Lardo Boffin wrote: ↑Thu Apr 22, 2021 1:47 pm
The 9V read 11.03V on mine at the regulator pins (once I found an original PSU). Will that vary from PSU to PSU?
Yes, as Sinclair used at least four different suppliers for the UK1400 PSUs. Also as they are unregulated, the output voltage is proportional to the mains voltage. So if the mains voltage is higher, the output will also be higher.
Lardo Boffin wrote: ↑Thu Apr 22, 2021 1:47 pmAlso would fitting a modern voltage regulator, e.g. a Murata, have any effect on this? Aside the being able to get rid of the heat sink that is.
Fitting a switching regulator / DC-DC converter reduces the current consumption from the mains PSU, which actually makes the problem worse!
The problem is the design of the DC-DC converter circuit formed by TR4, TR5, the coil and associated other components. TR5 compares the voltage on the +12V line (via resistors R58 and R59 which act as a voltage divider) to the +5V line (which here is being used as a reference voltage).
However, prior to the issue 4 boards, DC current could flow directly through the coil from the +9V (nominal) supply rail and feed the +12V line. This is not really a problem for the +12V rail if the +9V line is actually at, or near +12V (although any AC ripple on it from the unregulated PSU is undesirable).
However, if the 9V line is actually at 11.9V (approximately) or above, TR5 will start to turn off, and it will reduce the drive to, or even shut down TR4. TR4 drives pulses through the coil. With insufficient pulses from the coil, the coil will not be able to charge and discharge capacitor C46. Hence the system will not be able to generate the extremely critical -5V supply. Without this negative -5V supply to correctly bias the 4116 (or equivalent) DRAM chips, they are likely to be damaged.
If a switching regulator / DC-DC converter is fitted, the reduced current consumption from the unregulated UK1400 (or similar) PSU
WILL result in the output voltage on the +9V (nominal) rail increasing compared to when a 7805 voltage regulator is used.
The solution is to modify the board with
Sinclair’s latest revision of the DC-DC circuitry or to use a regulated +9V PSU. Although fitting this modification to an issue two board is considerably more difficult than in later issue boards.
If either the modification is done, or only regulated +9V PSUs are used, then, and only then, is it safe to change the 7805 voltage regulator to a switching regulator / DC-DC converter.
I hope this all makes sense...
Mark