillators are generated with code for within the voice PICs and realized with code in the ROMs that control the DSPs. The resulting output from the DSP is processed by the CODEC - an IC that has three separate stereo audio output paths, which likely just uses two (L & R) for audio output after the DSP has generated a final 'sound'. Much more difficult to debug than a straight analog board. My 'gut feel' is a DSP issue, but that's just from issues I've had with my MS2000.

Here's what I tried that didn't work:

Swapped PIC chips between voices
Swapped DSP chips (ROMs) between voices
Updated main, voice and DSP firmware
Reloaded all Programs

Here's what I tried that sort or works:

Choose a Program that uses Osc1; press and hold keys to consume voices 1-3; repeatedly press a key to activate voice 4. Eventually, osc1 on voice 4 will 'come to life'. If I continue to keep voice 4 active at this point, it will eventually deteriorate and then stop, at which point, holding down the key does not bring osc1 back on voice 4. If I repeat the process, I can make osc1 on voice 4 appear again (even for a few minutes of playing before is disappears again).

It seems slightly harder to make this work when the unit is cold: 100 keypresses or so, as compared to 30 or so for a warm unit. A laser thermometer reading shows no difference in temperature between the chips for all 4 voices.

Has anyone ever experienced something similar with their PEK (or Evolver)? At this point, I'm thinking it's going to result in a board swap, but I'm always interested to know what the root cause is.

Thanks for reading!
Andrew

[axmonti]

Well, as expected, DSI is offering an overly reasonable board swap - likely for the main and voice boards, since there's no way to know which board is at fault remotely. Although greatly appreciated, I do miss out on what the root cause is. At some point, PEKs will become scarce and components will be hard to find. The DSP is already discontinued and $50 on the resale market, with the CODEC soon to follow (plus the PIC chips). I guess at that point we'll have a PEK emulator!

Thanks again for reading...
Andrew

[Sacred Synthesis]

Thanks for the morbid reminder: our Poly Evolver Keyboards are not immortal.

Rather than a PEK Emulator, I'd rather have a PEK MK II.

[axmonti]

Ditto - I'd love to see a new version of this crazy machine - there's nothing like it (I consider it the successor the the PPG).

Small clarification: Sequential is repairing the board - not swapping it, so that's even better. Still would love to see what the issue is/was; how can a DSP generate a sound 'intermittently'? I suspect something's not telling the DSP to generate sound for OSC1. If I find out what the issue was, I'll post a follow-up.

[Sacred Synthesis]

I've wondered for a few years now if it was known how and approximately when our Poly Evolver Keyboards would begin to fail.

[axmonti]

I'm curious about that as well - nothing in the machine seems to run overly hot or is prone to failure, but in researching my issue, I came across a number of owners with dead voices (1-4). From a failure analysis point of view, it's difficult to determine what will fail on these machines - the critical components are the CPU and DSPs. There was/is a known issue with Korg MS2000 keyboards (also DSP-based), where the CPU would complain that communication with the DSP could not be established. These failed anywhere from 5-10 years into the life of the keyboard, and after the DSP was replaced, the keyboards worked again.

The PEK was released in 2005, and we'd see some evidence of failure at this point, but other than wonky encoders, there haven't been too many recorded issues, which is a great sign. Once DSI/Sequential can't repair the boards, maybe they'll provide firmware updates that 'skip' the dead voice, which means 3-key polyphony (or less), not great, but better than silence on a note press. There's likely a good supply of encoder boards because of the swap/PE versions, but there aren't too many 'spare' voice boards out there (I haven't seen any).

[axmonti]

 To wrap this up, Sequential didn't divulge what transpired (as expected), but here's what I found. Sometime in the early 2000s, Dave Smith apparently contacted Doug Curtis (of Curtis CEM chips) to develop a new version of the CEM3396 (synth-on-a-chip) for new keyboards. Dubbed the CEM3397, and PA397 specifically for Dave Smith, this new chip added stereo capabilities to the infamous CEM3396. There are two PA397 chips for each Poly Evolver voice, which seems to correspond to each pair of voices (1 & 2, and 3 & 4). Note that they don't make the PA397 chip anymore, but Behringer seems to have a clone chip available as of 2020. The chips use analog circuitry to generate oscillating waves based on various parameters (there's a datasheet available for the mono CEM3396 on the web), so the core oscillators are definitely analog - how the digital signal processor and analog to digital and digital to analog converters are combined with the oscillators is another story. For my particular problem, it's obvious that transistor Q26 was replaced, which I assume corresponds to Osc1 (left channel of the first oscillator). The replaced transistor is connected to the PA397 chip, and also the main DSP chip (which is out of production as well). Unfortunately, they don't make that particular transistor anymore (it's a MMBT3904 signal transistor), but there are a few equivalents that are close.

I also discovered another couple of issues, one of which was a cracked solder joint, likely from early (mid-2000s) versions of lead-free solder, which has been proven to degrade over time, plus my aftertouch cable developed a crack somehow (solved by some conductive epoxy).

Happily, all is well now and my Poly Evolver is back in business, but the bottom line is that not only are replacement components becoming scarce, the solder itself may be aging and cause issues, although reflowing solder is much easier than tracking down and replacing rare components.

 

[Sacred Synthesis]

In other words, if our Poly Evolver chips need replacing in the future, only Behringer will be able to do it!

Seriously, it makes me feel as if my Poly Evolver Keyboard and Rack are ticking time bombs.  What to do, what to do?  Such a fabulous and truly unique instrument.

[axmonti]

I know what you mean - I'm half waiting to find yet another issue, but in the meantime, I'd say enjoy playing and creating great music! There are many pre-2000 synths that face the same issues (leaky battery damage, dry capacitors, ICs that don't work anymore, transistors, etc.). I've been waiting two months for a BBD chip to arrive to fix my PolySix which had a multitude of other issues. Also, since I know the DSP on my MS2000 is prone to failure, I ordered a backup - just in case. If you are really concerned, you can still find most of the Poly Evolver components for sale (either in production or through eBay) and stock up on parts; I'm even thinking about picking up a programmer and spare PIC chips for the CPU and voices, but they're not cheap.

Realistically, the only things that have failed in 15+ years on the Poly Evolver have been: rotary encoders, LCD backlight, three capacitors, aftertouch ribbon, and one transistor, plus aging solder (which is easily solved by any repair shop). But yes, even Sequential doesn't have spare parts and has to rely on sourcing from 3rd parties for replacements. I'd say the chance of having an unrepairable failure are still pretty low at this time. If you're serious about stocking up on replacement parts, I'll put together a list of current sources for spares.

[Gerry Havinga]

I know what you mean - I'm half waiting to find yet another issue, but in the meantime, I'd say enjoy playing and creating great music! There are many pre-2000 synths that face the same issues (leaky battery damage, dry capacitors, ICs that don't work anymore, transistors, etc.). I've been waiting two months for a BBD chip to arrive to fix my PolySix which had a multitude of other issues. Also, since I know the DSP on my MS2000 is prone to failure, I ordered a backup - just in case. If you are really concerned, you can still find most of the Poly Evolver components for sale (either in production or through eBay) and stock up on parts; I'm even thinking about picking up a programmer and spare PIC chips for the CPU and voices, but they're not cheap.

Realistically, the only things that have failed in 15+ years on the Poly Evolver have been: rotary encoders, LCD backlight, three capacitors, aftertouch ribbon, and one transistor, plus aging solder (which is easily solved by any repair shop). But yes, even Sequential doesn't have spare parts and has to rely on sourcing from 3rd parties for replacements. I'd say the chance of having an unrepairable failure are still pretty low at this time. If you're serious about stocking up on replacement parts, I'll put together a list of current sources for spares.

Thank you for the research and your reporting back.

I am about to buy a second hand Poly Evolver Rack to complement my MEK and Evolver desktop. Unfortunately the seller does want (not an unreasonable) price well above 1000 Euros. It is impossible to guess what will happen in the future, but buying instruments of this complexity and age while some really great brand new synthesizers and Eurorack modules are being produced almost on a daily basis, might be a bad idea .......

[Sacred Synthesis]

I know what you mean, Gerry.  I've been seriously considering the replacement of my Poly Evolver with a Novation Summit.  But I have to say that it's not an easy decision.  Regardless of its age and the many poly synths that have appeared since, the Poly Evolver is an exceptional instrument.  To my ears it sounds superb, which is why - if I could have my way - i would much rather keep it than replace it.  In fact, I dread the disappointment once it's gone.  So, it's a difficult decision.

[Gerry Havinga]

I know what you mean, Gerry.  I've been seriously considering the replacement of my Poly Evolver with a Novation Summit.  But I have to say that it's not an easy decision.  Regardless of its age and the many poly synths that have appeared since, the Poly Evolver is an exceptional instrument.  To my ears it sounds superb, which is why - if I could have my way - i would much rather keep it than replace it.  In fact, I dread the disappointment once it's gone.  So, it's a difficult decision.

I totally agree, except for some small quirks, like naming patches  , the ease with which I create new patches from scratch on the Mono Evolver keyboard is astounding. The beauty is that I keep having the (wonderful) feeling I am only touching the surface of what the Evolver can do and that there is so much more to discover.

Another consideration for buying the PER is I then have 3 Evolver instruments. They can be each other's "backup". I have upgraded the MEK, two of it's three boards have been replaced. I have asked Tracy what the status is for replacement parts of the rack. Hopefully the answer will be coming soon, then I will make my decision. If the PER is sold by then, too bad for me.

Another benefit, from my perspective for getting the PER, is it doesn't have much of a learning curve for me regarding the user interface anymore. I am "cognitively limited" in learning more synthesizer interfaces because of lack of time. Switching a synthesizer back on after a few weeks of no use and then having to search through  it's manual once again if I want to achieve a particular sound / patch totally breaks my workflow and destroys my enjoyment.

Sound design is part of my composing tracks workflow, I rarely re-use patches I have created before. Creating a new sound often inspires me to write a track with that patch.

Ah decisions decisions decisions....... I am aware I am very privileged in this respect.

[Sacred Synthesis]

I have asked Tracy what the status is for replacement parts of the rack. Hopefully the answer will be coming soon, then I will make my decision.

Ah decisions decisions decisions....I am aware I am very privileged in this respect.

Could you include the substance of his answer in this thread?  It will be helpful information to many of us.  I've often thought of asking Sequential's advice on this: to keep or not to keep.

And yes, we are all quite privileged in having such "problems."  Or perhaps "spoiled" would be a better word.  We have so so much.

[axmonti]

Interesting question Gerry is posing to Tracy - Sequential is a great company and (in my experience) makes every effort to fix what they can, but after they run out of parts, suppliers, and donor/scavenge boards, they likely don't have much of an option but to say that the boards are not repairable. I think the PER voice board is identical to the PEK, so (according to Sequential), "...there are no factory [voice] boards to swap." There would be a small control panel board as well unique to the PER, possibly with wonky encoders. I wouldn't expect Sequential to start bidding for rare components on eBay or take chances with AliExpress suppliers on behalf of customers.

That being said, I'd still pick up the PER (I thought you could use an external controller on the PER and chain the MEK for additional polyphony, but maybe that's not true). Also, just to correct what I wrote above, Osc3 & 4 are digital, so AFAIK, not connected to the analog PA397 chips. More likely, to achieve panning, Osc1 and Osc2 are connected to a stereo chip each so the voices can independently move between channels (at least that's my guess).

[Sacred Synthesis]

I have a Poly Evolver Keyboard and a Poly Evolver Rack.  You chain the latter to the former and the Keyboard unit becomes the master controller of the Rack.  I never touch the Rack, except to turn it on and off.  Also, the audio of the Keyboard goes into the Rack, and the combination then goes into a mixer as only two stereo channels, rather than four.  On so many levels, the Poly Evolver is a masterpiece.

[Soundquest]

I have a Poly Evolver Keyboard and a Poly Evolver Rack.  You chain the latter to the former and the Keyboard unit becomes the master controller of the Rack.  I never touch the Rack, except to turn it on and off.  Also, the audio of the Keyboard goes into the Rack, and the combination goes into a mixer as only two stereo channels, rather than four.  On so many levels, the Poly Evolver is a masterpiece.

Hmmn,  Not in front of the instrument now to view ins/outs,  But I hadn't thought of that stereo routing to save mixer input space.  Perhaps I could run my PEK outputs thru the desk top Evolver.  But, other than board space or cable limits, I cannot think of another a reason to do that.

[Sacred Synthesis]

I have a Poly Evolver Keyboard and a Poly Evolver Rack.  You chain the latter to the former and the Keyboard unit becomes the master controller of the Rack.  I never touch the Rack, except to turn it on and off.  Also, the audio of the Keyboard goes into the Rack, and the combination goes into a mixer as only two stereo channels, rather than four.  On so many levels, the Poly Evolver is a masterpiece.

Hmmn,  Not in front of the instrument now to view ins/outs,  But I hadn't thought of that stereo routing to save mixer input space.  Perhaps I could run my PEK outputs thru the desk top Evolver.  But, other than board space or cable limits, I cannot think of another a reason to do that.

What I meant is that the PEK and PER amount to only two audio outputs, rather than four.  I don't think the Desktop can do this, though.

Soundquest, I'm still waiting for your brilliant reflections on the Summit as a replacement for a Poly Evolver.  I realize that nothing can actually substitute for a PEK; the instrument is too unique.  But I'm thinking of the Summit's ability to have each layer processed separately and sent to opposite channels for a PEK-esque stereo depth.  Come on, now; you gotta help me here.  Are the two instruments at all comparable?  I need to make some decisions soon.

[axmonti]

...You chain the latter to the former and the Keyboard unit becomes the master controller of the Rack.

Great setup; I've always wondered what happens to voice assignments in that configuration - do they move from 1-4 on the PEK, then 1-4 on the PER? Does unison use all 8 voices?

Funny, I wonder if Sequential and Novation will share designs and technology now that they're part of the same parent company? The Summit looks impressive - haven't seen one in person yet.

[Gerry Havinga]

I have a Poly Evolver Keyboard and a Poly Evolver Rack.  You chain the latter to the former and the Keyboard unit becomes the master controller of the Rack.  I never touch the Rack, except to turn it on and off.  Also, the audio of the Keyboard goes into the Rack, and the combination goes into a mixer as only two stereo channels, rather than four.  On so many levels, the Poly Evolver is a masterpiece.

Hmmn,  Not in front of the instrument now to view ins/outs,  But I hadn't thought of that stereo routing to save mixer input space.  Perhaps I could run my PEK outputs thru the desk top Evolver.  But, other than board space or cable limits, I cannot think of another a reason to do that.

The Evolver desktop will not route the audio from another Evolver when poly chained. I believe the gate to trigger the amplifier envelope only opens when the desktop Evolver receives a MIDI note over the poly chain connection. It doesn't open the amp envelop for the notes sent to the MEK (in my case). I had to use a small passive mixer to mix the audio from my MEK + desktop Evolver. But then it works beautifully.

My guess is the PEK and desktop Evolver combination will behave the same.

By the way I didn't get the PER in the end. Seller could get a lot more money (several 100's of Euros) from another buyer. I don't think I will go further down the route of expanding my current Evolver setup. Way to risky regarding component failures and there are so many wonderful new instruments on the market nowadays.

[Soundquest]

Ok thanks for the details Gerry.