We got an old Roland SH2 monophonic synthesiser in a couple of weeks ago.
It’s main fault was that the keyboard control circuitry was not working, it sounded like a Syndrum when the key was released with any form of long release time.
We have seen this several times this year, and the reasons were very similar, either a faulty chip in the keyboard Sample and Hold buffer or else the wrong chip put in to replace the faulty Sample and Hold chip. It was the latter.
Apparently this synth has been running for more than a decade on its CV and gate inputs.
The IC in question should be an LF13741 which is a 741 op-amp with JFET inputs. Someone many years ago saw the 741 number and replaced it with a regular 741 which explains the Syndrum syndrome.
We put in a CA3140 chip in a socket as we reckon this should work, but a backup plan is that the original LF13741 is available from Senso at www.vintageplanet.nl at a reasonable price.
The real crime in the condition of this synth is 30 years of neglect and abuse, many of the sliders were stiff and sticky and the rotary switches wobbled in use. The keyboard surface felt horrible to play, and the culprit was good old fashioned nicotine, we have seen this a great many times before, the keys on a keyboard should feel smooth and shiny and pleasant to put your fingers on.
A complete strip and rebuild of the keyboard was done, and several cleaning cloths were destroyed during this, the amount of dirty dust and fluff removed was considerable, along with the amount of liquid that had been spilt over the keyboard during its life. After this effort the keyboard at least felt like a place that you would want to put your fingers on.
The next phase was the front panel, there were several sliders that were almost unuseable and some that seemed faulty and this was probably the case, there were also a lot of bent slider stems.
Removing each of them from the PCB’s, the stripping and cleaning of the sliders is a laborious process (they take about 20 minutes each) and the black foam covers over all the sliders and switches was breaking up up into black crumbs, and had to be replaced by custom cloth tape replacements.
Straightening the slider stems can only really be done when the slider is completely dismantled, efforts to do so otherwise could lead to damage to the slider itself or to the PCB that it sits on which is terminally bad news on a product that is 30 years old, no spare boards are available in this case. The sliders themselves are long out of production and we bought the last remaining stock of JP8 sliders from Roland UK back in 1989. The sliders themselves are not used in a JP8, but they are of the exact same physical type, and parts of them are interchangeable, the stem/wiper assembly, the physical metal body, there is a bit of give and take on this, and one of the SH2’s sliders wiper was so badly damaged that it was replaced by a good one from a dead Jupiter 8 slider (thank goodness we stored the parts from even dead JP8 sliders).
The calibration of the SH2 was interesting, once the keyboard buffer issue was fixed and the keyboard interface calibrated, we were amazed by how good the oscillator calibration was after 30 years, only the tiniest tweak was needed to bring it to true. The other interesting thing was the filter cal. the maximum filter frequency at full resonance and the cutoff slider at 10 was about 8kHz which is way too low, the service manual says this should be 20 kHz which corresponds with what would be expected of several other Roland products of the time, and to an extent plain old common sense.
All in all a great result, this synth looks good, works perfectly and sounds great, we think the lively sounding oscillators are the big feature here, the filter seems a little less important to the sound.