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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ta7136 substitution

Some of the System 700 modules use Toshiba TA7136 SIP opamps (datasheet), which are long obsolete and semi-sought after by DIY guitar types because it was used to apparently pleasing effect in a Boss distortion pedal. An eBay seller is trying to get the best part of twenty quid for one.

A while back I bought a bunch from Utsource, and when I eventually got round to making a board to test them I found they were fake…

TA7136 vs fake

…booo.

I made up this adapter for an alternative op amp, ‘cos I don’t think using this particular chip is that important (for now, anyway).

TA7136 to TL071

Here’s a strangely moody photo of the resulting stripboard.

TA7136 > TL071 stripboard adapter

After this I cut one chunk out of one side of the board to indicate pin 1.

I’ve had this working in the 708 noise/ring mod and the 711 output module, and it works fine, although I’ve not had a chance to compare it directly to the 7136.

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i’d rather multiple jack

710 multiple clone

OK I know, but it’s a start.

3mm acrylic is bendy, so I reinforced the back with 1.5mm aluminium to make it much stiffer, but it’s still not as solid as I’d like it.

Clone System 700 multiple from the back, showing the metal reinforcement

I should have really made the metal plate wider than I needed on one side, and bent it up to form an L-shape to totally stop the bending.

No chance for recessed jack sockets like on the original, my metal working just isn’t that good. 4.5mm panel depth is really the limit for these jack sockets, and it’s borderline for rotary pots.

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