[Open_electroporator] repairing Mitchell's culture shock prototype

[John Griessen]

On 07/28/2017 07:52 PM, John Griessen wrote:
> It had blown out and was no longer 4 Ohms, but open.  Next I need to desolder the
> transistor on the side that doesn't pull, which must be failed shorted.

That can't be the reason:  those transistors are like shorts when they switch on,
and no pulling down the 20V, so I look elsewhere for a short.  I'll use my microvolt meter
with +/- meter readout while putting a little current into it from an adjustable lab supply.
There are 6 or so capacitors all in parallel and the transistors that could be the source of the short, so I'll
try to locate it with the microvolt meter probing all around looking for the lowest volts...

The transistor on the side that doesn't pull may still be bad, but not failed shorted.
There could be a solder blob problem at a transistor too, and not related to its switching action,
just a blob connecting tooo much together.

Why so much wrong with one pcb assembly soldered by me?  How did it get out the door?
I think maybe I did not use a 30k Ohm load on it for functional testing.

-- 
John Griessen
cibolo.com  Austin TX  building lab gear for biologists