[cctalk] Re: ADM3a screen rot.
> On Jan 29, 2024, at 8:47 PM, Jonathan Chapman via cctalk > wrote: > >> This apparently is true of some capacitors as well, I'm not sure which types. > > It is true of all capacitors (CRTs are intentional capacitors, after all) > designed for and subjected to sufficiently high voltage. It's referred to as > dielectric absorption, and is why HV caps ship from the factory with the > leads shorted. > > I have a 4.7 uF tens-of-kV capacitor in the shop for reasons. The shorting > jumper got knocked off once while moving stuff past it, and I noticed it the > next day. By that point, it had accumulated enough charge to register over > 200V on a Simpson 260 VOM (not a high impedance meter). I don't know if that > was accumulated static charge or from dielectric absorption. > > Thanks, > Jonathan Thanks for the confirmation. And also a good reminder that static electricity is one way to charge capacitors, at least ones with good dielectrics. After all, the famous Leyden jar is nothing more than a capacitor charged by static electricity, and it can certainly hand out substantial shocks. paul
[cctalk] Re: ADM3a screen rot.
> This apparently is true of some capacitors as well, I'm not sure which types. It is true of all capacitors (CRTs are intentional capacitors, after all) designed for and subjected to sufficiently high voltage. It's referred to as dielectric absorption, and is why HV caps ship from the factory with the leads shorted. I have a 4.7 uF tens-of-kV capacitor in the shop for reasons. The shorting jumper got knocked off once while moving stuff past it, and I noticed it the next day. By that point, it had accumulated enough charge to register over 200V on a Simpson 260 VOM (not a high impedance meter). I don't know if that was accumulated static charge or from dielectric absorption. Thanks, Jonathan
[cctalk] Re: ADM3a screen rot.
> On Jan 29, 2024, at 6:59 PM, Pete Turnbull via cctalk > wrote: > > On 29/01/2024 20:45, William Sudbrink via cctalk wrote: >> Sellam Abraham wrote: >>> I think you were fine. That's how you discharge them anyway. You >>> were just missing the grounding wire :) > >> I'd rather not be the electron sink In this case. I've never taken a >> hit off of a CRT have you? > > Yes, from a 26" CRT in a colour TV. It threw me across the room and > taught me a lesson I'll not forget. > > BTW, if anyone has CRTs lying out of circuit, don't assume they're safe > either. The EHT in normal use forces electrons into the glass, and when > disconnected they very slowly migrate back to the aquadag, producing a > substantial voltage. I remember seeing this demonstrated on an > electronics course several decades ago. Which is where I /should/ have > learned about discharging CRTs *through a suitable resistor* :-) This apparently is true of some capacitors as well, I'm not sure which types. paul
[cctalk] Re: ADM3a screen rot.
> On Jan 29, 2024, at 9:54 AM, William Sudbrink via cctalk > wrote: > ...the anode cap on the leaking CRT had gone rock hard and there was a small > "streak" on the back of the CRT that looked like the plastic that it was made > of had released some sort of oil… Often times dielectric grease is used around anode caps, and it’s not unusual to see streaks like this. Usually harmless, but you can clean it up and replace/freshen it if you replace the hardened rubber cap. Be careful with such cleaning on the tube, though — depending on the tube, there may be an external aquadag coating which you won’t want to damage/remove. —FritzM.
[cctalk] Re: ADM3a screen rot.
Is there a definitive guide for repairing screen rot. One of mine needs it. I have watched others but I have not attempted my own. I might try this at the Kennett Classic workshop this upcoming Feb 17th Bill On Mon, Jan 29, 2024 at 11:41 AM William Sudbrink via cctalk < cctalk@classiccmp.org> wrote: > A quick note on ADM3a screen rot... my vintage collection resides in a cool > (60-72 degrees F) dry basement. My "pride and joy" ADM3a (I have several) > was just starting to show a few bubbles at the corners last September. I > was pulling out some parts units on Friday and noticed that one had a much > better screen than I remembered. Thinking that I might swap screens, I > took > a close look at "PnJ" and discovered to my horror that most of the lower > half of the screen had "melted". "PnJ" was on a shelf, below eye level, > nowhere near a vent or other source of heat. I was so annoyed that I > immediately started cleaning/repair without taking any pictures (sorry). > Fortunately, there does not appear to be any corrosion from the "goo". I > completely desoldered and removed the keyboard assembly to get all of the > crud out of (and out from under) it. The mainboard is a fully socketed > example and the crud is down in several of the sockets. I'm still working > on that. Anyway, the take away is don't assume (like I did) that the > ruined > ADM3as you see are the result of temperature extremes. It can happen > anywhere. Keep a close eye on yours if you have one. > > > > Bill S. > > > > -- > This email has been checked for viruses by Avast antivirus software. > www.avast.com >
