Last Modified: 20 April 2013


ITA ELECTRONIC IGNITION AMPLIFIER circuit board rebuild due to losing the original, then first replacement design still not quite right.


First Version
This is the AB5 style case, with a date stamp '1979'. I originally bought a kit in 1985-ish for the Spada, but never put it on. So it later went onto the 850. To cut a long story short, in the interim I had taken the circuit board out to trace the circuit, then later lost it during a 'tidy-up' session where I disposed of a load of tat ~ including, unfortunately, the circuit board for this!




Later I got to look at a AB11 amplifier and trace it, so was able to make something using a BU208 high-Voltage power transistor. This was around 2001:











Second Version
Latterly I was able to source the correct BU941 Darlington power transistor as per the AB11 circuit I had traced (not possible through my usual suppliers in 2001), so was able to construct a new version of the board. The BU941 is of course designed to function as an ignition coil switch, the BU208 is a colour television LOPT driver, with poor gain, so the first version was a bit of a bodge:





Making the board fit the case is tricky, this is my second attempt with a new piece of matrix board:











Unlike its newer plastic packaged siblings (with 'Z' suffix), the TO3 BU941 does not include over-Voltage protection, hence the inclusion of this big blue Voltage spike suppressor type varistor (VDR):




The original Rita AB11 amplifier circuit that I based mine on ~ contrary to popular myth (via the Internet, of course) the circuit does not include any means for controlling ignition advance, that part is all done by the rotor and pick-up coil ('reluctor'), all the amplifier does is provide the reluctor with the means to make the ignition coil(s) work:




My version does not include C7, D3 & D4, which parts together function as a rev. limiter. It's ingeniously simple but I'm not overly convinced of its reliability so is something I'd rather do without:




Just for reference, the early Mistral Rita AB5 circuit board:




... and something more like the later 'improved' AB11 circuit board (early version, the latter had the BU941 power transistor instead of BU931), although to me this still looks a lot like the AB5 (?):



It's interesting that originally these appear to have the 'Z' suffix, this means it includes internal over-Voltage protection. It seems that the TO3 cased version is now being phased out in favour of the plastic alternatives, and according to the datasheet I have it no longer includes the internal protection Zener. Hence the addition of the big blue 385VDC VDR.

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