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I've been tinkering with a GEM Jumbo for a year or so, teaching myself how these beasts work (I have a couple of Farfisa Compacts that I actually play too, but I use the GEM to experiment with repairs). I fixed the vibrato by replacing a few capacitors, and a few days later the (internal) fuse blew and the whole thing has been borked ever since. I've replaced all the capacitors on the power board, which was fun but didn't change anything.
I recently replaced the wires that come out of the transformer, whose plastic coating had perished and crumbled. Now I know for sure that the transformer works fine. I unsoldered one end of each of the diodes in the rectifier arrangement so the power supply is disconnected from the rest of the organ, and verified that the DC supply is ok - 13.5V positive and negative, measured from ground (the 0V centre tap of the transformer).
That's as far as I've got. As soon as I reconnect the DC lines to the rest of the circuit the 250mA fuse on the power board blows. So there's a short somewhere, but I don't know where to start. I've checked all the solder joints and can't see any obvious problems there. What would you look at next? Disconnect and check transistors to see if they're shorting?
This is all probably trickier than it could be because the schematic I have for the organ (which was neatly folded inside) is totally wrong - it looks much more like the circuit in another GEM (mini) that I picked up recently. Every GEM Jumbo schematic I've found online is a duplicate of the one I have (the wrong one).
I'd love some tips!
I recently replaced the wires that come out of the transformer, whose plastic coating had perished and crumbled. Now I know for sure that the transformer works fine. I unsoldered one end of each of the diodes in the rectifier arrangement so the power supply is disconnected from the rest of the organ, and verified that the DC supply is ok - 13.5V positive and negative, measured from ground (the 0V centre tap of the transformer).
That's as far as I've got. As soon as I reconnect the DC lines to the rest of the circuit the 250mA fuse on the power board blows. So there's a short somewhere, but I don't know where to start. I've checked all the solder joints and can't see any obvious problems there. What would you look at next? Disconnect and check transistors to see if they're shorting?
This is all probably trickier than it could be because the schematic I have for the organ (which was neatly folded inside) is totally wrong - it looks much more like the circuit in another GEM (mini) that I picked up recently. Every GEM Jumbo schematic I've found online is a duplicate of the one I have (the wrong one).
I'd love some tips!