15 February 2017

Tektronix 7A13 plugin ... fixed!

Contary with what I stated in the previous post, I tried to fix the 7A13 plugin. I couldn't leave a burned capacitor in there.

Messages on the TekScopes list suggested a simple test: measure with an ohmmeter the impedance across the tantalum capacitor: if it is zero, then there is a problem. And it measured zero.

So I cut off the original C158 capacitor and promptly replaced it with a modern 100 uF 25V electrolytic. Why cutting instead of desoldering? Because pads might not sustain several re-heating and, in any case, that component will not be reused. Even if the board is densely populated, I could operate without disassembling it, that could have been a nightmare.

With the new capacitor in place, the meter reading was still zero. So I located another couple of tantalums and one of them was dead short even if it looked brand new. Off it came, and a shiny 10uF 16V electrolytic got in the scene.

No more 0 readings across those capacitors. Time to a test in the 7603 chassis ... GO! The baby lives, again.

Left: C158. Right: C165.
This 7A13 has at least one more tantalum that I should replace before I forget where it is located. Then I should do the same replacement in my other plugins, just in case!

Out of curiosity, the burned capacitor measures open circuit and does not smell: I suspect it went up in flames long ago. The smaller one measures short circuit.