I just started my first restoration project, and found two issues that have me stumped. Osc 2 on the Prodigy I am working on only outputs a low volume white noise when sync is turned on and the osc 2 waveform is set to the square wave setting. The square wave on osc 2 functions normally when sync is turned off.
Also, the pitch bend wheel freaks out and bends the note all over the place when I push it to the up position, and barely does anything when pushed down.
Any ideas?
What is the serial number ?
You are aware that there are two schematics version, depending on serial number above, or below 4160 ?
edit: first hand, I’d say it could be a defective or intermittent “sync” switch. It is sending pitch bend control voltage to both oscillators in the “sync off” position, and only to OSC 2 in the “sync on” position.
Thanks for the reply, thealien666!
Yeah, it’s the CV version and the serial looks like 10297. With sync on, the volume of Osc 1 set to zero, the volume of Osc 2 set to max, the wave shape of Osc 2 set to square, and pitch bend set to the middle position I get only the soft white noise. If I move the pitch bend up I actually can get it to make noise. Also, it seems like the two other wave shapes on Osc 2 in sync mode seem to be the same shape.
Here’s a video that demos the issues: http://vimeo.com/41466155
Thanks for the video, it helps a lot in understanding the situation.
When synching an oscillator to another one, you always want the oscillator that is being synched, OSC 2 in the case of the Prodigy, to be higher in pitch than the oscillator that is resetting it (OSC 1). For the simple reason that if the synchronization impulses, coming from OSC 1, are faster than the frequency of the synched oscillator, they reset it before it can even start a cycle (especially with a square waveform that starts abruptly) ! And that could possibly be what is happening here.
Although I’ve heard something else that doesn’t seem to be right in the video also, let’s just go at it one step at a time for now…
So let me ask you this: Does OSC 2 produce a sound with sync “on” (with square wave) when OSC 1 octave is set to 32’ instead of 16’ and OSC 2 is set to 8’ instead of 16’ ?
Thanks! I will check in on that as soon as I get back home tonight and let you know.
No it still doesn’t make a sound using those settings. There are two weird things going on.
With sync on Osc 2 doesn’t produce any sounds when set to square wave. It does produces sounds when set to saw and triangle, though the wave shape sounds the same when moving between triangle and saw with sync on.
The pitch wheel freaks out when pushed up. It’s not a smooth bend and it jumps up and down randomly about 2 octaves as I move the dial up. I have used deoxit on the pitch wheel pot to try to fix it and it has had no effect.
I can make another video if that would help. Thanks for being patient!
You would need your two hands but, can you show me what the pitch bend wheel does with sync on and osc 1 at 32’, and osc 2 at 8’ and square wave ?
I can’t seem to reproduce some of the problems I was having earlier using the same settings that were causing the problems before. Here’s a video showing some of the pitch bend problems using the settings you requested. Thanks again for the help!
Hello again Todd. Glad to see that your posts appear right away now.
Okay. I’ve had a look at the new video (thanks for that). There’s a positive thing coming out of it: you got sound out of OSC 2 even with the sync “on” now (with OSC 1 octave lower than OSC 2). But the pitch bend wheel definitely doesn’t behave correctly.
If you switch sync to “off” and move it, does it bend the pitch of OSC 2 up and down normally ?
EDIT: also have you tried moving the “interval” knob of OSC 2 with the sync “on” ? Does it produce the same behavior as with the pitch bend wheel ? (I know I’m asking a lot of questions, but I’m trying to narrow down the problematic area in the schematics)
With sync off the pitch wheel behaves the same as with sync on. I think my last problem with the square wave was somehow related because there seem to be dead spots that appear in random spots in the range of the pitch bend wheel. Would replacing the pitch bend pot fix this?
The interval knob behaves more like the frequency cutoff filter with sync on. I’ll try to post a video of this before I leave for work.
If the sync circuit is functionning correctly, the interval knob should produce the typical sync sweep sound.
It looks more and more like the pitch bend potentiometer might be defective. But I’ll confirm that after I see/hear what the interval knob does with sync on, in your next video.
Here’s the video:
https://vimeo.com/41706627
Good news: the sync circuit is working properly. What you hear in that video is the type of sound you typically get with sync.
The pitch bend wheel, when the sync switch is “on” should do the same as the interval knob.
The potentiometer is probably damaged, since you wrote that you’ve already tried to clean it but it didn’t fix the issue.
First, check that the wires from the main circuit board going to the pitch bend potentiometer are not broken, or loose, or have cracked solder joints. If not, then replace the pot with another new one.
Value is 10K ohms (most probably linear type, although the service manual doesn’t list resistor number 180 in the parts list, since the mod wheel came as a pre-assembled unit).