IBM 51xx PC Family Computers  -  Hazards Within

Power supplies

With the power supply open and connected to mains (house) power, there is the possibility that you may contact mains (house) voltage.
Depending on the scenario, that can be fatal.

The large aluminium electrolytic filter capacitors are considered hazardous, because of the combination of high voltage and high charge.  You need to 'treat them with respect'.

When you remove mains (house) power from the power supply, these filter capacitors take time to discharge.  In most (most, not all) models of power supply, the discharge is speeded up by the use of a bleeder resistor.

In rare cases, power supplies provide an indication (example) of how much time after power-off that they (that particular model) can be considered safe (from an electrical perspective), but what if some kind of failure in the power supply results in that time now being invalid !

To gauge discharge status, a voltmeter can be used to measure the voltage on the filter capacitors.

For information on manually discharging the filter capacitors, see here.

High voltage on CRT

The CRT is a large capacitor, charged to many thousands of volts.
It is often said that there is not enough stored energy to kill, but it is possible that a heart attack could be triggered.

Note that the CRT has very low leakage, and can hold charge for days after removal of power.

Some monitors have a bleeder resistor fitted to discharge the CRT after removal of power, however:
• Different models of monitor will have different discharge times.
• What if the bleeder resistor is faulty !

Additional CRT safety information is at here.

CRT implosion

I see people writing things like, "You don't need to worry about the CRT imploding, because if it does, the 'implosion protection' mechanism will stop glass flying about."
• Not all CRTs have 'implosion protection'
• Any 'implosion protection' is not perfect

Wearing safety glasses is a simple thing to do that should stop glass fragments from going into your eyes.

Beryllium oxide

At the time of the subject equipment, the thermal paste used between semiconductors and heatsinks, typically contained beryllium oxide (BeO).
Beryllium oxide is known to be carcinogenic.

I think it was in the nineties, that I started to see electronics stores advertising that their new stocks of thermal paste were now free of beryllium oxide.


The edges and corners on vintage heatsinks can sometimes be extremely sharp, delivering deep cuts.