5 ms·

I wonder if I just got lucky in my youth trying to repair a mac classic II (at the time it was about 10 years old), or if the 9" CRT that must have not been tur

by ido 5y ago

I wonder if I just got lucky in my youth trying to repair a mac classic II (at the time it was about 10 years old), or if the 9" CRT that must have not been turned on for years at that point didn't have enough of a charge? That machine eventually lived and functioned perfectly for quite a few more years (ended up just needing a new HDD) until finally dying.

formerly_proven 5y ago
The dangers of aquadag charge are enormously exaggerated. The real danger is not that the shock will instantly vaporize you and everyone you love, but that when you disconnect the anode improperly the spark jumps to something-not-quite-ground and destroys electronics, or, when it jumps to you, you hurt yourself because you hit something or drop something. Off-line SMPS are far more dangerous because their main caps actually contain 100-1000x more energy than a charged CRT tube. And unlike the aquadag on a tube these are very low impedance as well.
- jacquesm 5y ago
  You need to discount here for CRT size. A 9" will zap you, a big 30" Barco will happily kill you if you are not going to be careful, those things will put out HV at appreciable current. Ditto for old glass tube large screen color TVs.
  - formerly_proven 5y ago
    Yes, with big picture tubes there's certainly a potential for receiving a lethal shock (of course, depending on the circumstances even very weak shocks can become lethal). My point is more about risk, respect and understanding what you're dealing with instead of apocryphal stories and fear. The former is imho more conducive to safe procedures being followed.
dan_hawkins 5y ago
A level of high voltage in CRTs is directly related to the size of tube and whether it's B/W (monochrome) or color (higher for color as you need to attract 3 electron beams instead of one.) In small monochrome CRTs (like 9"-10") cathode voltages would be in single digit kilovolts range, while for large color CRTs (19"+) would be in tens on kilovolts (like 25kV.) Bottom line is that in 9" CRT lower voltages are a bit safer and dissipate quicker after power off.
- Sunspark 5y ago
  I plan to replace some RIFA caps on a 9" monochrome that hasn't been powered on for years but was still working last time it was powered on. Do you know how long it takes for an old CRT like that to bleed out all the power if it doesn't have a circuit to automatically do that?
  - dan_hawkins 5y ago
    Minutes to tens of minutes (depends of voltage and air humidity.) Couple of hours would be on quite safe side. There's a simple procedure to quickly discharge HV of just turned off TV/monitor but I'd recommend that only for someone experienced working with high voltages.
kn0where 5y ago
The Classic II (and others newer than the earliest Macs) had bleeder resistors to discharge the CRT automatically. Older monitors and TVs did not, so manually discharging was more important. The procedure was still to manually discharge the newer machines in case the bleeder resistor failed, but whenever I discharge the CRT of anything newer than a Mac Plus it’s already discharged well before I touch the grounded screwdriver to the underside of the anode cap.
- reaperducer 5y ago
  The Classic II (and others newer than the earliest Macs) had bleeder resistors to discharge the CRT automatically Was that why older/cheaper CRTs just turned off, but the more expensive and newer ones made kind of a "thunk" noise after the screen turned off?

by ido 5y ago

formerly_proven 5y ago

The dangers of aquadag charge are enormously exaggerated. The real danger is not that the shock will instantly vaporize you and everyone you love, but that when you disconnect the anode improperly the spark jumps to something-not-quite-ground and destroys electronics, or, when it jumps to you, you hurt yourself because you hit something or drop something. Off-line SMPS are far more dangerous because their main caps actually contain 100-1000x more energy than a charged CRT tube. And unlike the aquadag on a tube these are very low impedance as well.

jacquesm 5y ago
You need to discount here for CRT size. A 9" will zap you, a big 30" Barco will happily kill you if you are not going to be careful, those things will put out HV at appreciable current. Ditto for old glass tube large screen color TVs.
- formerly_proven 5y ago
  Yes, with big picture tubes there's certainly a potential for receiving a lethal shock (of course, depending on the circumstances even very weak shocks can become lethal). My point is more about risk, respect and understanding what you're dealing with instead of apocryphal stories and fear. The former is imho more conducive to safe procedures being followed.

dan_hawkins 5y ago

A level of high voltage in CRTs is directly related to the size of tube and whether it's B/W (monochrome) or color (higher for color as you need to attract 3 electron beams instead of one.) In small monochrome CRTs (like 9"-10") cathode voltages would be in single digit kilovolts range, while for large color CRTs (19"+) would be in tens on kilovolts (like 25kV.) Bottom line is that in 9" CRT lower voltages are a bit safer and dissipate quicker after power off.

Sunspark 5y ago
I plan to replace some RIFA caps on a 9" monochrome that hasn't been powered on for years but was still working last time it was powered on. Do you know how long it takes for an old CRT like that to bleed out all the power if it doesn't have a circuit to automatically do that?
- dan_hawkins 5y ago
  Minutes to tens of minutes (depends of voltage and air humidity.) Couple of hours would be on quite safe side. There's a simple procedure to quickly discharge HV of just turned off TV/monitor but I'd recommend that only for someone experienced working with high voltages.

kn0where 5y ago

The Classic II (and others newer than the earliest Macs) had bleeder resistors to discharge the CRT automatically. Older monitors and TVs did not, so manually discharging was more important. The procedure was still to manually discharge the newer machines in case the bleeder resistor failed, but whenever I discharge the CRT of anything newer than a Mac Plus it’s already discharged well before I touch the grounded screwdriver to the underside of the anode cap.

reaperducer 5y ago
The Classic II (and others newer than the earliest Macs) had bleeder resistors to discharge the CRT automatically Was that why older/cheaper CRTs just turned off, but the more expensive and newer ones made kind of a "thunk" noise after the screen turned off?