Some action today...
Dragon sent me knobs to dress the naked front panel of my HP 54645D MSO scoppy, looks much better now for sure, thanks !
Looks like a million dollar scope now
Also donated 4 little DC/DC step down converter boards with the MP1584 chip.
Then I received also today an envelope from Sweden, got RIFA caps to replace the one in the HP scope PSU, and also a set of lovely Hirshcmannnlong grabbers. Sadly the rfd one got broken during transport, sad.... the hard plastic hand piece is badly cracked at the base of the grabber, but still holding together somehow and still works, smoothly at that. It's still perfectly functional. Tried to bend it back into shape / straighten it, no joy, it wants to stay bent, ah well... maybe at least I could fill the gap/crack with super glue ? And maybe add a layer of two of red heat shrink tubing to restore thye electrical insulation ? because having my fingers right on that crack with HV inside the crack does not appeal to me very much, I must confess..
So I replaced that RIFA. PSU board was easy and quickly to remove from the scope. Paid extra attention to plastic rod operating the mains switch, since you said it could break easily... got lucky, it came out and back on, with no drama
You also warned me about the difficulty to remove the cap due to one lead being connected to a fat trace or ground plane. That was a big worry for me as my old Weller Magnastat iron is only 50 Watts
Luckily, it managed to remove the cap within a reasonable amount of time. 10 or 20 seconds top, nothing to crazy. Still, a much more powerful iron is on my wish list.
Replacement cap is twice the value 100nF instead of 47,F, so that package is larger, but still juuust about fits in the confined space that this cap is allowed to occupy, closely surrounded that it is by this tall heat sink and massive filter cap, mostly.
Pin pitch is larger as well, 20mm instead of 15mm. No big deal I bent one of the pins. However that meant that clearance / isolation with other surrounding discrete components, was reduced. So I had to chose which side of the capacitor was the safest for this reduction in clearance. Luckily one side was much safer than the other so I was relieved, no accident / sparking waiting to happen here.
Putting the new cap was a chore... had to clean the pad holes first, and it's not easy (don't have a desoldering gun) as the diameter is small and the pad hair thing as well, and my solder sucker doesn't work at all in that kind of scenario. So I had only solder wick at my disposal (maybe I should get a set of needles to unplug holes... will ad that to my shopping list). The easiest of the two was not too difficult, after a few attempts, but the hard one with the fat trace, my god.... took 15 attempts ! I couldn't even give up on it because I had no other means, I HAD to make it work !
So I could then put the new cap in place. Soldering was not cool because the pins are soldered on both sides and you just can't know for sure if the solder managed to reach the other side and make a good connection, since the component is sitting flush on the board (and access is so poor on this board with all the surrounding components !! ). So that's where I was paradoxically HAPPY that the new cap was bigger and I had to bend one lead... this meant the cap was NOT sitting flush onto the board any more, so with a good flash light I could just barely manage to see the solder joint under the cap !
So I put the scope back togetehr and what do you know, it still works, I have not killed it !!
Then for the grand finale I though I would gather all the bits together and make something out of it :
Used the scope to test Dragon's DC/DC board, connecting to my lab supply with Mansaxel's grabbers, and take some measurements for TEA content.
It just doesn't get any better than this eh ?
So I thought I could look at the ripple, and for example measure the switching frequency, see if it matched the theory. Why not.
So I pulled the datasheet for the chip, looked up the formula. Frequency is set by a single resistor. Found it on the board, but it's marked " 01D" which is a bit strange for a resistor eh ? What does that mean for a resistor.... so instead I measured it with a DMM. about 100kohms.
so I ran the numbers and if I get my math right I should get about 900kHZ, which sounds plausible given the tiny size of the board yet high current output... and ads up with the datasheet saying the chip can operate between 100kHz and 1.5MHz.
OK so I wire the board up, and scope the ripple. See for yourself.... TWO VOLTS peak to peak, and 40Hz.. yes 4x10 !
Eh ?!
Then I try to vary the output voltage with the tiny trimmer.. with careful use of a tiny screw driver... seemed to react OK for 10 seconds then suddenly it would not react anymore, I would get full output voltage permanently. I guess a bad joint on the trimmer, it's out of circuit now ?!
So not very good isn't it !
I got unlucky, I will try another board and report....
Thanks Dragon and Mansaxel !!