2019-11-16 Fixing a Driver Transformer With An Open Primary

Also FB group Building Transistor Radios

2019-11-16

I was working on a Realtone 10 transistor AM radio that had a driver transformer with an open primary. I slit open the wrap and found the tiny fine wire on the right was loose so I soldered it to the heavy wire. But it was still open. ☹️😱 So I checked the left wire and found it wasn’t connected, so I soldered it to the heavy wire. Checked with the DMM and it’s *still* open! I’m so sad! Buah-wuh-wah. 😭😭😭 Why is this so fouled up?? It’s crazy! Like it’s been hit by lightning! Now I’m just bummed out!

Update a few hours later – I tack soldered into the circuit a transformer with four 600 ohm windings. I connected two of the windings in series for the primary, and the other two in series with the ‘center tap’ to drive the bases of the two output transistors. The radio now works just fine – there was *nothing* wrong that could cause the driver transformer to have excessive current and burn out. What the hell caused the driver to go bad?? Some others in the FB group thought it might be the factory used corrosive solder. It seems to be a plausible answer.

Update Nov 21 – I ordered some LR-44 driver transformers from a UK seller, but they may not get here until next year. So I wired the temporary transformer into the circuit permanently, held in place with a piece of wire. This doesn’t look so hot but it works okay.

Final look at the old timer. A little worn but it’s working okay.

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