In this post I elaborate a bit more on the issue itself, and also give you some tips on how to figure out where is the defective potentiometer (pot) that is causing the issue. It could be a linear (slider) pot or a rotary (knob) pot. In my case, it was a knob pot (the AMP knob).
The ghost editing issue is a well known and common issue on the Roland JP8000 synths. Apparently, Roland tried to save up on components and didn’t use high quality pots in this particular synth, which results in some of them failing after almost 20 years.
The linear pots (the sliders) are easier to clean and lubricate, as they have a slot where the service can be done. Ideally, if the condition of the pot is way to bad, you should desolder it, disassemble and clean each component individually. But most of the time, a good cleaning with compressed air, isopropyl alcohol (99.9%) and a good fader lube (like Deoxit) does the job.
The rotary pots (knobs) are a bit trickier. Problem is that there is no good access to them. They are tightly closed and no cleaning can be done on them. I tried spraying some Deoxit on them, and it does get it. You can tell by the way they rotate after Deoxit has been sprayed. But in my case, the defective knob did not improve its condition after the “cleaning”. The video below shows my test after the cleaning. The issue remained.
Tip: To figure out which pot is generating random values at will, press and hold the SHIFT button. If any value is changed on ANY pot, you will see that in the LCD display. This is also an easy way to test some pots and see if the values they generate upon activity seem correct. The video shows this technique.
So, I had to desolder the pot and replace it. Luckily, finding replacement rotary knobs for the JP8000 is not that hard – and they are relatively cheap too. So, I went to eBay and bought 12 of those, 6 without center indent and 6 with it. Spent a total of 20 quid on them. Not too bad. They are not exactly the same, but they fit perfectly. The picture below shows on the left the original (defective) pot, and on the right, the replacement one. Case anyone is interested, they were bought from the user “synth-shack” at eBay.
So, pot replaced and new test done. Result: 100% OK this time 🙂 I also took the opportunity I was going to fully disassemble everything once again to replace the LCD display with a OLED one. Found a guy selling this at eBay for 50 quid and took a chance. The result can be seen in the below video.
So, there you have it…! A very nice vintage synth, with a lot of history to it, fully repaired and cleaned, and added to my collection. I am very pleased with the results!