There are some possible mods listed below. Both are non-critical "faults" and unlikely many will have these issues. Though in case someone does, then they can do the mods as listed below. If you do not have the issues, then there is no need to do the mods. You can however still do the mods if you so wish.
MOD1 - Trip voltage testing & fix (aka fuse blowing for no reason)
I have had problems with tolerances on the "4V" zener diode. Where I later soldered a resistor or 2 on the bottom of the zener to correct the trip voltage. However its possible some zeners might be even more out of spec than I initially found. No ill effects will happen other than the 5V trip voltage being "over sensitive" which results in random blowing of the fuse.
To check the trip voltage, a meter need to be placed on 0V and the "back end" of the 4V zener diode. One end will measure about 5volts, the other should measure somewhere around 0.20V-0.30V or lower. Due to tolerances on the SCR the trip voltage can be anywhere between 0.4V and 0.6V. So if the zener voltage is higher than about 0.3V then it may run the risk of blowing the fuse for no reason at all. The SCR should not trip until 0.6V though again with tolerances on parts its possible for it to trip at 0.4V. So I suggest anything over 0.3V is start to get a bit close to tripping.
Typically I worked out the values to trip at 5.5volts. Though I did not expect the tolerances to vary so much, so the "typical values" fitted might not suit all PSUs. Of course all PSU's have been tested before I send them out. Though as parts warm up over several hours of use, they may drift enough to blow the fuse.
Option 1-
Should the voltage be higher than 0.3V then I suggest increasing the resistor values on the bottom of the PCB (soldered under the 4V zener) by about 100R-200R and try the zener voltage again. This additional resistance should be in series with the existing resistors. Typically the value there already will be about 2.4K, so the value should end up being 2.5K and 2.6K. Do not increase the value to high otherwise the trip voltage could end up being over 6volts.
Option 2- Recommended
The zener diode and resistors are removed and a new zener used in its place. This zener is a BZX79-C5V1 diode and manufactured by NXP. DO NOT USE ANY OTHER BRANDS OF ZENER!
If you do not have a problem with the fuse blowing, then there is no need to do the above fix.
MOD2 - "soft start" power up fix.
Later revisions have this "feature" was removed. The fault would show up as showing 3volts on the 5V rail, simply because the regulator was turning off and on very fast. After about 30-60seconds the PSU would work normally.
If you have this problem then there is a 47K resistor near the 4V zener (near the left regulator itself) needs to be removed and a wire link placing there instead. This will make the regulator as "always on" so it removes the possible power up issue.
In later builds of the PSU, there is a 47K and small capacitor missing near the SCR on the top right of the board. Those parts are no longer fitted and no longer needed once the first 47K resistor is linked over.
If you do not have this issue then there is no need to do this fix.
- exxosmod.jpg (117.33 KiB) Viewed 1946 times
Much later revisions had the 2 regulators vertical and were later batches which are not affected by the above issues.
- exxosfalcpsu.jpg (176 KiB) Viewed 1946 times