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Wednesday, 31 July 2019

Lindos LA102 test set repair.

Morning ... coffee ... switch the power to the bench on and contemplate the day's work..

... hang on, the ever-faithful Lindos LA102 Audio measuring test set fails to start up.

It's dead, nothing. Thankfully I have a spare.


A cursory check round shows the mains transformer to have failed open circuit...

Fair enough, it's been in there since the 80's...











A suitable replacement looks like RS 121-3839, and is duly ordered...









Now it physically fits in the hole, we'll need to switch the phases on one of the primary windings, and one of the secondaries.. The original transformer is wired 0-120 0-120, and 0-6 0-6, the replacement is 0-115 115-0 and 0-6 6-0.  I'm not worried about the 5V difference in the primary.

So let's deal with the secondary first ... This is a picture of beneath the transformer.

















Cut the tracks as shown.


















And connect two insulated wires to cross the tracks over...

You can now fit the transformer.

Next to deal with the primary. Cut the original link, and carefully remove the trace completely. Note I'm working on a 240V UK unit here... if you're in that funny bit of the world where you use 110 or 115V, you'll need to work it out so both primaries are in parallel.















Then add a link wire in to couple put the windings in series, and in the correct phase...

















... and reconnect the mains supply.


















Switching on, and it's clear something is not right. The transformer is humming and getting warm rapidly!  (It's never easy, is it??)

Turns out the bridge rectifier is short circuit as well... poor thing is cracked too...

It's replaced, and now the unit powers up, but it's still not right, as the power supply is still under some considerable  load...













... and the final piece of the puzzle is the ancient nicad battery is almost short circuit. I disconnect it, as I haven't ever needed it.
















Now, let's get on with some work!

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