Prophet-600 - trying to get it all fixed up

[commodorejohn]

So I finally nabbed a P600, which I've been wanting for a while, but it's a bit of a fixer-upper. All the oscillators and filters work fine and the sound quality is good, so most of the major concerns with a vintage analog poly aren't a problem, but there's a lot of minor stuff: finicky slide switches, pots with weird response across their rotational range, a non-functional row in the switch matrix for the membrane panel, etc.

For starters, what I'm trying to do at the moment is run the scaling routine, since there's some definite issues with uneven scaling between oscillators. The service manual lays out the procedure for this, and it's quite simple, although I'm going to have to deal with the fact that some previous service tech helpfully capped the trimmers with some kind of sealant, apparently on the theory that it was physical jostling of these that causes drift :/ The bigger problem is that I can't seem to get it to boot into the scaling mode - when I jumper the two posts together and turn on the power, it just doesn't come up at all. Am I missing a step? Do I have a wrong ROM revision?

[Algorithm]

Funny timing! like a day or 2 before you got your p600 I got 1 too! haha
(still have not had the time to look at it, have like 4 or 5 different machines laying around in pieces, so Im trying to finish what I already started)

Man, that is a shame about the trimpot... My suggestion would be find the part # for the trimpot in question, and get a replacement. Might try to find out what sort of switches the membrane panel has under it, might as well replace those while you are at it.

I am unfamiliar with the scaling mode though, so I cant say what is up with that.

[commodorejohn]

The membrane panel is a custom board, unfortunately - a couple places apparently sold replacements at one point, but it doesn't seem like they do anymore. The good news is that, with a whole row of the matrix out of commission, the smart money's on it being that row-select line having a bad connection - I'm just not sure if it's the worn nature of the "fingers" on the ends of the ribbon cable, or if there's a break somewhere further in. Need to get some conductive paint and try touching up the connector and see if that helps.

The trimpots I think should be okay - the goop isn't thick, it's basically some kind of light foam sealant, and should be easy to break with a screwdriver. I just need to get it to boot into scaling mode before I know what direction to turn 'em.

[synthroom]

finicky slide switches, pots with weird response across their rotational range

Try some DeOxit on those.

I went through all the pots and slide switches on my Pro One last year and it feels brand new now.

[commodorejohn]

Yeah, I tried it with rubbing alcohol already, but I'll give DeOxit a try before I go ordering out for replacements.

[schmidtc]

The trimmer sealant is probably from the factory. pulling out the ribbon from the board connection cleaning with iso and reinserting it may fix the membrane if you're lucky. the pot rotational range could be the DAC gain relative to the 5V supply, there's a trim for it.

I've got a friends P600 here that's out of commish as well. The duty cycle into the enable pin of the multiplexer for the left board is only a few percent and so only channel 7 (arpeg speed) multiplexes correctly. All the digital lines check out so I'm grasping at straws with that one.

[lasynthco]

Hey, wanted to let you know that I have remanufactured the Prophet 600 membrane panel and it is for sale on our site. We have a few left in stock and our second order is on its way!

Boomer
lasynthco.com

[SeventhStar]

Hey, wanted to let you know that I have remanufactured the Prophet 600 membrane panel and it is for sale on our site. We have a few left in stock and our second order is on its way!

Boomer
lasynthco.com

Show some details about this new membrane panel you've made, instead of just a single photo.. I saw your ebay listing for one of these before you posted about it on here. If somebody's going to buy it, they want to know what they are getting for their money. You can start by explaining how yours is going to last longer and perform better than the original did.

[commodorejohn]

Well, I finally got the thing to boot into the scaling-adjustment mode and ran through the procedure outlined in the service manual, which has made a marked improvement in the consistency of the voices. However, according to the service manual my particular model of Prophet-600 doesn't have a trimmer for the DAC gain - however, the pots seem to be responding in a much more reliable fashion now, so it's not such a big deal. My biggest remaining question is, what exactly is the range of the filter on this thing? The main reasons I thought the pots were responding weird were the unusual range of the cutoff and resonance knobs - the cutoff even at maximum stops about an octave short of the filter being fully open, and the resonance starts verging on self-oscillation at about the halfway mark! Is this normal?

I should probably load the factory presets and see if they sound weird...

[commodorejohn]

A word of warning: be careful with "conductive paint" products! I bought a tube of silver-based conductive paint off eBay thinking I could repair the membrane-panel connector and get it working again for a few bucks - turned out the s**t corroded the rest of the contacts right off the goddamn plastic! Now I'm gonna have to shell out for a new one...f**k.

[silikon]

I'd suggest giving this guy a chance to get your P600 working correctly, with minimum cost -- if you're working with unknown problems, it might save you money down the road - as well as headache.

http://analogsynthservice.com/home

[commodorejohn]

Actually, the only problem at this point is the membrane panel - and that was actually easily diagnosable (one of the row-select lines for the matrix was out.) Unfortunately what should have been a simple fix (apply something to improve the contact and solve the issue) turned out to be a cautionary tale about buying chemical solutions on eBay.