Wall wart and pixel speckling
Posted: Mon May 13, 2024 7:03 pm
I've got a few Dragon 32s, one with the original internal PSU PCB and two with the internal SMPS supplied by dragonplus (thanks John). The first is powered by the original transformer, the other two by a 12V wall wart. All three use an external RGBtoHDMI (analog) to produce a colour display.
The facts are these:
a) The one with the original components produces a nice clean and crisp display with no artefacts.
b) The two with the internal SMPSs and external wall wart produce a correct display but there is additional dynamic and rapid pixel flickering/speckling all over the screen.
c) If I replace the wall wart with a suitably fused 12V car battery then the two machines with internal SMPSs produce a clean & crisp display with no pixel flickering/speckling.
That rather points to the display circuitry being sensitive to switching noise from the wall wart. I've tried a beefy filter, combining low pass and common mode circuitry after the wart, with no success. I've also tried several wart makes to no avail (most seem to produce around 400mV of switching spikes).
The question is: can anyone recommend a make/model of 12V 2-3A external SMPS that's as quiet as a mouse, or must I build a linear one?
Alan
The facts are these:
a) The one with the original components produces a nice clean and crisp display with no artefacts.
b) The two with the internal SMPSs and external wall wart produce a correct display but there is additional dynamic and rapid pixel flickering/speckling all over the screen.
c) If I replace the wall wart with a suitably fused 12V car battery then the two machines with internal SMPSs produce a clean & crisp display with no pixel flickering/speckling.
That rather points to the display circuitry being sensitive to switching noise from the wall wart. I've tried a beefy filter, combining low pass and common mode circuitry after the wart, with no success. I've also tried several wart makes to no avail (most seem to produce around 400mV of switching spikes).
The question is: can anyone recommend a make/model of 12V 2-3A external SMPS that's as quiet as a mouse, or must I build a linear one?
Alan