Main > Monitor/Video Forum
Zenith CRT TV - black horizontal bars
Zebidee:
Yeah I really should have given you some recommendations in the first place.
You use the ESR meter to measure Equivalent Series Resistance, not capacitance. All capacitors have some ESR, and it goes up as they age/fail. ESR is actually a better measure for failing electrolytic caps than capacitance.
Here is a decent guide featuring a few different types of ESR meter:
http://www.electronicrepairguide.com/esr-meter.html
This guy (EEVBlog) on Youtube demonstrates how to use an ESR meter to identify some bad caps and fix an LCD monitor he picked out of a dumpster. You'll see how easy the repair was. He checked all the caps but only needed to replace two, others in good condition still.
if long YT videos bore you, you might want to skip to 4 minutes in where he starts talking capacitors, or about 5 minutes in where he starts using the ESR meter. The meter he's using is exactly the same as the one I use (Dick Smith/Silicon Chip/Bob Parker design, built from a kit). I have used mine for about 15 years and the batteries last a very long time.
You'll see there how he takes the ESR reading, then compares value to the chart on front of meter to determine if capacitor is good. The chart values are a little rubbery, but bad caps will be obvious because they'll have wildly wrong values. With a little experience you won't even need to check the chart most of the time.
I actually have a few of those Dick Smith/Silicon Chip/Bob Parker kits (still in shrink wrap) in my shed that I got from Dick Smith (hobby electronics store like Tandy/Radio Shack) in Australia before the company finally went completely broke in the late 2000's. The only thing that is different is the screen-printing on the front. Nobody makes them anymore, though there are alternatives. I can happily sell one to you, except I worry that the postage from Thailand (where I am) to wherever you are might be a bit too much. If you are still interested, please PM me.
Zebidee:
--- Quote from: romshark on July 19, 2020, 08:42:37 pm ---What's the threshold for a "bad" capacitor? That first one is off by 10.2%, and the second by 4% (if I'm doing the math right). Or am I looking at this wrong?
--- End quote ---
It depends on the capacitor, but often they are rated to be +/- %20! But yeah, check the ESR. Capacitance can seem OK when you check it out of circuit, only to fail/falter while under load. A failing capacitor with high ESR will not be able to recharge/discharge as efficiently as before, create more heat and obviously will be putting extra resistance in the circuit where there wasn't anything (worth worrying about) before.
romshark:
Ok, I think I see what I was doing wrong. Since ESR is in the title for the LCR probe on Amazon, I thought it was using ESR by default when needed in automatic mode. Went into the menu, and set it to ESR mode.
Looking at my TV PCB, it looks like most of my capacitors are from Nichicon (though I did find a couple so far by Rubycon). The problem is, I can't find the manufacturer Impedance values for some of these. The charts online (and on the front of your meter) cover the common ones, but I can't find, for example, the expected value for a 330uF 200V Nichicon capacitor. Can't seem to find it in any datasheets I come across.
Is there some massive chart somewhere that I could use? Even if it's not exact to manufacturer, I know now that if the ESR is higher, then it's probably bad.
Zebidee:
--- Quote from: romshark on July 19, 2020, 11:29:15 pm ---The problem is, I can't find the manufacturer Impedance values for some of these. The charts online (and on the front of your meter) cover the common ones, but I can't find, for example, the expected value for a 330uF 200V Nichicon capacitor. Can't seem to find it in any datasheets I come across.
Is there some massive chart somewhere that I could use? Even if it's not exact to manufacturer, I know now that if the ESR is higher, then it's probably bad.
--- End quote ---
Don't worry too much about the datasheets.
For those edge cases you need to guestimate a little. Taking your 330uF 200v example, that's roughly halfway between 220uF and 470uF at 250v, and their best (worst) ESR values are 0.5 and 0.3 respectively. Best/worst ESR values for those same caps at 100v is 0.05 and 0.09 respectively, quite a bit lower. Anyway, I'd say that any ESR reading over 0.5 would definitely be suspicious. That estimate is maybe a little on the high side, especially for a Nichicon, but when caps go bad you'll normally see ESR go much higher/ out of normal range, often by factors of 10 or more. If you have potential replacement caps that you assume are good, you might compare.
Always try to use "low ESR" caps if available, especially in the power supply area.
romshark:
Sorry for the lack of updates, but I've only been able to work on this on Sundays (had to do overtime at work.)
I spent time measuring capacitors with my meter in ESR mode (100Khz). After some hunting, I found one reading 1.38K Ohms. Had to desolder it just to find the values (1.5uF 50V).
So, I found one possible issue. I don't have a spare of that value, so I'll have to order it, but I'm going to continue searching for more capacitors with bad ESR first. I may replace some of the other capacitors anyway (where the top looks slightly convex. Not anything like the above video, but still worth replacing. Also the ones that I desoldered last week during my testing.)
My spirits are up. I might be able to save this TV after all.
Edit: Found more with high ESR. It seems to be mostly the 1uF 50V Rubycon ones. I guess the smaller ones dried out first, which I guess makes sense. At least I can probably just order that one in a larger bulk.