Juno 106 tact switches

I've just spent the last hour replacing all of the tact switches on the Juno 106 panel board. I also took the opportunity to remove the remains of the foam dust guards that had become so brittle that they were themselves the major source of dust, and give the front panel a good clean - much easier without the sliders getting in the way.

The remains of the dust guards. They literally fell apart when I touched them.

Dust guards removed, board cleaned up and new tact switches installed.

All of the sliders were still functioning perfectly, so I left them alone. I'm currently waiting for the switch caps to dry after giving them a wash, and will reassemble the synth this evening.

MKS-50 finally repaired

Where did I leave off? Having replaced the BBD drivers and the 4051 that de-multiplexes the control voltages for chorus rate and volume, the noise remained. Replacing the underrated and likely half dead bypass capacitors improved the noise that the BBDs were injecting into the rails, but still didn't solve the problem.

I was on the verge of putting the MKS-50 back on the broken pile when I nudged something near the 4051 and the noise stopped! Initially I expected to find a broken trace or cold solder joint. When I first acquired it, the synth did not respond to MIDI messages due to a number of broken traces. Thanks to all but one of the mounting screws being missing, the PCB had flexed and cracked. The underside is a mess of bodge wires as a result. I had also found and reworked a few suspect solder joints.

Poking around with a meter revealed nothing, but a closer look at the component side found this.

Look closely at the capacitor array. Something isn't quite right.

The corner isn't supposed to be chauffeured and the common pin is missing!

Could this be the cause of the noise problem? The capacitor array is used to sample and hold the control voltages coming out of the 4051. These control the overall volume VCA as well as the VCAs and VCFs for each voice and the chorus rate. Without the common ground pin I'm amazed there weren't more problems. Rather than providing sample/hold the damaged array would capacitively couple all of these control voltages, meaning the chorus CV was being modulated by and indeed modulating filter cutoff, etc.

As the synth is already quite a mess (cracked PCB, bodge wires, missing case lid...) I made a repair horrible hack.

Awful hack, but it works!

As a point of interest, the construction of the MKS-50 is awful. A single layer board with a lot of jumpers, ICs seated at odd angles and some very poor soldering. The case design also makes it difficult to remove the PCB without flexing it. Roland gear tends to be fairly well made, so I'm not sure what happened here.

The MKS-50 lives and sounds great! Next up will be another horrible hack involving a sheet of aluminium and some extrusions to fashion a top for the case.

MKS-50 and Juno 106 update

Just a quick post. Having now restored the Juno 106 to working order, I've noticed several of the front panel tactile switches are rather intermittent. Unlike those on the MKS-30, these are of a standard and widely available design, so I'll replace them all.

The MKS-50 continues to annoy. Swapping the MN3101s had no effect and neither did changing the 4051 or the TL072 that feed them. Further probing reveals the power supply rails in the chorus area are significantly noisier than the rest of the synth. A close look at the board reveals that most of the bypass caps on the + and - 15V rails have the rubber bungs in their bases displaced. They are only rated for 16V, which really can't be a good idea on a 15V rail. In fact the whole construction of the synth is pretty poor. Roland really seem to have been penny pinching by 1987.