Home › Forums › Vintage EHX › Vintage polyphase calibration / alignment

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  • September 5, 2017 at 2:54 pm #84988
    Jimnirvana
    Member

    Hiya guys! First of all apologies for my first post being a cry for help but until now I’ve managed to find all the info I’ve ever needed by using the search function, this time however (unless I’m being blind) I can’t find any details at all regarding the above subject. Secondly thank you EH man, Scruffle and fenderEHX as over the years of me lurking on here you have been a huge help to me and no doubt thousands of others. I cringe everytime I see the countless threads asking how much their old EHX boxes are worth in your honour haha!
    But yes to the point of my topic, I recently bought an old beat up polyphase from ebay (’78 pots first board revision as far as I can tell) and although it sounds good I think something may be a miss and it could sound better. The feedback pot has a non original resistor soldered across lugs 1+3 and when removed the thing goes into crazy self oscillation at about 1/3 of the way up. I’ve not had the whole board out as of yet but looking at fresh solder joints it’s been recapped at some point but I’m thinking this was done to try and solve the problem?
    So my next point of call was going to see how to calibrate the thing but as stated earlier I can’t find any info? I know these pots aren’t for user fiddling but I’m assuming something is off and would just like to know if there’s a procedure for alignment and or points to check for certain voltages?
    I have a fair understanding of electronics but I’m by no means an expert. I have a scope and a signal generator and roughly know my way around them and have modest soldering skills so I’m hoping I can get this sounding as incredible as it was designed to be!
    Thanks for any help that you guys may bring and I will try and get a photo of my mini collection of vintage EHX uploaded asap :) happy Tuesday one and all!

    September 5, 2017 at 2:57 pm #123117
    Jimnirvana
    Member

    Edit: Also forgot to mention that with that resistor in place things get quite distorted when feedback is turned up. Using a Jazzmaster too so not really that hot of an input signal

    September 5, 2017 at 5:49 pm #123118
    Scruffie
    Member

    The resistor across the feedback pot may well be original, sometimes they were added if EHX were short of a pot value as it alters the value via parallel resistance.

    As for biasing, there’s only a single trimmer on the board and the schematic gives the bias point of 6.7V, a good place to test would be on pin 3 of the 324 opamp.

    September 6, 2017 at 9:26 am #123121
    Jimnirvana
    Member
    Quote:
    The resistor across the feedback pot may well be original, sometimes they were added if EHX were short of a pot value as it alters the value via parallel resistance.

    As for biasing, there’s only a single trimmer on the board and the schematic gives the bias point of 6.7V, a good place to test would be on pin 3 of the 324 opamp.

    Hey Scruffie!

    Thanks for the speedy reply, I was working til late on last night and I’ve only just seen this.
    I was thinking the same about the resistor but it’s a carbon comp and I’m pretty sure (from memory as I’m at work atm) the pot is a 100k (CTS with a white shaft) which the schematic calls for?
    And thanks so much for that biasing info, I’ve only had the thing a few days and not really ventured down the rabbit hole with regards to the schematic and I’ve not had the board out of the box as of yet. I assumed there would of been a few to tweak on the populated side as per the poly chorus but with your advice/looking at the schematic I see there’s only the one which is trace side on mine.
    As soon as I get back home this evening I’ll confirm the stuff I’ve recalled from memory and I’ll make sure I’m getting 6.7v on pin three of that opamp. Is this – https://sites.google.com/site/electroconducive/EHXPolyphase-Factorylessclear.jpgschematic all accurate? I know sometimes things get changed during production and schematics don’t get updated. Only reason I’m asking is I’m also going to check all the caps that have been replaced are the right value too.

    Thanks again for the reply Scruffie, your help is greatly appreciated :)

    September 6, 2017 at 12:20 pm #123123
    Jimnirvana
    Member

    Also would you say the distortion in the higher feedback levels is normal?

    September 6, 2017 at 4:51 pm #123126
    Jimnirvana
    Member

    Sorry to keep tagging things on here too but I also forgot to mention that the rate pot kind of gets to point which it goes super fast at about 3 o’clock then beyond that doesn’t phase at all? Don’t know if that helps with diagnosis?
    Just got home too and the pot is 100k and resistor in there is a 470k which actually measures at 570k!?
    The bias was a little hot at 7.6v so dialed the trim back to get the desired 6.7v. Seems to of helped with the distortion side of things but with that resistor removed it still squeals like hell beyond 2 o’clock ish?

    September 6, 2017 at 8:42 pm #123127
    Scruffie
    Member

    As far as I know, none of the production units quite match up to any of the schematics (there’s about 4 schematic variations, all subtly different) it’s close enough for jazz but I wouldn’t take values from it.

    Squealing with the feedback up is self oscillation and the reason a lot of effects with a feedback control have a trimmer inside to limit it before it reaches that point, that resistor was obviously added to limit the feedback to stop it oscillating so I still think it’s probably original.

    Yeah the rate control goes in to ring mod territory and at a certain point the vactrols are too slow to keep up, with value drift over the years your rate range might not be optimal but it sounds like it’s working as it should.

    All these vintage EHX effects were designed with the pickups of the day in mind which were mostly low output single coils, there’s a fair amount of gain at the front so with the addition of lots of feedback some distortion doesn’t sound like anything is probably wrong, but I couldn’t say with certainty without having it in person.

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