Sequential Circuits Prophet 600 key firing problem

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Sequential Circuits Prophet 600 key firing problem

Postby helloitabot » Mon Aug 31, 2009 4:14 pm

I have become the owner of a Prophet 600 which has been in a box since 1987. It is having an issue with the key mechanisms properly hitting the keyboard contact strips. When I powered it on, only three of the highest far right keys worked so I opened it up and cleaned the contact strips then plugged the keyboard circuit board back into the synth without reattaching the actual keys in order to test it. The rubber keyboard contacts fire notes just fine when depressed with a finger but for some reason when attached to the actual keyboard mechanism, most but not all are failing to fire. The top couple and the bottom three play and a few in between but that's it. The plastic keys don't seem to be warped or anything, it just looks like when they push the plungers down that they aren't descending far enough down in order to complete the circuit. I suspect this has something to do with it because I am able to make more of the keys play if I push down on the end of keys nearest circuit board, in other words the end of the key opposite to where you would normally hit it. I'm thinking maybe I can attach little pieces of cardboard to the bottom of the keys where they touch they silicone contacts so that they push down a little more, but I don't want to screw this instrument up...Anyone seen this issue before or know a better place/person/forum to ask? Any ideas?
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Re: Sequential Circuits Prophet 600 key firing problem

Postby schmidtc » Mon Aug 31, 2009 5:42 pm

Welcome to the VSE forum.

Did you verify that the rubber contacts all are sitting properly in the openings for the key mallets? Once I put a Prophet 600 keys back together I had a similar problem. It was just that some of the rubber contacts where smushed up against the sides of the openings and I just had to poke some of them through.
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Re: Sequential Circuits Prophet 600 key firing problem

Postby helloitabot » Mon Aug 31, 2009 6:01 pm

From where I can see them they don't seem to be caught on the sides of the openings. It just seems like they aren't being pushed down far enough to complete the circuit. But I could be wrong...
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Re: Sequential Circuits Prophet 600 key firing problem

Postby HideawayStudio » Mon Aug 31, 2009 8:22 pm

helloitabot wrote:From where I can see them they don't seem to be caught on the sides of the openings. It just seems like they aren't being pushed down far enough to complete the circuit. But I could be wrong...


I believe the Prophet 600 was fitted with either a Pratt-Reed J-wire or carbon pellet silicon membrane keyboard mechanism (very similar to the Juno-106) depending on when it was built. The membrane type keyboards were fitted to the later 600s. Unfortunately both forms of keyboard are not without their issues.

If yours is a later one see this video for contact cleaning:

http://www.vimeo.com/5565712

If it is an early one with a pratt-reed mechanism - like the ARP Omni-2 - then there are a number of failure modes but, since so many of your keys dont work I'm wondering if either the bus bar or the contact wires themselves have become heavily tarnished or if something is wrong eg. cabling.

Can you post a closeup of the offending article??

Promise not to botch your 600 - it deserves better than that :)
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Re: Sequential Circuits Prophet 600 key firing problem

Postby helloitabot » Mon Aug 31, 2009 8:42 pm

Mine has the carbon pellet silicon membrane. Looks exactly like in that video you posted. As I said I took off the membranes and cleaned the contacts just as in the video, put them back on and all the notes/keys played just fine when I reconnected the circuit board to the rest of the synth. It's just when I screw in the keyboard circuit board to the actual keyboard the notes don't fire properly, so It feels like the problem is mechanical.
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Re: Sequential Circuits Prophet 600 key firing problem

Postby HideawayStudio » Mon Aug 31, 2009 9:13 pm

helloitabot wrote:Mine has the carbon pellet silicon membrane. Looks exactly like in that video you posted. As I said I took off the membranes and cleaned the contacts just as in the video, put them back on and all the notes/keys played just fine when I reconnected the circuit board to the rest of the synth. It's just when I screw in the keyboard circuit board to the actual keyboard the notes don't fire properly, so It feels like the problem is mechanical.


Ahh got you....

It's possible there is a hairline crack in one of the tracks on your pcb which when screwed down is causing it go open circuit. Either this or a bad dry joint is opening up under pressure around one of your connector pins. Another possibility is that one of your screws is shorting something close to the screw hole eg. an adjacent track.

Are you sure all insulating material/nylon screws - are in the correct place? Is anything bent or distorted? Are any ribbons or cables trapped, split or broken internally?

I doubt it's the key to membrane orientation as this kind of mechanism is pretty forgiving to alignment issues as the membranes are located on the pcbs by rubber spigots. The key would have to be miles off the membrane to prevent operation.
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Re: Sequential Circuits Prophet 600 key firing problem

Postby helloitabot » Mon Aug 31, 2009 9:40 pm

I'll have to take a closer look at it when I get home in a couple hours. Another thing that makes me think it's mechanical is once I reconnected and screwed everything back together, I could get most of the notes to play if I pushed the keys straight down by applying pressure to the back part of the keys (closer to where the spring mechanisms are and the farthest part of the key from the person playing) instead of letting the key pivot from the end as it would if you played the key normally. Does that make sense?
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Re: Sequential Circuits Prophet 600 key firing problem

Postby helloitabot » Tue Sep 01, 2009 4:17 pm

I think the issue is just the gold contacts aren't conducting very well. There appears to be nothing wrong with the mechanism. The contacts are are shiny but they could be shinnier. I have to admit I did a crappy job cleaning them so I went and bought some compressed air and cleaned them again meticulously. Nearly all of they keys play now but they are still really spotty and cut in and out. Not sure what to do at this point. Thinking about trying this stuff:

pro gold contact cleaner
http://shop2.frys.com/product/2931175
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Re: Sequential Circuits Prophet 600 key firing problem

Postby schmidtc » Wed Sep 02, 2009 5:31 pm

Before you do that, try 99% iso-alcohol and a q-tip on the gold plated contacts and black round disc that completes the circuit when a key is pressed. This has worked for me on many synth tact switches and contacts. That pro-gold stuff should be a last resort. Check this guide, it even gives pictures for the prophet 600:

http://www.oldcrows.net/~oldcrow/synth/ ... clean.html
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Re: Sequential Circuits Prophet 600 key firing problem

Postby helloitabot » Wed Sep 02, 2009 7:57 pm

It's tough to tell if the problem is with the gold contacts, or the rubber dots. I also found this for reconditioning the dots:

http://www.chemtronics.com/products/pro ... &m=2&id=32
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Re: Sequential Circuits Prophet 600 key firing problem

Postby schmidtc » Wed Sep 02, 2009 8:20 pm

Old carbon and gold contacts will BOTH develop oxidization layers that break conductivity. You can't even see this oxide layer, but you just want to lightly and quickly rub these spots with a q-tip dipped in isopropyl alcohol 99% before you try something more harsh like chemtronics or add a new conductive layer. Using a fresh q-tip for every surface is best and the q-tip should slightly blacken when you rub the carbon black disc, too black and you're rubbing too hard.
I've heard some people have used a pink pencil eraser for rubbing off really oxidized surfaces, but I've never needed to even go that far. When you're putting the rubber strips back on, make sure that all the rubber pins are seated properly when you poke them back through.

Almost surely no need to mess with any type contact cleaner, and that might cause more problems then it solves.
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Re: Sequential Circuits Prophet 600 key firing problem

Postby HideawayStudio » Wed Sep 02, 2009 8:49 pm

schmidtc wrote:Old carbon and gold contacts will BOTH develop oxidization layers that break conductivity. You can't even see this oxide layer, but you just want to lightly and quickly rub these spots with a q-tip dipped in isopropyl alcohol 99% before you try something more harsh like chemtronics or add a new conductive layer. Using a fresh q-tip for every surface is best and the q-tip should slightly blacken when you rub the carbon black disc, too black and you're rubbing too hard.
I've heard some people have used a pink pencil eraser for rubbing off really oxidized surfaces, but I've never needed to even go that far. When you're putting the rubber strips back on, make sure that all the rubber pins are seated properly when you poke them back through.

Almost surely no need to mess with any type contact cleaner, and that might cause more problems then it solves.


+1 for this advice - I would not recommend contact cleaner on carbon pellet keyboards. The contact resistance does not have to be that low to trigger the keying circuits and so only mild cleaning should be necessary.
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Re: Sequential Circuits Prophet 600 key firing problem

Postby rhino » Wed Sep 02, 2009 10:01 pm

off the wall, but sill relavant

had a well-worn Akai AX-80 that had the same symptoms: push a key - no sound. remove the key and press the rubber bulb and sound. determined that the fconductive foam pads in the key bulbs had been compressed over the years (or, maybe shrunk). put stick-on 'dot' lables on the bottom of the keys and that extra 0.005 inch made it work. didn't like that junky fix, so ended up replacing the felt strip on the after-touch rail with new, thinner felt to let the keys go down a c**t-hair further. Someone will get rich someday reproducing the rubber key contacts for vintage gear.
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Re: Sequential Circuits Prophet 600 key firing problem

Postby helloitabot » Wed Sep 02, 2009 10:28 pm

I cleaned all the gold contacts and rubber dots with 70% Isopropyl and used a different q-tip for each but there was no black residue at all. Will 99% make that big of a difference? I've removed and reinstalled the keyboard circuit board twice now and I don't want to have to open it up again unless I'm positive that the keys will work permanently, or at least for a few years. I've read that some people who tried just cleaning with the alcohol had the same problem a few months down the line and had to periodically open the thing up and clean the contacts again.

This guy has had replacement gold contacts fabricated that can be applied with some sort of space age adhesive and a very steady hand:
http://sounddoctorin.com/synthtec/gente ... bberswitch

He will sell you a "limited number" of them if you email him. I'm considering this...
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Re: Sequential Circuits Prophet 600 key firing problem

Postby helloitabot » Wed Sep 02, 2009 10:35 pm

rhino wrote: put stick-on 'dot' lables on the bottom of the keys and that extra 0.005 inch made it work. didn't like that junky fix, so ended up replacing the felt strip on the after-touch rail with new, thinner felt to let the keys go down a c**t-hair further. Someone will get rich someday reproducing the rubber key contacts for vintage gear.



Yeah over at the prophet5.org forums someone suggested sticking cut off cigarette filters under the keys. the part of the key that actually touches the top of the rubber plungers is a hollow square shaped protrusion that could be the perfect size to stuff a filter in there without requiring glue. Might be worth a shot and I can't see it causing any damage.
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