#1870

Episode 1870
one bad cap. could that be all?
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[@andymouse]

Tip of the day ! use red and black pens for marking polarity !!Lol !.....cheers.

[@joeteejoetee]

ABSOLUTELY ! That Blue Scharpie shocked me, let alone the penmanship we've all seen...

[@andymouse]

@@joeteejoetee :)

[@ProtonOne11]

I'd always check with an oscilloscope how the ripple looks in that power supply too. If it can't regulate properly to a clean DC, the old analogue meters can react kinda funny, and your RMS digital meter might average that ripple out differently. And the ripple could be good indication if the bulk capacitors actually still work with DC bias, or if the capacitors are actually all kinda on the last leg. And of course, with something this old, it might just need a good old calibation sequence to display the proper values again. Potentiometers sitting for a long time are kinda prone to fail over time with the wiper oxydizing or digging into the resistor trace just from vibration (thanks to the humming noises of the mains voltage in the transformer).

[@fmashockie]

I was looking forward to the resolution of this repair! Love the ones that keep you guessing. I do have one problem with your diode test comment. Depending on your meter (for example my Fluke 117), in the diode test function, they will beep for anything below a certain forward bias voltage (say ~1V). For my Fluke 117, if I measure short across a diode in diode test function, it will give a long beep if you hold in there. So if you are going through quickly like that without looking at the meter, you could potentially miss a shorted diode. Which in my experience, is usually how they fail. Otherwise, great video! Look forward to part 3!!

[@larry785]

I'm going to assume all the caps are good. I had an HP power supply, the cap had a small hole and it looked like it oozed some stuff - I pulled it and tested it - it was just like new!

[@roscozone8092]

Was looking forward to the meter on that monster cap

[@featheredskeptic1301]

This could be due to power supply ripple. The thing just becomes an oscillator because of either a bad capacitor somewhere or some leakage that causes feedback. Error amplifiers are supposed to have huge gains, and something like this can happen very easy.

[@polaraligned1]

I remember axial by just thinking about the axis of the cylinder.

[@Mr.Leeroy]

Please, make a deep dive in explanation of this SCR pre-regulator and maybe rectifier cap inrush limiting at such output current levels. There are good vids on YT covering DIY LPSUs, but lacking on 6-10 amp >100W linear supplies w/o multi-tap transformers.

[@MSP_TechLab]

The bad thing about such old equipment is that it is not only capacitor can die, but also other components' parameters can drift away due to the age. So, once you replace that dead capacitor, it may happen that it is "too good" for other elements and you'll need to troubleshoot, re-calibrate or change another pile of parts 😢.

[@byronwatkins2565]

Use your oscilloscope. The electronic and mechanical meters respond differently to ripple.

[@IMSAIGuy]

part 3

[@jspencerg]

Better than finding that suspect component is good after you take it out.

[@iceberg789]

lol @ how they mounted that to-3 barebone on the pcb without any heatsink. did they have nothing in between the small and the big ? 🤣

[@SimEon-jt3sr]

I don't even have an adjustable buck boost converter with a V ammeter display... Or a bb bench supply:( :(

[@romancharak3675]

That meter does seem to "stick" in places. That may be your next exploration?

[@TonyBarr99]

I have the same power supply and it has served me well! I see that you have a jumper tying the ground and the negative posts. I never do this. Can you explain why you do this? Thank you!!

[@IMSAIGuy]

keeps the negative from floating way above ground (or below)

[@_nemo171]

I'd replaced all off'em. lol The power supply looks like 30+yo.

[@billharris6886]

I'm sorry the open cap didn't solve your problem (I guess that was too easy). Well... Looks like it's time to delve into the theory of operation some more to get better acquainted with the circuit. By the way, those tapped power resistors, the wattage rating assumes the entire resistor has voltage applied to it. When used in the tapped mode, you multiply the percentage of the resistor where voltage is applied to determine maximum power. For this reason, the max rating is usually expressed by a max current.

[@andymouse]

Is that how all rheostats are rated as this rings a bell and makes sense? .....cheers.

[@billharris6886]

@@andymouse Yes, that is correct, rheostats are rated the same way.

[@originalmianos]

Spoiler alert please¡ 😮

[@billharris6886]

@@originalmianos Sorry Rob, I don't know what "Spoiler Alert" means.

[@andymouse]

@@billharris6886 It means ' please warn me if you are going to ruin the ending of a story' so I can turn the volume down or something !