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Wanted: Schematics for 4780 Sequencer
http://www.paia.com/talk/viewtopic.php?f=8&t=610
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Author:  lucidsounds [ Tue Sep 16, 2014 1:39 pm ]
Post subject:  Wanted: Schematics for 4780 Sequencer

As subject, I'm currently restoring a 4700 system and the schematic for the sequencer is missing. Does anyone have a link to a copy please?

Thanks!

Author:  ModularMe [ Tue Sep 16, 2014 4:04 pm ]
Post subject:  Re: Wanted: Schematics for 4780 Sequencer

You will find it on this very site at:
http://www.paia.com/talk/viewtopic.php?f=6&t=296

Let us know how the restore goes!

Author:  lucidsounds [ Tue Sep 16, 2014 4:33 pm ]
Post subject:  Re: Wanted: Schematics for 4780 Sequencer

Ah...thank you for pointing me in the right direction.

The one here is interesting. It never steps past the 2nd stage, so I'm guessing a failed op-amp somewhere, so I'm currently changing them all out. Also, more curiously, Q1 transistor had been desoldered and was missing.

Any suggestions welcome and I'll post an update tomorrow.

Author:  PAiA-Scott [ Wed Sep 17, 2014 10:31 am ]
Post subject:  Re: Wanted: Schematics for 4780 Sequencer

It might be the LED or a failed connection for it. Three lockwashers work to space the LED board away from the panel so it doesn't flex or bow too much when it is tightened to the panel. The delicate little LEDs can break or break contact with the LED board when detaching/re-attaching the board. The LEDs are an integral part of the circuit.

I don't know where to find the MLS-750 LEDs, but here is a link to what looks like a functional equivalent on the Mouser website:

http://www.mouser.com/ProductDetail/Lum ... wr0xtW4%3d

At least it looks to be the same size, and red. Red LEDs generally have a 2v Vf and require a few to about 10 mA of current to light (generally, interchangeable).

Author:  lucidsounds [ Thu Sep 18, 2014 2:20 pm ]
Post subject:  Re: Wanted: Schematics for 4780 Sequencer

Thanks for the information.

The sequencer is now working perfectly apart from the LED on Stage 6. I'm guessing it's just the LED as the Stage itself works fine. Looks a lot of work to replace it, so I'm happy to live without that.

The fault in Stage 2 was one of the LM3900 op-amps, so I swapped all four out for good measure. The 2.2uF capacitors had gone bad which is why the clock speed and Glide were acting a little odd. I also added an RC network across the Step switch so that it doesn't 'jump' when stepping from one stage to another.

Seems a pretty cool little sequencer, especially with the variable pulse-width output. :)

Author:  PAiA-Scott [ Fri Sep 19, 2014 10:08 am ]
Post subject:  Re: Wanted: Schematics for 4780 Sequencer

Way to go!

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