Prophet 600 - No Response from controls

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Prophet 600 - No Response from controls

Postby Rik Bitter » Wed Aug 22, 2018 9:05 pm

My apologies if it's bad form for a newbie to post a question like this immediately after registering.

I'm very new to the analog synthesizer world, in fact this is my first synth of any kind. My hobby is antique electronics like old tube radios, amps and HiFis. Recently, i went go a guys house to look over a pair of old tube amps that he had for sale. While I was there I noticed a synthesizer propped up against some miscellaneous speaker cabs. Now I've always been curious about analog synths but never took the plunge because A) I don't know how to play a keyboard and 2) I find integrated circuits to be vexatious. Anyhow, I asked the guy about it but he didn't have a lot of info on it, it clearly wasn't his thing either--he didn't know if it even worked properly. After a little haggling I went home with my two (new to me) Traynor heads and a Sequential Circuits Prophet 600

Now, if you're still reading...

I've messed around with this synth for a few hours and I'm having a problem(s) with it. It could be something as simple as user error given my inexperience so I'll do my best to describe what's going on. The synth will power up and produce sound. All of the keys appear to be working properly. I can change patches using the touch pad and that will alter the sounds it produces. However, none of the multitude of controls on the front seem to effect the sound in any way. Not the wheels to the left, not the oscillator knobs and switches, none of the envelope controls, not even the volume knob.

I opened the synth up and looked for any obvious signs of failure like smoked resistors/transistors, cold solder joints, sketchy connections but nothing jumped out at me. Since this thing was pretty filthy when I got it, I decided to clean all the connections between the boards and ribbon cables. Unsurprisingly, that didn't solve my problem.

I'm pretty solid at repairing tube equipment, but this thing is far more complex than anything I've every delved into before. If anyone has any help/insight/condescending mockery that might point me in the right direction, I'd really appreciate it. Thanks.
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Re: Prophet 600 - No Response from controls

Postby Rasputin » Thu Aug 23, 2018 12:37 am

There are a couple revisions of the P600, and this is a bit of a shot-in-the-dark, but:

I think there's a 4174 logic chip on the panel board, or the largest connector going between the panel board and the mainboard which may lead to that symptom.

I'm not P600 expert, by any means, so I could be totally wrong. But I know some other people on here know the Prophet 600 fairly well, so I'm pretty sure you can count on some help and get this sorted.

Maybe check pin 16 of the 4174 for 5VDC first?
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Re: Prophet 600 - No Response from controls

Postby Rik Bitter » Sun Aug 26, 2018 9:37 pm

I'm not sure what revision I have. Hopefully from the information below it will be apparent which one I'm dealing with.

Board 1 Rev B
Board 2 Rev 0
Board 3 Rev F
Main Board is dated 5-18-84 (I think, the year is a little sloppy)

Just for good measure I cleaned all the main connector between Board 1 and Board 3. Also checked for cold joints in those connectors with a magnifying glass. No change.

I didn't see a 4174 chip, but I did locate a MC14174B chip (located between the oscilator A & B Frequency controls)
Voltage on 16 was 5.05 VDC.

Thanks for your response.
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Re: Prophet 600 - No Response from controls

Postby Rik Bitter » Sat Sep 01, 2018 2:50 pm

Hmmm... I was hoping this was a common problem.

Since none of the controls are responding I figure there's some common point that the signal runs through that might be the issue or possible the control boards simply have a power problem. I can't seem to locate a complete set of schematics for the 600. Does anyone have a PDF or know where I can get one?

Thanks.
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Re: Prophet 600 - No Response from controls

Postby Rik Bitter » Wed Sep 19, 2018 2:17 pm

I still haven't had any luck finding a schematic. Is there a site that sells this sort of information?

I was hoping this would be a cool project to learn a little about analog synths, but if I can't locate some basic documentation for this thing, I guess I'll have to unload it.
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Re: Prophet 600 - No Response from controls

Postby Rasputin » Wed Sep 19, 2018 6:02 pm

Rik Bitter wrote:I still haven't had any luck finding a schematic.


I don't know how... they're all over the place:

https://elektrotanya.com/sequential-cir ... nload.html
https://archive.org/details/synthmanual ... schematics

Even in a previous thread on this very forum:

viewtopic.php?f=5&t=96348
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