polychorus trim pots?

[flipperbaby]

hey guys first post for me.

i opened up my polychorus and noticed 8 little scattered holes on the back of the board. you can adjust them so i went right int oit and @#$%ed it up...the filter matrix and flanger became a fuzz

luckily i tweaked it back to the original sound but i wanted to know if any of you know which thing does what :?

thanks

-mike

[Mark Hammer]

Not sure about what each of the trimpots does in a Polychorus, but here is a listing of what they *might* do.  These are all common functions in that sort of pedal:

1) In order to pass signal, delay chips require a DC bias voltage that the audio signal sits "on top of".  Some companies take an educated guess and do this with fixed resistors.  Other companies use a trimpot and adjust the bias voltage to optimize it for that delay chip.  In some instances, it is possible to use a single trimpot to set the bias *once* and distribute that bias voltage to multiple chips.  In other cases, companies will use a separate bias trimpot for each BBD chip on board.  The trimpot is generally located near the BBD chip.  Setting it improperly will get you a distorted delay signal (just off the bias voltage it needs to be) or no delay signal (way off where it needs to be).

2) Because of their internal architecture, delay chips generally provide two complementary outputs driven by the two clock signals: one for "tic" and another for "toc".  The sample that passes through the delay chip in interleaved fashion is then stitched together to make a whole sample by mixing the two outputs together.  Most companies will do this by simply using equal-value resistors from each output to a mixing point.  Because the two complementary paths are pushed along by opposite phase clock signals, though, if you mix the outputs in *just* the right proportions, you can get the clock signals to cancel out at the mixing point.  Consequently, many delay-chip-based pedals have trimpots linking the two BBD outputs so that maximum clock cancellation and lowest noise can be achieved.  If this strategy is adopted, there would be one for every BBD chip.  If the chips happen to be a SAD1024, there will be two such trimpots for output pairs A/A' and B/B' (see the datasheet).

3) Because of component tolerances, it is often difficult to nail what the part values should be for the MOST regeneration before you get runaway feedback.  Consequently, many companies, in an effort to get as much regeneration as is feasible, use a combination of a regen control on the panel and a trimpot on board.  During setup, the Regen is set for max, and the trimpot adjusted to the point where there is runaway feedback and then eased back just until it stops.  There is normally only one such trimpot.

4) Maximum "effect" is achieved for many types of effects when the dry and wet signals are mixed inthe right proportions.  Some companies will provide you a control for mixing, but the majority will save on panel space, pots, knobs, etc., by having a trimpot on-board that sets the wet/dry balance.  In the case of E-H pedals, this is often something to set how much wet signal is mixed in.  If all the other trimpots are set right, what you'd hear when twiddling this one is a reduction in the intensity of the effect so that the delay signal moves to the background.

5) Chorus sounds require that the clocking produce a certain range of delay times.  E-H has included "clock-trim" controls on board in several of their designs, such that the delay time can be adjusted to achieve the prototypic sound for that particular product/effect - delivering the same sound all the time, no matter what other deals on parts resulted in component substitutions.

6) The Polychorus is one of E-H's interesting combinations of delay and envelope control.  As is discussed here quite often, matching the sensitivity of an envelope follower to produce the right amount of parameter change in response to picking differences and instrument differences is a common problem.  It wouldn't surprise me a bit if one of the trimpots were associated with the sensitivity of the envelope follower.

That's not all the possibilities, but probably most of them.  Tell us what's in the pedal in the way of chips, as well as the physical poximity of the trimpots to the chips, and we can probably tell you what the trimpots do and how to tinker with them.

[Mark Hammer]

So this is an old one.  A buddy brought over a little mountain of ehx pedals to bring back to life, and one is a Polychorus.  I have a schematic for the reissue that uses an MN3008 and 3009, but this one is an earlier issue with two SAD1024 chips.  Anybody know where to find a schematic for that one?

[DiscoVlad]

Have you tried searching for the Echoflanger, or Polyflange? The Polychorus has had several names over its lifetime.

[Mark Hammer]

Thanks.   Helpful.  I had naively thought the name change reflected a change in design, but looking at the Echoflanger, Polyflange and Polychorus, I do see that they are all essentially the same thing.  I think what threw me off was the absence of any obvious 4-position switch in the schematics for the Echoflanger that I had.  The CMOS sections for switching are certainly there, just nothing that looks obviously like a 4-position rotary switch.  I see the 4-way switching in a different Echoflanger schematic, though.

I was obviously very mistaken in my post above, back in 2004, naively assuming that because the Polyphaser had envelope control, therefore the Polychorus must have the same prefix for the same reason.  Wrong, wrong, wrong.  ironically, the fellow who brought over the Polychorus for repair, brought over a Polyphaser for the same reasons.  It'll be fun rejuvenating these beasts.  I'll try and post some useful gutshots.  IN particular, identifying the various trimpots on the Polychorus board, and what they do may be useful for folks.

[RickL]

Hopefully, for me, the problem with the Polychorus will match the problem I have with an Echoflanger I bought years ago and your solution might help me fix mine.

When it's first turned on it works fine but gradually the delay sound starts to distort and finally fades completely out. Letting it sit for a day or two or shorting the power rail to ground restores it for a few minutes but eventually it fades again.

A leaky cap seems to be the obvious problem but in several tries I haven't been able to find it. Any ideas on which cap is the likely suspect or could it be something else?

[Mark Hammer]

As always, anything that systematically varies with time suggest origins in either heat buildup or bad electrolytic caps.  The transition from decent to distorted to absent delay strongly suggests one or more bad caps impacting on the bias voltage, since tweaking the bias trimpot feeding the BBD input gets one that precise pattern:  from nothing, to a distorted delay signal to a clean signal, as you approach the optimal bias voltage for the chip.

From the Echoflanger schematic I have, there are a pair of 10k trimpots providing bias voltages at pins 2 and 15 of the Reticon chips, via 100k resistors.  In many designs, one will generally see a smallish electro (1-10uf) between wiper and ground on such bias trimpots, but i this instance, none is shown on the schematic.  What IS shown is an electro of unspecified value on the pin 7 output of the only LM324 on the board, and a 15uf cap on the input to that op-amp section.  The bias trimpots take their voltage off that op-amp output.  So it seems reasonable that a productive first step is to replace both of those caps, since they may well be old and crunchy by now.

I took some pics last night, and hope to finish annotating them with chip and trimpot labels tonight or tomorrow night.  I'll pop them off to you once done.

[Paranoid57]

Hello,
This is an old topic but I just received a bunch of all battered pedals for repair.
One of them is EHX Polychorus reissue with MN 3008 and MN3009.
Power supply missing, missing and bad connection inside, 4-pole switch broken, obviously someone tried before.
After I made a new PSU, rewired the connection and fixed the switch. (I would rather change it but very slim chance of finding it).
After assembling and testing I found that signal is going through but effects do not works.
I checked the path but but all components seems to be fine and that someone mess around with ALL trimpots.
I have adjusted clock frequency and can get effects working by moving trimpots but iI doubt I can figure out a proper setting or it will take extremly long time.
Where I can get a procedure for setting the trimpots by using signal generator and oscilloscope ?

Sending the pedal somewhere is not feasible, I live in Egypt and it will just cost more then peal is worth plus dealing with unpredictable Egyptian customs.

Thanks.

[Scruffie]

The only set up info in the factory schematic is for clock adjustment;
Sweep Filter: Off
Width: CCW
Rate: CCW
Tune/Delay: CW
Mode: Filter Matrix
Adjust trim 8 for 100uS @ U10 (4013) Pin 1

For biasing the rest of the circuit it says to connect a signal at 400mV p-p 400hZ to the input, for the BBD bias (Trim 1 & 4) & cancel/balance trims (2 & 5) you can set them up as any BBDs but the BBD gain (3 & 6) and the mix trimmer (7) you'll have to use your best judgement, keeping in mind a 50/50 dry/wet mix will provide the 'best' chorus and flanger sounds..

Hope that helps a bit.

[Paranoid57]

That helps a lot in fact.Thanks.