Flashing lights of the 100M


Continuing with some stuff from my ongoing silly giant-sized clone 100M – here is the flashing lights bit from the VCA and VCF circuits in action.

It’s unlikely it’ll be of any use to anyone, ‘cos it uses a (possibly unregulated) 22V power line from the 100M PSU, but, just in case – here’s the layout:

Roland System 100M level indicator stripboard

The lights on the 100M are meant to be useful, telling you when signal is coming out of the output, and if you’re pushing the level too hard. In practice, the overdriving seems to kick in slightly before the red light comes on.

It’s lot of effort for some pretty lights, but the whole thing has been a ridiculous endeavour so far anyway; so in for a penny, in for a pound.

The 100M level detector works more reliably than I’ve so far managed with the System 700 equivalent, which seemed to need very specific transistors, and even then occasionally gets stuck.

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System 100M VCA clone

I’ve had a bunch of Curetronic 100M PCBs for ages and done nothing with them, so finally decided to start building them up as a test for Schaeffer UV-printed panels… only because I wanted to test out my version of the OpenMusicLabs clown/clone BA662, I built a single one up on stripboard.

System 100m 130 VCA stripboard photo

For stripboarding I’ve switched to Veecad, which is much less pretty than DIY Layout Creator but at least allows for verifying the layout using a netlist exported from Kicad. Here’s the Veecad layout using a BA662 as the OTA:

Roland System 100m 130VCA stripboard layout

In case you don’t have a BA662 clone or original to hand, here’s a stripboard adaptor for a BA6110. Without any other compensation it’ll be slightly quieter than the ‘662. I measured 2.68dB difference between the two, everything else being the same.

ba662 > ba6110 adaptor

If you build the VCA, don’t be tempted to socket the transistors – I ended up frying a small pile of my clone ‘662s due to dodgy connections.

Here’s my redrawing of the VCA schematic.

As for the clone (…of the clone) BA662 – the OTA part of the chip seems to work OK. The 100M VCA doesn’t use the onboard buffer, so I need to check that as well.

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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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