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Original mono EHX Polychorus investigation

 
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NickS
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Joined: 14 Jan 2008
Posts: 9459
Location: Nr. Basinggrad, UK

PostPosted: Wed Jan 23, 2019 2:08 pm    Post subject: Original mono EHX Polychorus investigation Reply with quote

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Someone found it and asked me if I had any suggestions about why his wasn't working.
Mk 1 Polychorus, from the web (a site in Greek if I remember)

Enquirer's Polychorus

Windows and the browser are arguing about jpg metadata so I might replace those pics with PNGs to get them consistently the right way round. First step is to ID the components. Hang on, someone already did an EchoFlanger overlay that's been archived here.
Schematics for the near-identical Echoflanger:.
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Note, these are copyright EHX but contain devices that can no longer be obtained so I don't see any commercial value to them. They are posted here in the interests of education.


What I get from the schematic is:
1. When the unit is powered on, I would expect the LED to be lit and the buffered guitar signal should be available at the Direct out. The LED goes straight to the supply pins of the SAD1024A bucket-brigade-delay (BBD) chips. If this is not lit either the 15v supply has shut down or something drastic has happened with the SAD1024As. The Bypass switch should cause the buffered guitar signal to appear at the Effect Out.
2. When in Filter Matrix mode with Blend engaged, only one BBD is used, IC4 - which is the one nearer the rotary switch; for the signal path to Effect Out to work practically everything has to be working except the SAD1024 nearest the edge of the board, IC3, half of IC1 and the low-frequency oscillator (two of four op-amps in IC3). If no output on Effect Out I would switch off and allow power to decay, carefully remove IC4 (observe antistatic precautions) and jumper pin 15 to pin 11; if the signal now appears there's something up with the BBD or clock.
3. When in Flange mode again only IC4 is used and the LFO is engaged.
4. In the other modes both BBDS are used.
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NickS
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Joined: 14 Jan 2008
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Location: Nr. Basinggrad, UK

PostPosted: Wed Jan 23, 2019 9:27 pm    Post subject: Reply with quote

The EchoFlanger PCB is different, though..
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NickS
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Joined: 14 Jan 2008
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Location: Nr. Basinggrad, UK

PostPosted: Mon Jun 17, 2019 7:03 pm    Post subject: Reply with quote

OK, here's a little follow-up.

It turns out that the PolyChorus PCB on that Greek website (with the square rotary switch) is different to the PCB on both the enquirer's (let's call him/her K for purposes of data protection) pedal and Speedfish's. The circuit design is also slightly different. K has also sent his/her pedal off to a professional repairer with disappointing results.

On the PCB with the square rotary switch, the LED is connected to the power rail and provides a voltage drop to the SAD1024 delay chips. It's just a power LED. The bypass footswitch is connected to the input of the CD4053 (or whatever) triple switch chip and a 1 Meg pull-up which is shorted to ground when the switch is closed. Both these features are as per the EchoFlanger schematic I've seen.

On the PCBs of the other two, the traces are different from the other in the area of the SAD1024 chips and rotary switch. Instead of using a voltage drop across the Power LED to bias the delay chps, there are lands to insert ordinary diodes, placed between the rotary switch and the SAD1024s. Speedfish's PCB has two diodes inserted; K's I can't tell from pics whether it's one or two but I'll guess it's two. The pull-up resistor value is changed to 2K; this lower value would allow an LED to be operated from the pull-up resistor, but as the 4053 chip is wired to select the effect when the input is pulled low, that wouldn't really work as the LED would be lit in bypass and off with effect engaged. [edit - see below]
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NickS
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Joined: 14 Jan 2008
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Location: Nr. Basinggrad, UK

PostPosted: Tue Jun 18, 2019 12:55 pm    Post subject: Reply with quote

OK, teh Intawebz tell me that the LED is only a Power indicator. You could use it as an effect indicator by wiring the LED wiring between 15v and the side of the foot-switch that goes to the switch IC (there should be a series resistor in the LED wiring) so that it's only grounded when the foot-switch is pressed, but that would be unauthentic.
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NickS
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Joined: 14 Jan 2008
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Location: Nr. Basinggrad, UK

PostPosted: Wed Jun 19, 2019 12:44 pm    Post subject: Reply with quote

K tells me that on his Polychorus the LED is marked "Status". OK, so given that the input to the switch IC (CD4053) is considered "Low" when <4V with supply volts of 15V, and the voltage across the LED should be around 2v-3v, the existing wiring* should work to both light the LED and pull the input low, different to other versions of Polychorus pedals though it is.

The PCB is EH7700G for anyone tracking these changes.. on most pics the detail is hidden under an inspection sticker Sad

*Pin 10 of CD4053 -> Foot-switch tag 1
Foot-switch tag 2 -> LED anode
LED cathode -> 0v
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