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.

Repairs

Long time no post! I have been meaning to post something for a while, but I didn't realise that it had been quite that long.

So what have I been up to? I have built a MIDIbox Seq V4 and MIDIbox CV. Neither are completely finished, but they are usable. I have also acquired some more retro toys in varying states of (dis)repair....

Only one of these actually works, so lots to do. Oh and my Juno 106 is now suffering from the dreaded 80017A VCA/VCF plague. I have removed these and will be giving them the acetone bath treatment in due course.

Shruthi-1 Build

Another day, another build. The Shruthi-1 4 Pole Mission kit arrived from Mutable Instruments a few days ago and the remaining parts (resistors, capacitors, etc) turned up yesterday. Too inpatient to wait for the latter, I soldered anything I could as soon as I had the kit.

By the time the remaining parts arrived, it looked like this:

Shruthi-1 PCBs partially populated

A couple of hours later it looked like this:

Analogue board

Digital board (top)

Digital board (bottom)

A quick test before putting it in the enclosure and then the most awkward part - putting the case together.

Done!

A final addition was the LEDs for the dog's eyes. These had to wait until this morning when I found some header connectors. The filters still needs tuning, but other than that, it works.

Shruthi-1 coming

I have ordered a Shruthi-1 - more precisely the limited edition Shruthi-1 4 Pole Mission. Hopefully I'll get a chance to build it next weekend.

In other news. I now have all the parts for the Gristleizer and Echo Base - except the PCBs. <rant> Deliveries from the US to UK seem to be getting slower - and more expensive what with the post office's "admin fee" for paying import duty that is often several times the cost of the duty itself........ </rant>

x0xb0x.

I built this in a few days, whilst waiting to acquire some rare transistors to finish the 9090. The 9090 took weeks.

The x0xb0x is a clone of the infamous Roland TB-303 "Transistor Bassline", designed by Adafruit Industries and originally supplied by them. Kits are currently available from several sources. I got mine from http://www.x0xsh0p.de.

The underside of the main PCB. This is split into two sections - towards the front is the synthesiser, using mainly original '80s "rare parts" and at the back is the sequencer. The ATmega for the latter is missing in this shot.

The top of the same PCB, with buttons and LEDs for the sequencer in the foreground and potentiometers for the synth at the back.

The main PCB mounted in the case, and wired to the IO board which contains the MIDI, DINSYNC and USB interfaces, CV and Gate out and the audio jacks. The power supply is also on this board.

The completed x0xb0x.