System 100 LFO fixing

System 100 102 module VCF/VCA board

While making a godawful racket with the System 100 on Saturday, I switched to PWM on the 102 VCO and the pulse width stayed unmodulated. The LFO light didn’t light, no matter how much I waggled the slider. Hmph.

Should be easy to sort out a broken LFO, right? I’ve built all the bits of a 102 on stripboard, so surely I should be able to fix it?

Fretting that it wouldn’t be as easy as all that, I peered at the schematic.

102 LFO schematic

None of the outputs were working, so I reckoned the problem was in the tri-core, either the capacitor or the op-amp. Just to check, I measured the output of IC301, and it was pinned high.

At this point feeling strangely optimistic, I enlisted the help of my son, who was bored enough to come and watch/help/encourage/take the piss as appropriate. Is dad gonna be able to fix this thing?

I yanked out the CA1458G in IC301 with a desoldering gun:

Dead CA1458

I bunged a socket in, and replaced it with an MC1458 I had in the bits box. So far, so good.

Replacement MC1458 in place

Switched it on, and…

…it worked straight-away. WHOOOP.

I probably should have measured all the voltages and given it a bit of a once-over before I put it all back together, but my son was keen to start making some horrible noises, so I just had time for a couple of slightly wobbly bonus photos: here’s the VCO board – see all the keyboard specific components left unpopulated:

System 100 102 VCO board

and here’s the ring-mod/S&H board

System 100 102 ring mod board

It’s missing the circuit board annotations that my 101 keyboard has, and the soldering on the back is without solder resist (more like earlier System 700s, or my SH-5). The serial number starts with 46, so according to that Boss serial number lookup chart, it was made in May 1976. My 101 keyboard serial is 60xxxx, which works out to be July 1977.

Here’s my Sys101 VCF/VCA for comparison.

System 100 101 keyboard VCF/VCA board

I’ve not gone over the board to see if there are any differences in terms of components; there’s nothing extra soldered on to the back of the 102 VCF/VCA board, which suggests probably not. I pulled out the circuit board for my 104 sequencer recently, and it has a different circuit board layout to the one in the manual, with a load of bodged extra capacitors soldered to the back.

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5 comments

  1. 24th November 2019ak47ngo says:

    First of all, thank you for this great blog and sharing precious information about this wonderful
    instrument! I am getting a crackkling sound out of the high and low outputs. Do you think
    its safe to replace all the 1458 chips with the newer MC1458’s? Thank you so much for your help!

  2. 24th November 2019ua726 says:

    I don’t know if there are any differences between the MC1458 and the CA1458 – you’d have to have a look at the datasheet to see if you can spot anything. I’d think it should be fine though.

    Having said that, don’t just blindly replace all the 1458s in your System 100. You’d be much better off trying to track down the source of the problem first by trying to narrow down where the crackle could be coming from. Handily, the System 100 has outputs for each section so it’s worth listening to each to see if the crackle starts at a particular point.

    Don’t forget that dirty potentiometers and jack sockets can be a source of noise too, it’s worth checking/waggling those too.

  3. 25th November 2019ak47ngo says:

    Thanks! it will take a while but i will hunt down the source of the crackle.
    I found this synth and an SH 5 in an abandoned building. i got bored of the SH 5
    and threw it in the bin. Something I’ll regret for the rest of my life.

  4. 25th November 2019ua726 says:

    oh nooo that’s painful 🙁

    Hope you get the System 100 sorted out, it’s a pretty simple synth, and the service manual is super clear.

  5. 25th February 2020ak47ngo says:

    Thanks for your advice! I cleaned out the VCF sliders and the crackle is almost all gone!
    Now its a matter of finding the right grease to revive the slider action!

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