Even if it always seems to be bad luck.
I seem to be collection repair/maintenance failures, and now have to clear some work space.
Taking up a surprising amount of desk space, the recent triple fail of two watches and a clock (at least not my fault, but not viable fixes) still was not a great start, even if they moved off the bench.
Didn’t really help, since they got replaced by three LARGE items (not mentioned yet as just become ‘work in progress).
Ye Olde Power Supply
Might have mentioned this last year, after a slipping clip killed a 13.8 V 8 A power supply I was using. As usual, bad luck played a part, when the two giant croc clips I was using to connect to the supply manage to touch when the wire were pulled. Although I’d checked that the bare metal of the clips couldn’t bridge the terminals, I should have remembered my #Bad Luck Only’ option, and checked the scenario where the clips could be pulled by their wires, and somehow move in OPPOSITE directions, and short on each other.
Of course, the output stage blew and protected the fuse, which was fine 🙂
Because it was in use, I didn’t have time to look at it then, so it got shelved as I dug up a replacement to get things going again.
However, it did get opened up shortly afterwards, and has been sprawling all over the bench since, waiting to be fixed.
That just happened, and I found the output transistor, and one used to monitor/limit the current had expired, but no other damage.
Simple fix? Of course not
This circuit was so simple it was easy to check everything. The main hassle was the TO3 transistor which I had mounted on an external heatsink, and had to be insulated, with lots of messy heatsink compound keen to get everywhere.
I forget how the transistor was originally mounted, but my external heatsink had fins at leat 20 times the size of the bent metal of the original (inside the case)l, yet STILL got too hot to touch when this supply was only running at half its rating, 4 A.
Not wanting to get caught by something unexpected, I powered this supply up in stages, hoping to avoid a BANG!
It actually worked fine, with all the expected voltages present, and just one small problem – NOTHING at the output terminals, even though there was voltage present on the output transistor.
I stared at this for a LONG time before the solution presented itself, and that only came after I started writing down the circuit and the connections to the PCB and components, again!
I’d done this at the start, expecting things to get messy as I took the thing apart.
What I hadn’t noticed was that the wires extending the output transistor changed colour at a connection almost out of sight, hidden by the heatsink fins.
Once I wrote these changes down, and identified which was which as the colours changed, and connected the output transistor to the PCB correctly – the thing was working fine (and always had been, but for the wiring).
At least the crossed wiring didn’t lead to the thing being shorted, and going up in smoke.
Looking at the circuit, I don’t think it could, at least if there wasn’t a load connected – and that wasn’t going to happen until I’d proven it was working first.
Job done? Don’t be so silly.
I’d like to finish this by saying I buttoned the thing up and it’s sitting on the shelf ready to go the next time it’s needed.
Of course, with my luck, I can’t.
Those clever designer decided to mount the PCB on the back of the output terminals, and they have large metal contacts that mate with a similar area on the PCB, and are bolted down to it.
One of the terminals appears to have been badly machined, and while the thread was not found to be stripped, the thread seems to have been cut to a slightly smaller diameter than it should have been. While both nuts tighten securely on the other terminal, on this one they just spin over the widest part of the thread on the terminal, and don’t grip.
Of course, I don’t have any PCB mounting terminals. All my stock is for mounting on panels, with the mounting hardware insulating the metal from everything around it. And, there’s no room to mount such a terminal in this supply – so I’ll just have to look for a replacement.
SpongeBob? Is that you?
This power supply is not photogenic. A black box with a black heatsink, and even blacker after I slapped black car underseal on the case in an effort to cut down the hum coming from the steel case. This supply was in use on a nearby table for some years, and the mains hum was considerable as there was not much space between the sides and the sizeable transformer. I even added stiff rubber mounts to stop the sides vibrating.
Just for fun, I asked an AI image source to provide an image of a scary/crazy exploding transistor.
It came up with the SpongeBob family album!
Well…
Off I go, onto the NEXT failure (no worries, It’s already well underway).
Secret Scotland 