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(ET) SCRs on card 1




I've been trying to identify the failure on card 1 that's keeping it from properly controlling the reverse relay, and have hit a snag - where the heck is SCR 106 on card 1? This is the semiconductor switch that pin 8 is directly connected to. None of the SCRs on the card seem to be labelled. I would tend to think that SCR 106 should be the big black semiconductor switch (I think that's what it is) just below pin 8, based on proximity, but that's clearly not a reasonable approach, since many of the other components on the little time delay circuit are spread all over the card, rather than nearby the other components in the same circuit. I could remove card 1 so I can flip it over and look at the paths on the underside, but that would be a royal pain if it's not necessary (unplugging and keeping track of all those wires). From looking at the topside, and trying to see the paths underneath, it *looks* like pin 8 is in fact connected to the black chip (which I assume is an SCR) just below it - the problem is that using an ohmmeter, it doesn't seem to be connected (based on the schematic, there should be 0 ohms resistance between pin 8 and one of the legs of SCR 106 (the anode side). And when pin 8 has battery voltage due to the speed lever being moved to reverse, one leg of the SCR should have battery voltage. But, that SCR just below pin 108 doesn't appear to have any leg connected to pin 8, and when in reverse, only has voltages of around 4 or 5 V, none at batt voltage. So - if that IS SCR 106 right below pin 8, and it's not showing a connection to pin 8, then perhaps the problem is either wire 39 (I think that's what it is, can't rememebr for sure right now) isn't making a good connection, or the SCR has lost its connection to the board.

Mike


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Michael S. Briggs
UNH Physics Department
(603) 862-2828
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