PAiA Proteus I - distorted LFO

I built one of the last few PAiA Proteus I keyboards ages ago and was disappointed in one of the oscillators - I believe it was the LFO.

I could not tell much difference between the square wave output and the triangular wave. On my oscilloscope, I discovered that regardless which waveform I selected both were present. A triangular waveform showed many spikes leading to the rails to form a square, and vice versa. While one waveform was prominent, it would look like the area between was shaded in.

At the time, I concluded that it was a defect in the design of one of the ICs used. I’d noticed the same behaviour with the XR2206 function generator and somehow got it in my mind that that is what the Proteus used for the LFO. Looking at the schematics now, I’m suspecting something is wrong with C33 and that first op-amp.

Has anyone encountered something like this?

I repaired one when I was in high school; a very long time ago. If I remember, it was the analog switches, either 4051 or 4066, and when I replaced those the LFO and other modulation came to life. Lucky you! I wish I had that old Proteus.

Oh my gosh… Thank you!

I focused on the op-amps and VCA, and never would have thought about the switches that came after it. It makes sense, because it sees both waveforms. I’m not sure if I even looked at the modulation.

It’s been at the bottom of the closet for all of these years - forgotten. The weird LFO made it kind of useless. I’m going to bring it out and see if I can’t get it going again.

Have you heard of Tethys? Someone actually made a stand-alone emulator for it on the PC side and made a few enhancements. Not quite the same, but it will give you some of that distinctive sound.

I’m working to restore my Proteus! I pulled it out, and it looks very good - almost like new. Keys are nice and white - no yellowing.

It just goes to illustrate how many problems you can have, though…

I found my ribbon cable floating around inside. Once put together, the keyboard didn’t work - it sounded like one key being held down. In the process of narrowing it down, the AC connector broke off of the back panel. I finally got that fixed, and realized that the ribbon cable was in backwards on one end - the manual was not clear on this.

Now dealing with a problem with the knobs: none of them make a difference on the output - although it did work before I removed the boards from the case to fix the connector issue. I’ve finally decided the digitization is okay, and that there is an issue with the data getting to the voice card’s D/A circuitry.

One LED would not light. When I went to replace it, I found that it is loose due to a cold solder joint. A cold solder joint could also be behind the issue with the D/A issue.

Preset advance is flaky: it skips numbers, and the number displayed bobbles at times. Could be an intermittent connection between the display board and the control board. Broken wire? Or another bad connection?