Roland VK-1 Organ Drawbar hum and keys continuously playing

[couch]

I picked up this roland organ pretty cheap knowing that it was making a humming sound, thinking it could be an easy fix like recapping. i figured that each of the drawbars produce a hum. most starting humming over time after being powered on, but two always hum right away. the other problem is the G#4 A4 A#4 and B4 will keep producing a tone after being played, and this doesnt happen right away only after being powered on for a while. ive replaced some electrolytic caps on the switch gate and tone generator, but i still have these problems. should i replace all the caps? does it sound like the preamp board could be the problem? ive been looking through the service notes and was trying to determine what might be letting the hum/sound leak through and what determines on/off for the keys. i appreciate any feedback because this is the first major repair project involving soldering

[Hugh Robjohns]

Random swapping of capacitors isn't likely to help. Ideally you will need an oscilloscope to work through the circuitry and find the faults.

But at a guess, I'd start by checking the PSU voltages for the source of hum, and the relevant gate circuits for the sticking notes (which might also reveal if it's a key contact issue).

For anyone interested, the service manual is here: https://www.synthxl.com/wp-content/uploads/2018/04/Roland-Vk-service-notes.pdf

[James Perrett]

You are likely to cause more problems than you solve by wholesale recapping - especially as you don't have much soldering experience so you could easily be damaging tracks as you remove and replace components. There are very few circumstances where wholesale recapping is appropriate (mainly gear used at high temperatures and gear with known bad batches of capacitors) although it seems to be fashionable nowadays with certain technicians.

The first thing you need to do is to check all the supply voltages and connectors. Humming could well be down to a poor ground connection somewhere caused by a dirty connector.

Edit to add: As Hugh says, an oscilloscope can be really handy and you can buy a basic USB oscilloscope that uses your PC as its display relatively cheaply.

[couch]

i should add that there was visible leakage from the caps (C2 and C3 on the gate switch board) so i replaced all those caps first. also when i have all the drawbars in the hum is gone. thank you guys for the feedback ill go through and test all the connections with my digi multimeter, starting with the switch gate board. also all the soldering looks like it hasnt been touched since it was at the factory. what should i know when looking at 40 year old electronics?

[James Perrett]

also all the soldering looks like it hasnt been touched since it was at the factory.

That's good - properly made solder joints only break if they come under mechanical stress, thermal cycling or chemical attack. Old Japanese gear tends to be pretty reliable so I'd be looking at switches and connectors first before suspecting anything else. Do you have any switch cleaner? A can of Servisol Super 10 is always handy.

[couch]

i dont have any switch cleaner, but i can go pick some up! thanks for the recommendation! just to be clear, are the connectors the cables that split and plug into the boards? and switches are black chips on the switch board? then do i check to make sure all the INs and OUTs of these chips can carry voltage to confirm they work? just wanna make sure im checking the right thing