Hello synth nerds! Two voices (3 and 6) my JX-3p have a low pitched buzzing to them when I first power it on, and it takes about 10 to 20 minutes for it to fade away. I’m trying to refurbish this unit and recently replaced a few NAND chips on three channels (3,4,5) to get them working, but I assume this is not the cause. I’m wondering if anyone can give me some advice on where to search next. Could this sort of issue be caused by old capacitors or resistors warming up on channels 3 and 6? Or perhaps the oscillator chips? I’ve never heard of DCO synths needing time to settle.
Upon further investigation I think I’ve changed my diagnosis. What actually appears to be happening is that each voice has a low pitched drone when using the OSC 1 saw wave. The volume of this drone differs depending on the voice. When I first turn the Synth on, voice one and two are the worst, but once I play voice four it becomes the worst and voice 1 and 2 become normal. It’s seems like something is going wrong with the master clock? I imagine I’m going to have to get my hands on an oscilloscope to work this out, but if anybody could tell me if it’s worth checking capacitors that would be useful.
Hey, I have done a thorough investigation with an oscilloscope and I believe it could be the 6x 733 transistors (I think TR07 or TR08, the service manual mentions them in the circuit description section) near the bottom of the voice channels on the main board. These generate the osc 1 saw waves and they are all fed from the same pulse signal, and the distorted saw wave is present on the output pin. The only thing that confuses me is why only osc 1 voices are playing up and not osc 2 I have the spare 733’s but haven’t changed them over yet because I got distracted. .
I would love a tech to comment to see if I am on the right track here!
The odds all 6 transistors are bad is low. I would investigate common elements, especially the usual suspects, caps. Electrolytics for sure and other types as well. I’ve seen polystyrene caps go bad and they supposedly rarely fail. Good luck!
Totally agree. It would be very strange if it was just the 6 transistors on osc 1. There are EL caps connected to those transistors which could be the issue, I’ve tested those however. It is very odd that osc 2 are all normal as well. Logically you’d think it was something earlier in the signal chain, but what is particular confusing is that the six voices are all fed from the exact same signal, so it doesn’t seem possible that there could be variations pre-voice channel, It has really stumped me and that is why I have given up temporarily.