There have been many articles concerning the cause as well as cure for QL crashes. These articles have basically attributed these crashes to:
1) Glitches on internal power and ground PC runs; the fix is to add capacitors.
2) Internal heat build up; the fix concerns the computer’s 5 volt regulator.
3) External AC power problems; the fix an external power supply.
This article provides an easy cure for random occassional lockups. As this is an easy fix, it is suggested that you try this one first.
My Qls, in accordance with previous articles, do have the capacitor modifications. One of my QLs however would still have an occasional random crash, several times a day. Because of its randomness I suspected “noise” from the external power source rather than internal heat.
My original solution was, in accordance with an “Update” article, to add a high grade Coleco external power supply and including, to eliminate internal heat, bypassing the QL’s regulators.
My intentions were good however I was pressed for time and decided to try replacing the QL’s external power supply with a low grade substitue that I was no longer using. This power supply produces 9 volts from a simple transformer / rectifier / capacitor circuit, similar to the QL’s. A seperate transformer was added to produce the required 15VAC.
This simple short term fix worked; no more crashes for over a year. I wondered why, but never pursued it further.
One day while reviewing the QL’s power supply schematic in a back issue of “QUANTA” I knew why. The QL’s power supply with one exception is similar to mine. In fact, it was intended to be much better! The Ql’s power supply has an added component, a SCR to clip voltage spikes.
SCR’s are neat devices but sometimes treacherous to apply. If not applied properly they can turn on at the wrong time. Also, once turned on, they stay on (even if clipping is no longer required) until their current becomes zero.
The SCR does have a calibration adjustment, however I removed it by clipping off its 3 leads. I don’t believe that the loss of clipping will damage the QL, if unsure use an external spike protector.
I’m now back to using the QL’s power supply; no problems. Clipping the SCR’s three leads is simple, see the attached sketch. The procedure consists of:
1) Unplug the power supply (very important)
2) Turn the P. S. upside down. Insert a Phillips screw driver into the four holes and remove the four screws.
3) With the P. S. right side up, lift the top off.
4) Clip the three SCR leads, see the sketch.
5) Reverse steps one through three; put the P. S. back together
This simple fix should eliminate those random lock-ups which are power supply related.
